Published by Shoucheng DING1,2, Limei XIAO1,2, Rui HUANG1, Jie LIU1, Shizhou YANG1, Xiao CHAI1, College of Electrical and Information Engineering, Lanzhou University of Technology (1), Key Laboratory of Gansu Advanced Control for Industrial Processes (2)
Abstract. This paper designed a small terminal of a 35kV substation; it can meet the substation area electricity and the future development of the needs of the vision of growth in electricity demand. The article presents the design of transformers, substation main connection, and lightning protection. The practice shows that it applies to the township (town) of agricultural load and small-scale processing enterprises in electricity.
Streszczenie. W artykule przedstawiono projekt przyłącza podstacji elektroenergetycznej 35kV. W opisie uwzględniono projekt transformatorów, głównego przyłączenia podstacji oraz ochronę odgromową. W praktycznym ujęciu, projekt może mieć zastosowanie w rejonach agroturystycznych i niewielkich zakładach. (Podstacja elektroenergetyczna niskiej mocy na potrzeby małego skupiska odbiorców).
Keywords: miniaturization; transformer; substation main connection; integrated automation. Słowa kluczowe: miniaturyzacja, transformator, główne przyłącze podstacji, automatyka.
Introduction
In order to meet the electricity needs of the agricultural load and small processing enterprises of the township (town), we designed a 35kV terminal substation. When the substation is put into operation, it can improve the grid structure, reduce the power supply radius, reduce losses and improve the power supply capacity and quality of power supply, and there is a lot of adjustment of the layout of the entire grid. Area altitude 1750, the annual average temperature is 6℃ -9℃, the highest temperature is 35℃, over the years the minimum temperature is -22 ℃ and average annual rainfall is 450mm, the annual average wind speed is 1.6m / s. The instantaneous maximum wind speed and wind: 40m / s dominant wind northwest wind, over the years the maximum depth of frozen soil is 0.8m; underground 16 m depths is no groundwater [1-5].
The main transformer
In order to ensure reliable power supply and maintenance, running facilitate the design of the two main transformers. Taking into account the maximum capacity of a line transmission, the maximum load is 2400kVA. When a main transformer is outage, the remaining capacity of the transformer should be able to meet 70% -80% of the total load. Therefore, a single main transformer capacity shall be: S=2800×70%≈1680kVA. Taking into account the quality of power supply voltage, choose S9-1800/35 three-phase three-winding naturally air-cooled power transformers with OLTC voltage ratio: k =35 ± 8×2.5% / 10kV. Parameters Uk = 6.5%, the main transformer to use low-noise, low loss, oil spills, totally enclosed free hanging core main transformer, installed over-voltage, over temperature, light and heavy gas protection and on-line monitoring device.
Substation transformer
35kV line back to design a transformer connected to the incoming line isolation switch on the outside line, so to ensure the normal power supply are within the power outage, maintenance, emergency lighting, floating power supply. The design capacity is calculated at 0.5% of the main transformer capacity, so it can choose a 30kVA transformer as the transformer model S9-30/35, 35 ± 5% / 0.4, Uk = 6.5%, Y-yn0.
Substation main connection
According to the demand for electricity, this system 35kV line is a loop, qualifying four loops 112,113,114,115; its main wiring scheme is shown in Fig. 1. The 35kV voltage circuit into line, substation 35kV into line with isolation switch. The 10kV-side used the single bus connection mode. 35kV into line with isolation switch (GW5-35D), high and low pressure side of the main transformer installed 35kV, 10kV switch, 35kV, 10kV side single busbar connection. 111 interval as a backup interval, the capacitors are mounted on the the the 10kV bus side 103 intervals. The two main transformers are run in parallel
Fig.1. The main substation wiring diagram
Short-circuit current calculation
Short-circuit current calculation of the connection is shown in Fig. 2.
Known system power from the new substation 23km select a baseline capacity Sj=100MVA, the reference voltage Uj = Uav. XL=0.4×23×100/372 = 0.6720; Xc= (0.1+ XL ) /2= (0.1 +0.6720) /2= 0.3860; UK1=U1- 2/2= 6.5/2 = 3.25; XT1= (UK1/100) × (Sj / Se) = (3.25/100) × (100/3.6)= 0.9028.
Fig. 2. Short-circuit current calculation wiring diagram
35KV side of the current reference: Ie1=Se / (√3×37) = 1.56 kA. The 10KV lateral current reference: Ie2=Se / (√3×10.5) = 5.5 kA. 35KV side of the maximum operating current: Imax=1.05× (3600 /√3×37) = 59kA. 10KV side of the maximum operating current: Imax =1.05× (20000 / √3 × 10.5) = 208kA. Short-circuit points are shown in Fig. 3. Substation 35kV system, 35kV bus short circuit, short circuit current for 35kV outgoing switch, when the export of short-circuit, the distance from 35kV bus of more recent, can be seen on the bus short circuit, so as to chose d1-point as the selection and validation of 35kV switch, disconnecting switch, current transformer and voltage transformer short-circuit current calculation points.
Fig.3. Short-circuit points
Substation 10kV system, 10kV bus short-circuit, short circuit current for 10kV outgoing switch, when the export of short-circuit, the distance from 10kV bus of more recent, can be seen on the bus short circuit, so as to chose d2- point as the selection and validation of 10KV switch, disconnecting switch, current transformer and voltage transformer short- circuit current calculation points.
The d1-point short-circuits: Impedance: X * =0.3860, I * = 1 / X * = 2.59; Short-circuit current: Id1 = Ie1×I * = 1.56 × 2.59 = 4.04 kA; Full Current: ICH = 1.5×Id1 = 1.5×4.04 = 6.06 kA; Short Circuit Current: ICH = 2.55×Id1 = 2.55×4.04 = 10.30 kA; Short-circuit capacity: S = √3 Id1Ue = 1.732×4.04 × 37 = 258.9 MVA; The d2-point short-circuits: Impedance: X * = X1* + X2*/ 2 = 0.3860 +0.9028 / 2 = 0.8374, I * = 1 / X * = 1.194. Short-circuit current: Id2 = Ie2×I * = 5.5×1.194 = 6.567 kA, Full Current: ICH = 1.5×Id2 = 1.5×6.567 = 9.851 the KA. Short Circuit Current: ICH = 2.55×Id2 = 2.55×6.567 = 16.75 kA. Short-circuit capacity: S = √3 Id2Ue = 1.732×6.567×37= 119.43 MVA.
Main transformer protection
35kV main supply power substation, 35kV side of the three-stage phase current protection as the line of the main protection and back-up protection, protection Selection is PLP66-01 type the microcomputer line direction of the current voltage protection cabinet side. Back-up protection: complex voltage start-up over current protection; overload protection; light gas alarm; over temperature alarm. 10 kV shunt capacitor bank protection: the installation of trip current protection; definite time over current protection; over voltage protection; low-voltage protection; zero sequence over-voltage protection. 35kV, 10kV power distribution unit area and the main transformer area, each has a 600*600 cable trench, outdoor 600*600 summaries of the main ditch backward master control room and carrier machine room.
Substation lightning protection design
The substation suffered lightning damage from two aspects: Ray watch at the substation, lightning lines along the road to the substation invasion. Line lightning, lightning wave intrusion along the road is the main reason for substation mine-stricken, the line insulation level is relatively higher than the insulation level of the substation, the voltage level of incoming and outgoing in substation import and export the installation of zinc oxide surge arresters, which can be used to limit the invasion of the amplitude of the lightning, over-voltage on the device does not exceed the impulse with stand voltage value. The main transformer 35kV side of the commonly used 8.5m door frame, is the protection of a high degree of the hx = 8.5m. The use of a single lightning rod is clearly not meeting the requirements. The use of a lightning rod for two 30m distance of 119m, the axial height of 8.5m equipment to meet the requirements can also be protected.
Integrated substation automation
Scheduling system management principles and the specific circumstances of the 35kV substation remote information and communication using POLLING way, the communication protocol should be coordinated with the county dispatch automation system.
Integrated automation system is a typical distributed architecture, centralized assembly system configuration. Action protected with alarm, the prompt box to display the content. The remote control functions: switch can be remotely operated and print and save records the name of the operator, operating time, the nature of the operation and other information; should have anti-disoperation locking function. Remote functions: remote operation can be carried out on the switch and print and save records the name of the operator, the perk time, the nature of the operation and other information; and should be anti-disoperation. The integrated automation system features include measurement of accumulation, from time to time, at any time print, online maintenance functions, various types of data processing and computing. Substation lighting and maintenance of power by the AC and DC powered control panel. The main control room ceiling and embedded fluorescent lighting, and other room lighting is simple fluorescent lighting. 35kV, 10kV outdoor distribution equipment lighting spotlights lighting set to. Spotlights mounted on independent lightning rod, and the roof of the main control building.
Conclusions
Based hierarchy substation structure is compact, substation automation systems and equipment technology is advanced, and its running is reliable. It can meet the unattended requirements, and covers an area of less practical. It can effectively improve the grid structure, reduce losses, and increase the power supply capacity and quality of power supply, so that it can meet the demand of the area of electricity and the future growth in electricity demand.
REFERENCES
[1] J.Q. Pang. New Developing Trend of Substation Integrated Automation Technology, Automation Application, 4(2010), 49- 50. [2] Rajakanthan Thurairajah, Meyer Alan S. and Dwolatzky Barry. Computer generated transformer zones as part of township electrification design software, IEEE Transactions on Power Delivery, 15(2000), No. 3, 1067-1072. [3] Z.J. Liu, H.Y. Chen, K. Chen and M. Ye. The Analysis of Substation Earthing, Friend of Science Amateurs, 11(2012), 36-36. [4] S.C. Ding, J.H. Li, L.M. Xiao, R. Huang and S.Z. Yang, Intelligent Digital Multi-purpose Vehicle Instrument, Przeglad Elektrotechniczny, 88(2012), nr5b, 64-67. [5] J.F. Li and C.X. Zhang. A 2D-Role Based Universal Dynamic Configuration Management Infrastructure, Journal of Convergence Information Technology, 7(2012), No. 1, 188-196.
Authors: Shoucheng Ding, associate professor, College of Electrical and Information Engineering, Lanzhou University of Technology, Lanzhou 730050, Gansu, P.R. China, E-mail: dingsc@lut.cn
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 3b/2013
Published by Electrotek Concepts, Inc., PQSoft Case Study: Distribution Substation Capacitor Bank Evaluation, Document ID: PQS1014, Date: October 15, 2010.
Abstract: This case study presents a utility distribution substation capacitor bank harmonic analysis. The simulations were completed using the SuperHarm program. The investigation included frequency response and harmonic distortion simulations for a utility substation that included a step-down transformer and a number of distribution feeders with a significant number of small capacitor banks and several industrial customers.
INTRODUCTION
A utility distribution substation capacitor bank harmonic analysis case study was completed for the system shown in Figure 1. The 12.47 kV utility substation included a 36 MVA, 161 kV/12.47 kV step-down transformer and several distribution feeders that included a significant number of small capacitor banks and several industrial customers.
There were two 5,400 kVAr, 12.47 kV capacitor banks at the substation bus, a 1,200 kVAr capacitor bank at one customer location, and a number of 300 kVAr and 600 kVAr capacitor banks on four distribution feeders. Each distribution feeder also included a number of distributed harmonic producing nonlinear loads representing a variety of customer facilities.
The customer loads included a 450 kVA step-down transformer supplying a 69 kVAr, 480 V power factor correction capacitor bank and a 15 MVA, 12-pulse induction furnace. The harmonic characteristics of the various customer nonlinear loads were determined from field measurements at the meter point for each customer. The case study was completed using the SuperHarm® program. The accuracy of the simulation model was verified using three-phase and single-line-to-ground fault currents and other steady-state quantities, such as steady-state voltage rise.
Figure 1 – Illustration of Oneline Diagram for Distribution Capacitor Bank Evaluation
SIMULATION RESULTS
The utility was experiencing blown fuses on feeder capacitor banks when the 15 MVA induction furnace was in operation. Capacitor banks that were switched on to improve the voltage regulation for the distribution feeders caused harmonic resonances that caused very high voltage and current distortion levels that also led to the fuse failures.
The configuration of the distribution system capacitor banks included one larger substation capacitor bank that was switched in two steps, one capacitor bank connected at the customer primary, and 14 smaller feeder capacitor banks. The feeder capacitor banks included five that were temperature-controlled, six that were fixed, and three that were fixed, but were de-energized during the winter months. The simulation analysis for the case study included 12 distinct capacitor bank configurations that were investigated.
Fault currents at various points on the distribution system and the voltage rise at 12.47 kV bus with the 5,400 kVAr capacitor banks in-service were used to verify the accuracy of the harmonic simulation model. The steady-state voltage rise at the substation bus for each 5,400 kVAr, 12.47 kV capacitor bank was approximately 1.8%.
Figure 2 shows the harmonic current characteristic for one operating point of the 12-pulse induction furnace. The current had a fundamental frequency value of 650 A, an rms value of 652 A, and a THD value of 3.3%. The highest harmonic current components were the 11th at 2.75% and the 13th at 1.65%. The waveform shown in Figure 2 was created using an inverse DFT with 256 points per cycle.
Figure 2 – Customer Induction Furnace Current Waveform
Figure 3 shows the impedance vs. frequency simulation result with the 1,200 kVAr, 12.47 kV feeder capacitor bank in service (Case 8b). The frequency scan was placed at Customer 1. The base-case result with no utility or customer capacitor banks in-service (Case 8a) was also shown on the graph so the two conditions can be easily compared. The simulated parallel resonance due to the addition of the shunt capacitor bank was 708 Hz (11.8th harmonic). A simple expression may be used to validate this result:
.
In addition, the simulated steady-state voltage rise with the 1,200 kVAr, 12.47 kV capacitor bank in-service was 0.8%. This value may be validated using the following expression:
.
where: hr = parallel resonant frequency (x fundamental) ΔV = steady-state voltage rise (per-unit) MVA3ϕ = three-phase short circuit capacity (MVA = √3*12.47 kV*7.17kA≈155MVA) MVAr3ϕ = three-phase capacitor bank rating (MVAr)
Figure 3 – Illustration of Frequency Response with Feeder Capacitor Bank In-Service
A more thorough investigation of the effect of the various substation and feeder capacitor banks was completed using a batch solution capability that allows multiple data cases to be completed consecutively. A batch identification variable was used to create a number of different operating conditions with the repeated solution of the same date case. Different distinct frequency scan output files were also created for each set of system conditions. This simulation process allowed the implementation of sophisticated techniques to simplify certain analysis procedures.
The batch solution involved 12 different cases, representing three different substation capacitor bank conditions, two winter capacitor conditions, and two temperature-controlled capacitor bank conditions. Figure 4 shows the impedance vs. frequency simulation results for the different operating conditions for the two 5,400 kVAr, 12.47 kV substation capacitor banks. The frequency scan was placed at Customer
The base-case result with no utility or customer capacitor banks in-service was also shown for reference. The simulated parallel harmonic resonant frequency with neither substation capacitor bank in-service was 708 Hz (11.8th harmonic). Addition of the first 5,400 kVAr capacitor bank reduced the resonant frequency to 390 Hz (6.5th harmonic), while the addition of both 5,400 kVAr capacitor banks further reduced the resonant frequency to 312 Hz (5.2nd harmonic).
The magnitude of the simulated impedance at the lower-order harmonics was significantly reduced with the substation capacitor banks in-service. This would suggest that the voltage distortion at the customer and substation buses would indeed be somewhat lower with the substation capacitor banks in-service. The resulting voltage distortion levels will be shown later in the case.
Figure 4 – Illustration of Effect of Substation Capacitor Banks on Frequency Response
Figure 5, Figure 6, and Figure 7 show the impedance vs. frequency simulation results for various feeder and substation capacitor bank configurations. The base-case result with no utility or customer capacitor banks in-service was also shown for reference. The codes used for the simulation legends included:
“W” Winter Season Feeder Capacitor Banks, Either Off or On (e.g., W Off, W On) “T” Temperature-Controlled Feeder Capacitor Banks, Either Off or On (e.g., T Off, T On)
Figure 5 shows the simulated frequency response characteristics for the cases with no substation capacitor banks (5,400 kVAr, 12.47 kV) in-service. The most severe resonances were near the 11th and 19th harmonics.
Figure 6 shows the corresponding simulated frequency response characteristics for the cases with one 5,400 kVAr substation capacitor bank in-service. The most severe parallel resonances were near the 19th harmonic.
Figure 7 shows the corresponding simulated frequency response characteristics for the cases with both 5,400 kVAr substation capacitors bank in-service. The most severe parallel resonances are near the 17th harmonic.
Figure 5 – Illustration of Feeder Capacitor Banks with No Substation Banks
Figure 6 – Illustration of Feeder Capacitor Banks with One Substation Bank
Figure 7 – Illustration of Feeder Capacitor Banks with Both Substation Banks
Table 1 summarizes the harmonic distortion simulation results that corresponded with the frequency scans shown in the previous three figures. The table includes the same simulation legend codes, as well as the voltage distortion (VTHD) at the 12.47 kV substation bus and at the 12.47 kV bus at Customer 1. For reference, the voltage distortion values with no utility or customer capacitor banks in-service was 2.25% at the substation bus and 4.13% at the customer bus.
As was illustrated with the previously simulated frequency response characteristics, the worst case harmonic distortion levels were when neither of the 5,400 kVAr substation capacitor banks were in-service. The severe distortion levels for these operating conditions resulted in a number of feeder capacitor bank fuses failing when the induction furnace was operating near capacity.
Distortion cases 8c through 8f corresponded to the frequency response characteristics previously shown in Figure 5. These were the most severe harmonic distortion levels at both the customer and utility substation buses. The high distortion levels were primarily due to the 11th harmonic resonance along with the 11th and 13th harmonic current components created by the 12-pulse induction furnace. Distortion cases 8g through 8j corresponded to the frequency response characteristics shown in Figure 6, while distortion cases 8k through 8n corresponded to the frequency response characteristics shown in Figure 7.
Table 1 – Summary of the Voltage Distortion Levels with Various Capacitor Banks In-Service
Case Number – Case Description
12.47 kV Substation Bus
12.47 kV Customer #1 Bus
Case 8c – 0x5400kVAr, W Off, T Off
8.09%
16.29%
Case 8d – 0x5400kVAr, W Off, T On
7.66%
15.01%
Case 8e – 0x5400kVAr, W On, T Off
8.68%
16.18%
Case 8f – 0x5400kVAr, W On, T On
8.17%
15.18%
Case 8g – 1x5400kVAr, W Off, T Off
2.79%
2.95%
Case 8h – 1x5400kVAr, W Off, T On
4.35%
4.72%
Case 8i – 1x5400kVAr, W On, T Off
3.09%
3.25%
Case 8j – 1x5400kVAr, W On, T On
4.86%
5.33%
Case 8k – 2x5400kVAr, W Off, T Off
4.81%
5.62%
Case 8l – 2x5400kVAr, W Off, T On
3.50%
4.16%
Case 8m – 2x5400kVAr, W On, T Off
5.93%
6.83%
Case 8n – 2x5400kVAr, W On, T On
3.18%
3.86%
.
The power conditioning solution alternative that was investigated during the study was converting one of the existing feeder capacitor banks into a passive shunt single-tuned harmonic filter. Passive filters are made of inductive, capacitive, and resistive elements. They are relatively inexpensive compared with other means for eliminating harmonic distortion, but they have the disadvantage of potentially adverse interactions with the power system. They are employed either to shunt the harmonic currents off the line or to block their flow between parts of the system by tuning the elements to create a resonance at a selected harmonic frequency.
Filters are generally tuned slightly below the harmonic frequency of concern. This method allows for tolerances in the filter components and prevents the filter from acting as a direct short circuit for the offending harmonic current. It also minimizes the possibility of dangerous harmonic resonance should the system parameters change and cause the tuning frequency to shift slightly higher. The filter design involved converting the existing 1,200 kVAr, 12.47 kV capacitor bank at Customer 1 into a 4.7th harmonic filter.
Figure 8 shows the impedance vs. frequency simulation results with the Customer 1 power factor correction capacitor bank reconfigured as a 4.7th harmonic filter. The previous worst-case frequency scan and the base-case with no utility or customer capacitor in-service banks was shown for reference. As can be observed in the figure, the frequency response characteristic shows the very low impedance at the filter tuning frequency. Figure 9 shows the effect of various feeder capacitor banks on the frequency response with the harmonic filter in-service.
Figure 8 – Illustration of Frequency Response with Feeder Harmonic Filter In-Service
Figure 9 – Illustration of Effect of Feeder Capacitor Banks with Customer Harmonic Filter
Table 2 summarizes the harmonic distortion simulations with the 1,200 kVAr, 4.7th harmonic filter in-service. The table includes the same simulation legend codes, as well as the voltage distortion (VTHD) at the 12.47 kV substation bus and at the 12.47 kV bus at Customer 1. The results shown in the table highlight the fact that all of the voltage distortion levels were below the commonly-used voltage distortion (VTHD) limit of 5%
Table 2 – Summary of the Voltage Distortion Levels with the Harmonic Filter Bank In-Service
Case Number – Case Description
12.47 kV Substation Bus
12.47 kV Customer #1 Bus
Case 8n – 0x5400kVAr, W Off, T Off
2.02%
3.04%
Case 8o – 0x5400kVAr, W Off, T On
2.08%
3.27%
Case 8p – 0x5400kVAr, W On, T Off
2.35%
3.57%
Case 8q – 0x5400kVAr, W On, T On
2.84%
3.97%
Case 8r – 1x5400kVAr, W Off, T Off
3.05%
2.42%
Case 8s – 1x5400kVAr, W Off, T On
2.55%
1.88%
Case 8t – 1x5400kVAr, W On, T Off
4.17%
3.60%
Case 8u – 1x5400kVAr, W On, T On
2.45%
1.95%
Case 8v – 2x5400kVAr, W Off, T Off
2.03%
2.07%
Case 8w – 2x5400kVAr, W Off, T On
3.12%
2.68%
Case 8x – 2x5400kVAr, W On, T Off
2.31%
2.39%
Case 8y – 2x5400kVAr, W On, T On
3.43%
2.90%
.
SUMMARY
This case study summarized a utility distribution substation capacitor bank harmonic analysis. The investigation included frequency response and harmonic distortion simulations for a 12.47 kV utility substation that included a 36 MVA, 161 kV/12.47 kV step-down transformer and a number of distribution feeders with a significant number of small capacitor banks and several industrial customers.
The utility capacitor banks included two 5,400 kVAr, 12.47 kV capacitor banks at the substation bus, a 1,200 kVAr capacitor bank at one customer facility, and a number of 300 kVAr and 600 kVAr capacitor banks on four distribution feeders.
The customer loads included a 450 kVA step-down transformer supplying a 69 kVAr, 480 V power factor correction capacitor bank and a 15 MVA, 12-pulse induction furnace. In addition, each distribution feeder also included a number of distributed harmonic producing nonlinear loads representing a variety of customer facilities.
The power conditioning mitigation alterative selected was to convert the 1,200 kVAr, 12.47 kV capacitor bank at one customer facility into a shunt passive harmonic filter tuned to the 4.7th harmonic which, in turn, reduced the harmonic voltage distortion levels for all of the simulated contingencies to below the specified limit.
Due to the excessive component duty requirements, the capacitor bank units that were used in the shunt harmonic filter design were rated at 14.4 kV for application on the 12.47 kV customer primary bus.
REFERENCES
1.IEEE Recommended Practice for Monitoring Electric Power Quality,” IEEE Std. 1159-1995, IEEE, October 1995, ISBN: 1-55937-549-3. 2.IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, IEEE Std. 519-1992, IEEE, ISBN: 1-5593-7239-7. 3.R.C. Dugan, M.F. McGranaghan, S. Santoso, H.W. Beaty, “Electrical Power Systems Quality,” McGraw-Hill Companies, Inc., November 2002, ISBN 0-07-138622-X.
RELATED STANDARDS IEEE Std. 1159, IEEE Std. 519
GLOSSARY AND ACRONYMS ASD: Adjustable-Speed Drive CF: Crest Factor DPF: Displacement Power Factor PF: Power Factor PWM: Pulse Width Modulation THD: Total Harmonic Distortion TPF: True Power Factor
Published by Liu Liqun, Liu Chunxia*, Taiyuan University of Science & Technology
Abstract. At present, an optimal maximum power tracking (MPPT) and grid-connected control methods for a PV power system are very important to improve the output efficiency. However, most of literatures only introduce the MPPT or PLL method, respectively. A novel MPPT method is proposed by improving the convention perturbation and observation (P&O) method in this paper, which can track the real peak of PV array at different irradiation and partial shading. To analyze the locked performance, the Single Synchronous Reference Frame Software Phase Locked Loop (SSRF-SPLL) is discussed at different grid faults such as single phase voltage drop, two phase voltage drop, and frequency discontinuity.
Streszczenie. W artykule przedstawiono algorytm MPPT oparty na zmodyfikowanej metodzie perturbacji i obserwacji, umożliwiający śledzenie punktu mocy maksymalnej zespołu paneli PV w warunkach niejednorodnego nasłonecznienia i częściowego zacienienia. W celu analizy działania systemu podłączonego do sieci elektroenergetycznej, zastosowano algorytm synchronizacji fazowej PLL o pojedynczej ramce. (Sterowanie zespołem paneli PV podłączonym do sieci elektroenergetycznej – algorytmy MPPT i SSRF-SPLL).
Keywords: PV array; Maximum Power Point Tracking; Partial shading; Phase Locked Loop. Słowa kluczowe: tablica paneli PV, MPPT, częściowe zacienienie, PLL.
Introduction
With the increasing concern about global environmental protection, the need to produce pollution-free natural energy such as solar energy has received great interest as an alternative source of energy for the future since solar energy is clean, pollution-free and inexhaustible. In an effort to use the solar energy effectively, a great deal of research has been done on the grid-connected photovoltaic generation systems [1]. Grid interconnection of PV power system has the advantage of more effective utilization of generated power. However, the technical requirements from both the utility power system grid side and the PV system side need to be satisfied to ensure the safety. Normally, there are many problems need solve to satisfy of the PV installer and the reliability of the utility grid such as islanding detection, harmonic distortion requirements and electromagnetic interference, etc [2]. A Grid-Connected PV Power Systems (GCPPS) consists of the PV array, the inverter, the convert, test module, and protect module, and so on. Here the output direct current (DC) of PV power system is converted to the alternate current (AC) to generate power to utility. In order to extract the maximum power from the costly PV modules, the MPPT control method is necessary by controlling the duty cycle of the switch of the DC–DC inverter. Furthermore, the DC-AC converter is used to convert the DC current to sinusoidal current and inject into the grid, and to satisfy the need of grid utilization, the output voltage and frequency of AC from converter must be same to that of the grid, which is easy to realize. However, the instantaneous voltage, frequency, and phase is difficult to gain, certainly, the grid-connected between PV power system and the grid becomes a difficult task. A Phase-Locked Loop technology is necessary to satisfy the grid-connected need.
Many synchronization techniques have been presented over the recent years. In synchronous reference frame based Phase Locked Loop (PLL) based systems, the phase angle estimation is adaptively updated by a closed loop mechanism whose objective is to track the actual frequency and phase angle [3]. The three-phase grid-connected converter is widely used in renewable and electric power system applications. Traditionally, control of the three-phase grid-connected converter is based on the standard decoupled d–q vector control mechanism [4]. Moreover, there are many factors affect the grid-connected characteristic such as the change of the temperature, irradiation and partial shading. And the different MPPT and PLL methods affect the output efficiency of PV power system and the safety of the grid utilization. Many international application examples have been introduced in some literatures [5]. Nevertheless, most of them only introduce the PLL or MPPT control method. This article presents a design case of PV power system which considers the MPPT and grid-connected to improve the output performance and reduce the loss of PV system and satisfy the need of grid-connected. Firstly, the topology structure is described. A novel improved P&O MPPT method is introduced in the next section. Then, the SSRFSPLL is used in this GCPPS, and the output characteristic of SSRF-SPLL at different grid faults are described [6]. Finally, simulation results demonstrate the correctness of the proposed method.
Topology structure of the GCPPS
Fig.1 shows the topology structure of the GCPPS. Here, multiple series-parallel PV modules become a PV array, and the DC bus is connected with the DC-DC inverter, then the DC output is injected into the DC-AC converter. Finally, the AC output is injected into the grid. Certainly, test module and control module are necessary to insure the satisfy performance. For example, the output voltage and output current signals of PV array are sampled to calculate the optimal Pulse-Width Modulation (PWM) and track the maximum power point at the time. Furthermore, the three-phase output voltages of grid are sampled to gain the optimal grid-connected PWM using the PLL technology.
Fig.1. Topology structure of the grid-connected PV power system
Proposed MPPT method
There is only one maximum power point (MPP) of PV array at uniform irradiation, and the MPPT method is very easy such as P&O, incremental conductance, fuzzy logic, etc. Current–voltage and power–voltage characteristics of large PV arrays under partially shaded conditions are characterized by multiple steps and peaks [7]. In these reasons, tracking the MPP is difficult under the partial shading or non-uniform conditions [8]. Rapidly changing shadow conditions increase the difficulty of MPPT [9]. In order to track the real MPP of PV array at partial shading, the conventional P&O method is improved in this section.
Fig.2. P-I-V output characteristic of PV at partial shading
Fig.2 shows that there are three MPP at partial shading, and the B point is the real MPP (called the global peak (GP)) and the A and C points are called the local peaks (LP). Certainly, the conventional P&O method has been described in many literatures which can track the LP (A point) as can be seen from Fig.2. However, which can not track the GP (B point) under the partially shaded conditions. An easy improvement can improve the output performance of the conventional P&O method. When the conventional P&O method tracks the A point and the PWM reaches the stable value PWMA . A perturbation is added in the PWMA , and the perturbation range is ± 30% of PWMA and the perturbation step is 0.5% of PWMA . Fig.3 shows that the proposed method have better output efficiency and sensitivity and response speed of improved method as compare with the conventional P&O method.
Fig.3. Output characteristic curves at partial shading
Grid-connected output using the SSRF-SPLL
In grid-connected systems, accurate phase angle and frequency of utility voltage is essential since the voltage or current reference is synchronized with the phase of the utility voltage for various applications such as power factor correction, active/reactive power control [3]. The phase angle estimation of SSRF-SPLL is adaptively updated by a closed loop to track the actual frequency and phase angle, and the structure is very simple, which is widely accepted synchronization algorithm for grid connected systems.
The three phase voltages Va , Vb , Vc are expressed as equation (1).
.
The three-dimensional coordinates are transformed into the stationary reference frame signals Vα and Vβ as can be expressed in (2). Where, Vγ= 0 because of Va , Vb , Vc are symmetrical.
.
The stationary reference frame signals are transformed to the rotating reference frame signals Vd and Vq .
.
Where θ‘ = w’t , and w’ is the angular frequency of the rotating dq frame, and t is the time. θ is the actual phase and θ‘ is the accurate estimate. When θ‘ – θ = 0 , the PLL gets locked to the utility voltage. Vm is the magnitude of voltage. And a proportional-integral (PI) controller is used in Vq . The model diagram of SSRF-SPLL can be seen from Fig.4. PI controller is equivalent to a loop filter, and Kp and Kiare the parameters of PI, which are 10 and 802, respectively.
Fig.5. Frequency and voltage locked under ideal conditions
There are some questions affect the grid-connected performance of PV power system, such as the change of output voltage amplitude of PV array due to the change of solar irradiation, the single-phase voltage drop or the two phase voltage drop or voltage offset or the frequency discontinuity due to the grid failures. It is very important to consider different faults because of which affects the output efficiency and performance.
To clarify the correctness and effectiveness of SSRF-SPLL, the ideal situation, single-phase voltage drop, two phase voltage drop, and frequency discontinuity are considered in this section. As shown in Fig.5, the SSRF-SPLL can track the frequency and voltage of grid, which has high precision and rapid response speed. Fig.6 (a) shows the tracking performance of frequency locked at different grid faults, and two phase voltage drop has the largest frequency oscillation. Fig.6 (b) shows the voltage locked at voltage drop, and the tracking performance is worst at two phase voltage drop. In other words, the SSRF-SPLL is different to gain the correct frequency of grid when single-phase or two phase voltage drop. The performance of voltage locked at frequency discontinuity can be seen from Fig.7. The SSRF-SPLL technologies can rapid lock the Voltage and frequency as shown in Fig.7 (a). Here, Fig.7 (b) shows the partial enlarged drawing at frequency discontinuity, and the change span is from 50Hz to 40Hz.
Fig.6. Comparison of frequency and voltage locked at voltage drop
Fig.7. Voltage locked at frequency discontinuity
Fig.8. Phase locked under different conditions
The comparison of phase locked of different situations can be seen from Fig.8. The phase locked performance is excellent under ideal or frequency discontinuity conditions as compare with that of single-phase voltage drop or two phase voltage drop.
Conclusions
The total control methods of MPPT and PLL using GCPPS is described in this paper. First, the proposed MPPT method can track MPP at different irradiation or shading. Second, the output performances of SSRF-SPLL at different grid faults are described to pay attention to select the optimal PLL technology based on different grid faults.
Acknowledgments: this work was supported by the Program for the Industrialization of the High and New Technology of Shanxi province (NO: 2010016), Youth Science Foundation of Shanxi province (NO: 2011021014-2), Doctor Fund of Taiyuan University of Science & Technology (NO: 20122018).
REFERENCES
[1] L. Hassaine, E. Olias, J. Quintero, M. Haddadi. Digital power factor control and reactive power regulation for grid-connected photovoltaic inverter, Renewable Energy, 34(2009) No.1, 315-321. [2] Mohamed A. Eltawil, Z. Zhao. Grid-connected photovoltaic power systems: Technical and potential problems—A review, Renewable and Sustainable Energy Reviews, 14(2010) No.1, 112-129. [3] B. Indu Rani, C.K. Aravind, G. Saravana Ilango, C. Nagamani. A three phase PLL with a dynamic feed forward frequency estimator for synchronization of grid connected converters under wide frequency variations, International Journal of Electrical Power and Energy Systems, 41(2012) No.1, 63-70. [4] S. Li, Timothy A. Haskew, Yang-Ki Hong, L. Xu. Direct-current vector control of three-phase grid-connected rectifier–inverter, Electric Power Systems Research, 81(2011) No.2, 357-366. [5] J. H. So, Y. S. Jung, G.J. Yu, J. Y. Choi, J. H. Choi. Performance results and analysis of 3 kW grid-connected PV systems, Renewable Energy, 32(2007) No.11, 1858-1872. [6] S. K. Chung. A phase tracking system for three phase utility interface inverters, IEEE Transactions on Power Electronics, 15(2000) No.3, 431-438. [7] H. Patel, V. Agarwal. Maximum power point tracking scheme for PV systems operating under partially shaded conditions, IEEE Trans. Ind. Electronics, 55 (2008) No.4, 1689-1698. [8] Karatepe, E. Syafaruddin, T. Hiyama. Artificial neural network-polar coordinated fuzzy controller based maximum power point tracking control under partially shaded conditions, IET Renewable Power Generation, 3(2009) No.2, 239-253. [9] L. J. Gao, Roger A. Dougal, S. Y. Liu, Albena P. Iotova. Parallel-connected solar PV system to address partial and rapidly fluctuating shadow conditions, IEEE Trans. Ind. Electronics, 56 (2009) No.5, 1548-1556.
Authors: Assistant prof. L.Q. Liu, college of electronic and Information engineering, Taiyuan University of Science & Technology, Waliu road 66, Wanbolin district, Taiyuan, China, Email: llqd2004@163.com; Assistant prof. C.X. Liu, College of computer science & technology, Taiyuan University of Science & Technology, Waliu road 66, Wanbolin district, Taiyuan, China, Email: lcx456@163.com.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 3a/2013
Published by Masoud FARHOODNEA1, Azah MOHAMED1, Hussain SHAREEF1, Hadi ZAYANDEHROODI1, University Kebangsaan Malaysia (UKM) (1)
Abstract. This paper presents a dynamic PQ analysis on the effects of high-penetrated grid-connected photovoltaic (PV) systems in a distribution system under different weather conditions. To track practical considerations, all information on PV units and weather conditions given in this paper were collected from different solar panel producers and from the Malaysian Meteorological Department (MMD), respectively. A 1.8 -MW grid-connected PV system in a radial 16-bus test system is modeled and simulated using Matlab/Simulink software to study the effects of this technology on the system under different levels of solar irradiation. The simulation results proved that the presence of high-penetrated grid-connected PV systems could cause power quality problems such as voltage raise, voltage flicker, and power factor reduction.
Streszczenie. W artykule przedstawiono analizę dynamicznych zmian mocy w dużym systemie fotowoltaicznym dołączonym do sieci, w przypadku rożnych warunków pogodowych. W badaniach wykorzystano rzeczywiste informacje pogodowe. Model sumacyjny stworzono w programie Matlab- Simulink, a następnie sprawdzono jego działanie w warunkach różnego poziomu zadanego nasłonecznienia. Wyniki pokazały, że duże zespoły paneli PV, podłączone do sieci, mogą wywołać problemy z jakością energii. (Analiza jakości energii w sieci elektroenergetycznej z dołączonymi systemami fotowoltaicznymi).
Keywords: power quality; distributed generation; renewable energy; photovoltaic systems; voltage fluctuation; flicker Słowa kluczowe: jakość energii, generacja rozproszona, energia odnawialna, systemy fotowoltaiczne, wahania napięcia, migotanie.
Introduction
The use of photovoltaic (PV) systems as a safe and clean source of energy from the sun has been rapidly increasing. The application of PV systems in power systems can be divided into two main fields: off-grid or stand-alone applications and on-grid or grid-connected applications. Stand-alone PV systems can be used to provide power for remote loads that do not have any access to power grids while grid-connected applications are used to provide energy for local loads and for the exchange power with utility grids [1]. The first large grid-connected PV power plant with 1 MW capacity was installed in Lugo, California, USA. The second plant with 6.5 MW capacity was installed in Carissa Plains, California, USA. Currently, many large grid-connected PV systems with different ranges of power are operating in various countries, such as Switzerland, Germany, Australia, Spain, and Japan.
PV systems can enhance the operation of power systems by improving the voltage profile and by reducing the energy losses of distribution feeders, the maintenance costs, and the loading of transformer tap changers during peak hours [2]. Nonetheless, in comparison with other renewable technologies, PV systems still face major difficulties and may pose some adverse effects to the system, such as overloading of the feeders, harmonic pollution, high investment cost, low efficiency, and low reliability, which hinder their widespread use [3]. Moreover, variations in solar irradiation can cause power fluctuation and voltage flicker, resulting in undesirable effects on high penetrated PV systems in the power system [4]. Some control methods, such as Maximum Power Point Tracking (MPPT) can be used to improve efficiency of PV systems. In such controllers, both the produced voltage and the current of the PV array should be controlled. This may complicate the PV system structure with increased possibility of failure while tracking maximum power in unexpected weather conditions [5]. With respect to system protection scheme, the PV system-based distributed generations (DGs) should energize the local loads after the system has been disconnected from the utility grid during faulty conditions [6]. In these situations, any unintentional islanding may increase the risk of safety problems or damage to other parts of the system components, which can decrease system reliability [7].
These problems mean that accurately analyzing the effects of installing large grid-connected PV systems on the performance of the electric network is necessary. This evaluation is important because it can provide feasible solutions for potential operational problems that grid-connected PV systems can cause to other components in distribution systems. In the literature, many works focus on steady-state modeling and analysis of PV systems [8–11]. However, no attempt has yet been made to study the effects of grid-connected PV systems on the dynamic operation and control of the system before real-time implementation.
This paper aims to accurately analyze the effects of installing large grid-connected PV systems on the dynamic performance of distribution networks. To conduct practical analysis in the absence of field measurements, all PV unit modeling data were obtained from various solar panel manufacturers. To investigate the effects of different weather conditions on the produced power of the PV units, the required Kuala Lumpur meteorological data was obtained from the Malaysian Meteorological Department (MMD) [12]. Simulation was performed on a modified radial 16-bus test system with a 1.8-MW grid-connected PV system, using the Matlab/Simulink software to study the effects of the PV system on system performance under sunny and cloudy weather conditions.
PV System modeling
High-penetrated grid-connected PV systems, which are known as a type of DG in the megawatt range, are rapidly developed. These cover the majority of the PV market in different countries worldwide. The main components of a grid-connected PV system includes a series/parallel mixture of PV arrays to directly convert sunlight to DC power and a power-conditioning unit that converts DC power to AC power; this unit also keeps the PVs operating at maximum efficiency [13]. Figure 1 shows the general diagram of grid-connected PV systems. Notably, in many cases, energy storage devices such as batteries and super-capacitors are also considered the third component of grid-connected PV systems. These devices enhance the performance of PV systems, such as power generation at night, reactive power control over the PV systems, peak load shifting, and voltage stabilizing of grids [14].
Fig. 1 Simplified diagram of the grid-connected PV system.
To provide proper interface between grid-connected PV systems and the utility grid, some conditions must be satisfied, such as phase sequence, frequency and voltage level matching. Providing these conditions strongly depends on the applied power electronics technology of PV inverters.
The electric characteristics of a PV unit can generally be expressed in terms of the current-voltage or the power voltage relationships of the cell. The variations in these characteristics directly depend on the irradiance received by the cell and the cell temperature. Therefore, to analyze the dynamic performance of PV systems under different weather conditions, a proper model is required to convert the effect of irradiance and temperature on produced current and voltage of the PV arrays.
Figure 2 shows the equivalent electrical circuit of a crystalline silicon PV module. In this model, I is the output terminal current, IL is the light-generated current, Id is the diode current, Ish is the shunt leakage current, Rs is the internal resistance, and Rsh is the shunt resistance. In practice, the value of Rs strongly depends on the quality of the used semi-conductor. Therefore, any small variation in Rs value can dramatically change the PV output [15].
Fig. 2. Equivalent circuit of the PV module.
Following Fig. 1 the output current, I, of the PV module can be expressed as
.
where Vo is the voltage on the shunt resistance.
The diode current, Id, can be obtained using the classical diode current expression [15], thus
.
where Io is saturation current of the diode, q is electron, n is curve-fitting constant, K is Boltzmann constant, Tr is temperature on the absolute scale, and n is the ideality factor, whose value is between 1 and 2.
By substituting (2) in (1) and ignoring the last term, the output current, I, can be rewritten as
.
where the saturation current, Io, at different operating temperatures can be calculated [16], thus:
.
and,
.
In (4) and (5), Vg is the band gap voltage, Voc-Tr is the open circuit voltage, and Isc-Tris the short circuit current at the rated operating conditions.
The photocurrent IL, in (3) is directly proportional to the solar radiation level, G (W/m2), and can be expressed as
.
where,
.
where, αIscis the short circuit temperature coefficient. The open circuit voltage Voc, which is sensitive to temperature, can be also obtained [17], thus:
.
where, βVoc is the open circuit temperature coefficient. Notably, all coefficients should be determined under a standard rated condition of 25 °C cell temperature and 1000 W/m2 solar radiation level [18]. Using the provided coefficient by the manufacturers and the mathematical equations (3–8), any PV module can be modeled for dynamic analysis.
The produced DC voltage of a PV module can be raised to a specific level using a DC-DC boost converter, and an MPPT technique can be used in the boost converter to efficiently control the produced power of PV arrays. The produced DC power is then converted into AC power by using a three-phase three-level Voltage Source Converter (VSC). The power is then injected into the system using a coupling transformer at the desired voltage level.
Possible effect of grid-connected PV systems on distribution systems
Renewable energy sources, especially PV systems, have become more significant sources of energy, attracting considerable commercial interest. Nonetheless, the connection of large PV systems to utility grids may cause several operational problems for distribution networks. The severity of these problems directly depends on the percentage of PV penetration and the geography of the installation. Hence, knowing the possible impact of large grid-connected PV systems on distribution networks can provide feasible solutions before real-time and practical implementations. The aim of this section is to introduce possible effects that PV systems may impose on distribution systems.
Inrush Current The small inevitable difference between PV systems and grid voltages may introduce an inrush current that flows between the PV system and the utility grid at connection time, and decays to zero at an exponential rate. The produced inrush current may cause nuisance trips, thermal stress, and other problems [19].
Safety Safety is one of the major concerns in PV systems due to unintended islanding at the time of fault occurrence at the grid side. Here, PV systems continue to feed the load even after the network is disconnected from the utility grid, which may lead to electric shock of workers [20].
Over-voltage PV systems usually are designed to operate near unity power factor to fully utilize solar energy. In this case, the PV system only injects active power into the utility grid, which may change the reactive power flow of the system. Therefore, voltages of nearby buses can be increased because of the lack of reactive power [14]. The produced over-voltage can have negative effects on the operation of both the utility and customer sides.
Output power fluctuation The fluctuation of the output power of PV systems is one of the main factors that may cause severe operational problems for the utility network. Power fluctuation occurs due to variations in solar irradiance caused by the movement of clouds and may continue for minutes or hours, depending on wind speed, the type and size of passing clouds, the area covered by the PV system, and the PV system topology. Power fluctuation may cause power swings in lines, over- and under loadings, unacceptable voltage fluctuations, and voltage flickers [1].
Harmonic Harmonic distortion is a serious power quality problem that may occur due to the use of power inverters that convert DC current to AC current in PV systems. The produced harmonics can cause parallel and series resonances, overheating in capacitor banks and transformers, and false operation of protection devices that may reduce the reliability of power systems [21].
Frequency fluctuation Frequency is one of the more important factors in power quality. Any imbalance between the produced and the consumed power may lead to frequency fluctuation. The small size of PV systems causes the frequency fluctuation to be negligible compared with other renewable energy-based resources. However, this issue may become more severe by increasing the penetration levels of PV systems. Frequency fluctuation may change the winding speed in electro motors and may damage generators.
Simulation results
To investigate the various effects of grid-connected PV systems on distribution systems, a modified 16-bus test system [22] (Fig. 3) is simulated using the Matlab/Simulink software. The system, which is fed through two 69-kV utility grids, comprised of eight loads with a total power of 10 MVA and 0.8 power factor and three inter-tie circuit breakers. In addition, a 1.8-MW grid-connected PV system, consisting of three 600-kW units, were placed in bus 11 to provide the required power for local loads and to exchange the rest with the system. Two types of commercial PV arrays, SunPower SPR 305 [23] and Sanio HIP 225 [24], were modeled using company data sheets and the described equations in section 2. The produced DC voltage by each PV array was raised using a 5-kHz DC-DC boost converter. An MPPT [25] is implemented in the boost converter to efficiently control photovoltaic energy conversion. Furthermore, the boosted DC voltage is converted into AC voltage using a three-phase three-level VSC. In this analysis, the required information related to solar irradiance under different weather conditions within a year were collected from the MMD [12] and were mixed to create different solar irradiances for sunny and different cloudy weather conditions with slow and fast variations (Fig. 4).
Fig. 3. Single-line diagram of the 16-bus test system.
Fig. 4. Solar irradiance pattern.
Fig. 5. Injected power by PV system at bus 11.
Fig. 6. Utility grid1 active power at bus 1.
Fig. 7. Utility grid1 reactive power at bus 1.
Fig. 8. Utility grid1 power factor at bus 1.
Fig. 9. Utility grid2 active power at bus 2.
Fig. 10. Utility grid2 reactive power at bus 2.
Fig. 11. Utility grid2 power factor at bus 2.
The PV system starts to inject 600 kW power, which is equal to 6% of the total load demand for the first penetration level at 350 ms. In this case, PV continues to feed loads with produced power under 1000 W/m2 solar irradiance until 560 ms. The PV system then feeds through solar irradiance with slow and fast variation at 560 and 1000 ms, respectively. This process is repeated under medium and high PV penetration levels by injecting 1200 kW (12% of the total load demand) and 1800 kW (18% of the total load demands), respectively. Figure 5 shows the injected power by the PV system at bus 11 under these three penetration levels. Figures 6 to 11 show the effect of injected power of the PV system on active power, reactive power, and power factor of utility grid1 and grid2 at bus 1 and bus 2, respectively.
A portion of consumed active power by the loads are covered by the PV system as its penetration level increases, whereas the reactive power consumption continues to be provided by the utility grid (Figs. 5 to 11). Therefore, the power factor of the grid decreases to 70% at 1000 W/m2 solar irradiance. Notably, when irradiance is low, the produced active power of PV unit is low. In this case, the PV unit must draw a very small amount of reactive power from the system because of a small difference between line voltage and reference voltage in the PV controller.
As the penetration level of PV system and the produced active power increases, the system voltage also increases (Fig. 12). Figures 5 and 12 indicate that by increasing the injected active power of PV unit during sunny weather and at high penetration level, the voltage magnitude at bus 6 increases to 1.06 pu, and it is considered as overvoltage based on the IEEE Std 1159-2009 [26]. Voltage flicker occurs at 1000 ms due to the fast power fluctuation of PV together with cloudy weather (Fig. 12). The measured flicker index ( ΔV /V ) at bus 6 under the worst condition exceeds over 6% of its limit as defined in IEEE std 519 [27] (Fig. 13).
Fig. 12. System voltage magnitude at bus 6.
Fig. 13. Measured flicker index at bus 6.
Power fluctuation and voltage variation, which are harmful to sensitive loads, also caused slight variation in total active and reactive power demands of loads, (Figs. 14 and 15, respectively). These variations may cause cable and transformer overloading.
Fig. 14. Total load active power.
Fig. 15. Total load reactive power.
To assess harmonics generated by the PV inverter, the current harmonic spectrum of the current injected by the PV system at bus 11 was measured (Fig. 16). The current THD was calculated to be 15.06%. This value is inconsistent with the THD limit of 5%, as defined in IEEE Std. 519 [27], due to the absence of proper harmonic filter in the PV inverter.
The impedance vs. frequency curve is plotted to investigate the effects of produced current harmonics on system resonance (Fig. 17). The figure shows that the probability of resonance occurrence in the test system in Fig. 3 is very low because of the high R/X ratio of radial systems.
Fig. 16. Current harmonic spectrum at bus 11.
The simulation results indicate that power and voltage fluctuation are the most important effects of PV systems. This fluctuation occurs due to solar irradiance variation and excessive real power produced by the PV unit, which may cause severe problems on system components. Therefore, proper use of capacitor banks or active power conditioning devices to control reactive power and voltage magnitude of the system, in close electrical proximity with PV units, is necessary. In addition, proper harmonic filters should be used for PV inverters to reduce THD and resonance probability, especially in systems with high X/R ratio.
Fig. 17. System impedance vs. frequency curve.
Conclusion
This paper presents an investigation on possible effects of high-penetrated grid-connected PV systems on power quality in distribution systems under varying solar irradiances. All information related to the modeling of PV units and solar irradiances were obtained from different solar panel producers and from the Malaysian Meteorological Department (MMD), respectively. A 1.8-MW grid-connected PV system in a radial 16-bus test system was simulated using Matlab/Simulink software under different solar irradiances. The results show that the active power produced by PV system causes voltage rise, voltage flicker, and power factor reduction, which may create severe problems on the system components.
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[1] Eltawil, M.A. and Z. Zhao. Grid-connected photovoltaic power systems: Technical and potential problems—A review. Renewable and Sustainable Energy Reviews, 14 (2010), No.1, 112-129. [2] Omran, W.A., M. Kazerani, and M.M.A. Salama. A study of the impacts of power fluctuations generated from large PV systems. in IEEE PES/IAS Conference on Sustainable Alternative Energy (SAE), 2009. 1-6. [3] Chowdhury, B.H. Effect of central station photovoltaic plant on power system security. in Conference Record of the Twenty First IEEE Photovoltaic Specialists Conference, 1990. 831-835 vol.2. [4] Caamaño Martín, E., H. Laukamp, M. Jantsch, T. Erge, J. Thornycroft, H. De Moor, S. Cobben, D. Suna, and B. Gaiddon. Interaction between photovoltaic distributed generation and electricity networks. Progress in Photovoltaics: research and applications, 16 (2008), No.7, 629-643. [5] Seok-Ju, L., P. Hae-Yong, K. Gyeong-Hun, S. Hyo-Ryong, M.H. Ali, P. Minwon, and Y. In-keun. The experimental analysis of the grid- connected PV system applied by POS MPPT. in International Conference on Electrical Machines and Systems 1786-1791. [6] Zayandehroodi, H., A. Mohamed, H. Shareef, and M. Farhoodnea. A novel neural network and backtracking based protection coordination scheme for distribution system with distributed generation. International Journal of Electrical Power and Energy Systems, 43 (2012), No.1, 868-879. [7] Gyeong-Hun, K., S. Hyo-Rong, J. Seong-Jae, P. Sang-Soo, K. Sang-Yong, K. Nam-Won, P. Minwon, and Y. In-keun. Performance analysis of the anti-islanding function of a PV-AF system under multiple PV system connections. in International Conference on Electrical Machines and Systems, 2009. 1-5. [8] Anwari, M., M.I. Hamid, M.I.M. Rashid, and Taufik, Power quality analysis of grid-connected photovoltaic system with Adjustable Speed Drives, in IEEE PES/IAS Conference on Sustainable Alternative Energy, 2009. p. 1-5. [9] Yi-Bo, W., W. Chun-Sheng, L. Hua, and X. Hong-Hua, Steady-state model and power flow analysis of grid-connected photovoltaic power system, in IEEE International Conference on Industrial Technology, 2008. p. 1-6. [10] Yang, B., W. Li, Y. Zhao, and X. He. Design and Analysis of a Grid-Connected Photovoltaic Power System. IEEE Transactions on Power Electronics, 25 (2010), No.4, 992 -1000 [11] Yibo, W., W. Chunsheng, L. Hua, and X. Honghua, Steady-State Model of Large-Scale Grid-Connected Photovoltaic Power Generation System, in Proceedings of ISES World Congress. 2009, Springer Berlin Heidelberg. p. 1623-1627. [12] Malaysian Meteorological Service. Online: http://www.met.gov.my. [13] Messenger, R.A. and J. Ventre, Photovoltaic systems engineering. 2004: CRC. [14] Bin, W., H. Tianxiao, J. Bo, D. Xinzhou, and B. Zhiqian.Dynamic modeling and transient fault analysis of feeder in distribution system with MW PV substation. in 45th International Universities Power Engineering Conference (UPEC), 2010. 1-5. [15] Patel, M.R., Wind and solar power systems: design, analysis, and operation. 2006: CRC. [16] El-Saadawi, M.M., A.E. Hassan, K.M. Abo-Al-Ez, and M.S. Kandil. A proposed framework for dynamic modelling of photovoltaic systems for DG applications. International Journal of Ambient Energy, 32 (2011), No.1, 2-17. [17] El-Saadawi, M.M., A.E. Hassan, K.M. Abo-Al-Ez, and M.S. Kandil. A proposed dynamic model of Photovoltaic-DG system. in 1st International Nuclear & Renewable Energy Conference(INREC), 2010. 1-6. [18] De Soto, W., S. Klein, and W. Beckman. Improvement and validation of a model for photovoltaic array performance. Solar Energy, 80 (2006), No.1, 78-88. [19] Kageyama, H., T. Yamada, T. Oozeki, K. Kato, and Y. Hishikawa, Measurement of Inrush-Current Waveforms for Modeling Reactance Characteristics of PV Modules, in 26th European Photovoltaic Solar Energy Conference and Exhibition, 2011. p. 3430-3433. [20] Sharma, S. and B.R. Parekh. Impact of PVPS (PhotoVoltaic Power System) Connection to Grid in Urban Areas. in National Conference on Recent Trends in Engineering & Technology, . Gujarat, India, 2011. 1-5. [21] Farhoodnea, M., A. Mohamed, H. Shareef, and H. Zayandehroodi. An enhanced method for contribution assessment of utility and customer harmonic distortions in radial and weakly meshed distribution systems. International Journal of Electrical Power and Energy Systems, 43 (2012), No.1, 222-229. [22] Abdul Kadir, A.F., A. Mohamed, and H. Shareef. Harmonic Impact of Different Distributed Generation Units on Low Voltage Distribution System. in IEEE International Electric Machines and Drives, 2011. 120-125. [23] SunPower®. Online: http://us.sunpowercorp.com. [24] SANYO North America Corporation. Online: http://us.sanyo.com. [25] Kadri, R., J.P. Gaubert, and G. Champenois. An Improved Maximum Power Point Tracking for Photovoltaic Grid-Connected Inverter Based on Voltage-Oriented Control. 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Authors: 1- Masoud Farhoodnea, Email: masoud@eng.ukm.my; 2-Professor Dr. Azah Mohamed, Email: azah@eng.ukm.my; 3-Dr. Hussain Shareef, Email; hussain_ln@yahoo.com; 4-Dr. Hadi Zayandehroodi, Email: h.zayandehroodi@yahoo.com; Department of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia (UKM), Bangi, 43600, Selangor, Malaysia.
The correspondence address is: Masoud Farhoodnea (E-mail: masoud@eng.ukm.my)
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 2a/2013
Published by Electrotek Concepts, Inc., PQSoft Case Study: Distribution Feeder Resonance and Harmonic Filter Evaluation, Document ID: PQS1006, Date: March 15, 2010.
Abstract: Utility power system harmonic problems can often be solved using a comprehensive approach including site surveys, harmonic measurements, and computer simulations.
This case study presents the results for a utility distribution feeder resonance and harmonic filter evaluation. The analysis was completed using the PSCAD program. The study evaluated the effects of distribution capacitor banks on the frequency response characteristic and the resulting harmonic distortion levels.
INTRODUCTION
A utility distribution feeder resonance and harmonic filter evaluation was completed for the system shown in Figure 1. The case study was completed using the PSCAD program. The accuracy of the simulation model was verified using three-phase and single-line-to-ground fault currents and other steady-state quantities, such as capacitor bank rated current.
The circuit model for the case involved a 25.56kV utility substation and a single 25.56kV feeder with three 2,500 kVA step-down transformers supplying 500 hp ac drive loads.
Figure 1 – Illustration of Oneline Diagram for Feeder Resonance Evaluation
SIMULATION RESULTS
The case study evaluated the effect of distribution feeder capacitor banks on the frequency response characteristics and the resulting voltage distortion levels. The mitigation alternative of passive harmonic filters was also evaluated. Figure 2 shows the simulated customer current waveform and spectrum for the 500 hp ac drives. The current has a fundamental frequency value of 497 amps, an rms value of 561 amps, a power factor of 80.3%, and a current THD value of 52.5%. The highest components were the 5th harmonic with a value of 48.2% and the 7th harmonic with a value of 17.5%.
Figure 2 – Customer AC Drive Current Waveform and Spectrum
Figure 3 shows the results for the three frequency scan simulations. Case #1 was the base case with no utility capacitor banks included in the model. Case #2 was the case with all of the 25.56kV capacitor banks in service. Case #3 was the case with all of the 25.56kV capacitor banks reconfigured as 4.2nd harmonic filters. The scan was at the end of the distribution feeder. The parallel resonances for Case #2 were approximately 276 Hz (4.6th) and 768 Hz (12.8th).
Table 1 summarizes the results for the three simulations. The table includes the simulated voltage distortion (THD) at the end of the feeder near the 1,200 kVAr capacitor bank for the three different operating conditions. A single case exceeded the voltage limitation of 5% THD. Reconfiguring the capacitor banks as 4.2nd harmonic filters in Case 3 reduced the voltage distortion to below 5% THD.
Figure 4 shows the transformer primary current waveform for the customer drive current previously shown in Figure 2. The waveform shows the effect of the transformer connection and phase shift on the drive current characteristic.
Table 1 – Summary of the Simulation Results
Case Number
25.56kV Feeder VTHD
25.56kV RMS Bus Voltage
25.56kV RMS Feeder Voltage
25.56kV PCC 5th Current
1
2.90%
25.2kV
24.5kV
11.7A
2
5.34%
25.9kV
25.7kV
32.1A
3
1.46%
25.9kV
25.7kV
3.3A
.
Figure 5 shows the utility point of common coupling (PCC) current for the two cases with the capacitor banks (Case #2) and harmonic filters (Case #3) in service. Converting the capacitor banks into harmonic filters reduced the current total harmonic distortion from 12.2% to 1.5%. As summarized in Table 1, the 5th harmonic PCC current was reduced from 32.1A to 3.3A when the harmonic filters were applied.
Figure 3 – Simulated Customer Frequency Response Characteristics
Figure 4 – Customer Transformer Primary Drive Current Waveform
Figure 5 – Utility Point of Common Coupling Current Waveform
SUMMARY
This case study summarizes the results for a utility distribution feeder resonance and harmonic filter evaluation. The case study evaluated the effects of distribution capacitor banks on the frequency response characteristic and the resulting harmonic distortion levels.
The simulation results showed harmonic resonances that increase voltage distortion levels above the assumed 5% THD limitation when the utility substation and feeder capacitor banks were in service. The mitigation solution was to convert the capacitor banks into harmonic filters tuned to the 4.2nd harmonic. Adding the harmonic filter banks reduced the voltage distortion to below 2.0%.
REFERENCES
1.Power System Harmonics, IEEE Tutorial Course, 84 EH0221-2-PWR, 1984. 2.IEEE Recommended Practice for Monitoring Electric Power Quality,” IEEE Std. 1159-1995, IEEE, October 1995, ISBN: 1-55937-549-3. 3.IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, IEEE Std. 519-1992, IEEE, ISBN: 1-5593-7239-7.
RELATED STANDARDS IEEE Std. 519-1992 IEEE Std. 1159-1995
GLOSSARY AND ACRONYMS ASD: Adjustable-Speed Drive CF: Crest Factor DFT: Discreet Fourier Transform DPF: Displacement Power Factor PCC: Point of Common Coupling PF: Power Factor PWM: Pulse Width Modulation TDD: Total Demand Distortion THD: Total Harmonic Distortion TPF: True Power Factor
Published by Tomasz SAMBORSKI, Andrzej ZBROWSKI, Stanisław KOZIOŁ, Instytut Technologii Eksploatacji – Państwowy Instytut Badawczy
Abstract. The introduced device allows performance of the research in the rotary testing of the resistance to the surface discharge and erosion of the polymeric insulators according to PN-EN 62217:2007. The applied control system of the operation of the device allows controlling of the parameters of testing (rotary speed, duration time) in the wide range. The performance of tests of resistance to the surface discharge and erosion of the polymeric insulators in the wider range of the parameters than defined by the standard allows more effective detection of the weak points of the construction, which could cause the damage of the insulator during exploitation.
Streszczenie. W artykule przedstawiono metody badań odporności na wyładowania pełzne i erozję polimerowych izolatorów energetycznych zgodne z PN-EN 62217:2007 ze szczególnym uwzględnieniem próby kołowej. Opisano zaprojektowane i wykonane w Instytucie Technologii Eksploatacji – PIB w Radomiu urządzenie do próby kołowej. Umożliwia ono przeprowadzanie długotrwałych badań izolatorów wsporczych i liniowych wyposażonych w elementy mocujące dowolnej konstrukcji. (System do testowania odporności na wyładowania pełzne i erozję polimerowych izolatorów energetycznych)
Keywords: support insulators, line insulators, creeping discharge effect, insulators testing. Słowa kluczowe: izolatory wsporcze, izolatory liniowe, wyładowania pełzne, badania izolatorów.
Introduction
The insulators made of polymeric ceramics are technical and economical alternative for the insulators made of the porcelain, particularly for their perfect electro-insulating properties, low material costs and the energy saving production process. Most often, the polymeric line insulators are smaller in the diameter than the porcelain or glass insulators. Thanks to that higher resistance to the breakdown through shape or to the water absorption. Taking into consideration the insulation properties of the insulators the hydrophobic properties of insulator are the key factor [1].
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Fig. 1. The hydrophobic properties of the surface of the polymeric insulator [1]
The processes of dynamic losing and regaining of the hydrophobic properties and their strong dependence on the material (including basic components, filler and additives) and on the production technology are the areas of the permanent development. If the hydrophobicity is lost, the insulator “should be protected” against intense wear-out (erosion, surface currents) by the second protection mechanism of the material. It is best assessed by performing tests for resistance to surface currents and erosion.
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Fig. 2. High voltage testing (PFISTERER SEFAG)
For ensuring the high level of technical safety of the power systems the standard obligatory tests are performed for testing the resistance of the construction to the surface discharge and erosion of the polymeric power insulators. The performed certification tests allow elimination of such construction of insulators, materials or production technologies that are not good enough for application in high voltage techniques occurring in standard work conditions.
In the electrical aspect the task of the insulator is the insulation of the high voltage elements from the grounded elements or mutual insulation of two high voltage elements against the spark passing. Simply put, the passing of spark might be the result of too high voltage or pollution. When the passing route determines the action during the overvoltage, the shape (geometry) of the insulator and the reaction of the surface of the insulator to the water absorption are the critical factors for the resistance to pollution.
The polymeric insulators are made of single insulation material (synthetic resin insulators) or of two or more insulation materials (composite insulators). The construction materials are most often the cross-linked organic materials made of the coal of silicon components. The insulation materials might be composed of the organic materials with additions of different organic and non-organic materials such as fillers and solvents.
The construction of the polymeric insulator is generally defined by:
– materials for the core and the cover and method for their production, – materials and construction of the holding elements and the method for their montage, – the thickness of the cover on the core (including the middle layer, if exists).
For the elimination of the construction of insulators, materials or production technologies that are not suitable for applications in high voltage the construction tests are performed. The range of the construction tests is selected to make possible the estimation of the time influence on the electrical properties of the complete polymeric insulator and its components (core material, cover material, border surfaces etc.) that ensure the expected life-span in normal work and environmental conditions (Table 1).
Table 1. Normal environmental conditions
Conditions
Indoor insulation
Outdoor insulation
Maximal ambient temperature
Not higher than 40°C, and its average value measured within 24 hours not higher than 35°C
Not higher than 40°C, and its average value measured within 24 hours not higher than 35°C
Minimal ambient temperature
-25°C
-40°C
Vibration
Insignificant vibration caused by external phenomena or by underground vibration
Insignificant vibration caused by external phenomena or by underground vibration
Sun radiation [1]
Insignificant
Up to 1 000 W/m2
Air pollution
No visible pollution with dust, smoke, flammable gas or corrosive gas, fumes or salt
Possible pollution with dust, smoke, flammable gas or corrosive gas, fumes or salt (Not higher than “high level” defined by IEC 60815 [4])
Humidity
Average relative humidity, measured within 24 hours not higher than 95%; measured within a month not higher than 95%. Steam condensation might occur
.
The tests of resistance to the surface discharge and erosion that are part of the construction research are treated as the test resulting in selection and rejection of improper materials or constructions. The tests include only a part of complex loads occurring during the work of the insulator and are not the basis for assessment of the durability of the insulators [3].
Testing the resistance to the surface discharge and erosion
The composite insulators are used both for alternating current and direct current. There is no developed method for testing the insulators and their resistance to surface discharge and erosion under the direct current. The latest standard PN-EN 62217:2007 “polymeric insulators for indoor and outdoor use with a nominal voltage above 1000 V – general definitions, test methods and acceptance” defines the tests of resistance to surface discharge and erosion under the alternating current. The conducted tests are used for determination of the minimal requirements for the resistance of the cover material to generation of the conductive paths. The standard allows the choice between three methods of testing of the resistance of the construction of the insulator to the actions of the electric discharge.
The first test is the test in the salt mist, second test includes various cyclic loads and the third is a rotary test. The test in the salt mist during 1000 hours consists in the time limited effect of the salt mist on the insulator under the alternating current with net frequency and the constant voltage value [5]. The test is performed in the water and corrosion proof chamber with volume not greater than 15m3, equipped with the opening not greater than 80cm2, for natural ventilation.
In the test with different loads the tested insulators are placed in the water and corrosion proof chamber with volume not greater than 20m3. The normalised load cycle – repeated for 5000 hours – includes: testing alternating current of net frequency, simulation of the sun radiation, artificial rain, dry and wet hot air and pollution simulated with the salt mist [5, 6].
The rotary test consists in cyclic loading (30 000 cycles) of the insulators placed on the rotating holder (Fig.3). During one cycle the insulators go through four positions. Each piece stays motionless in each of four positions for about 40s. The rotation by 90° from one position to another lasts about 8s. In the first part of the cycle the insulator is immersed in the saline solution. The second part of the cycle allows the surplus of the solution to drain which in turn allows that in the third part of the cycle the discharge occurs on the moist surface crosswise to the drying stripes. In the third part of the cycle the sample is exposed to the alternating current of the net frequency. In the last part of the cycle the surface heated by the discharge is cooling down. The testing current is connected from the testing transformer. With the load of the active current of 250mA on the high voltage side of the testing circuit the maximal drop of the output voltage should not exceed 5%.
.
Fig. 3. The diagram of the testing stand for the rotary testing of the insulators for the resistance against the surface discharge according to PN-EN 62217:2007
In each of the presented testing methods two insulators are picked from the production line, both of identical construction and the leak path in between 500 mm and 800 mm. If there are no such insulators in the production line, a substitute should be made from the available types by installing normalised holding elements in such a way than the leak path is in the mentioned range. For the inspection reasons it is possible to stop the continued test once a week. None of the pauses should be longer than 1 hour, and its duration is not counted into the test time. One longer pause is acceptable, but no longer than 60 hours. The test time should be extended by triple the pause duration. The final report should include all the details about the pauses.
The result of the test is considered positive if on both samples:
– there are no signs of the surface discharge (megaohmmeter should be used by the voltage of 1 kV or higher, putting electrodes in the distance between 5mm and 10mm from each other along each suspected path; the resistance below 2MΩ means the damage occurred); – in case of composite insulators – the depth of the erosion is smaller than 3mm and does not reach to the core (if that is possible); – in case of resin insulators – the depth of the erosion is smaller than 3mm; – there are no breakdowns of the cover or the border surface.
The device for rotary testing
Despite the free choice of the testing method, the most popular method among the producers is the rotary method, considered to be most reliable. For the sake of inexistence of the testing apparatus for such tests and interest demonstrated by the certification unit (Institute of Power Engineering), the Institute for Sustainable Technologies in Radom has designed, produced and introduced for use in the Institute for Power Engineering in Warsaw a unique, specialised system for rotary testing according to the PNEN 62217 standard.
The construction of the device for testing of resistance to surface discharge and erosion of the polymeric insulators is presented in the figure 4.
.
Fig. 4. The construction of the device for testing of resistance to the surface discharge and erosion of the polymeric insulators: 1 – frame, 2 – scissor lifter, 3 – lifter drive handle, 4 – saline solution tank, 5 – insulator positioning module, 6 – geared drive, 7 – belt drive, 8 – testing voltage supply module, 9 – cover, 10 – wheels, 11 – vibroinsulators, 12 – tested insulators
The stand consists of the steel frame, which is the chassis of the device, bearing the mechanical loads from the tested objects. In the front, horizontal part of the frame the scissor lifter is fixed, powered with screw drive. In the frame of the lifter the tank, of the volume of 500 dcm3, is fixed and filled with the saline solution, in which the insulators are immersed.
The precise, vertical movement of the lifter allows proper expected level of immersion of the tested insulators. The tested insulators are fixed in the positioning holder, mounted in the vertical part of the frame. The positioning holder (Fig. 5), to which the testing voltage is connected through the tested insulators, is fixed on the four support insulators connected to frame.
.
Fig. 5. The positioning module for the insulators: a) the model, b) the view, 1 – hub, 2, 3 – holders, 4 – support insulators, 5 – brush module, 6 – position sensor
For the sake of different constructions of the holing elements of the tested insulators (Fig. 6) used for bearing of the mechanical loads, the positioning module is equipped with universal prismatic holders intended for fixing of the hanging insulators and with specialised holders for the standing insulators.
Fig. 6. Sample constructions of the insulator holders
The holders are fixed to the hub, which angular position corresponding to the particular cycles of the test is identified by the induction proximity switch. The grounding of the hub is realised by the brush module cooperating with the sliding ring.
The rotary movement of the hub of the positioning module is performed by the geared motor fixed in the vertical part of the frame through the cog belt gearbox that guarantees the insulation of the drive system from the tested object (Fig. 7). The applied drive system is intended for powering with the power inverter which allows fairly wide range of time duration for each test cycle.
Fig. 7. The mechanism of the holder drive a) front view, b) back view
For the sake of the long time testing (30 000 cycles) it was necessary to ensure the high durability of particular construction node. The important module which decides on the proper testing process is the module of testing voltage supply (Fig. 8). The task of the module is supplying of the high voltage to the clamp of the insulator after the draining of the insulator from the saline solution. In the developed solution the role of the contacting element with the tested insulator, exposed to the mechanical wear (abrasion) and burnout, is the carbon slider used in traction networks. The slider is fixed to the swinging arm, which allows good contact to the insulator clamp, which position is controlled by the screw positioning mechanism.
For ensuring of the necessary mechanical safety related to the maintenance of the device it is equipped with the cover made of colourless polycarbonate, which is removed for the inspection of the insulators. Supplying of the device with high voltage requires placement of the device in the separate zone. To achieve that the device is equipped with the wheels and adaptable stands (vibroinsulators) that allow its levelling.
For the sake of the corrosive character of the environment in which the tests are performed, the device is made mostly of the acid resistant steel. The power supply circuits and the control circuits for the drive were put in the mobile control box (Fig. 9).
Fig. 9. Control box for the device
On the door of the box the control elements were placed for the control of the work of the drive module of the holder. The control of the work is performed by means of programmable relay and the inverter. Such solution allows:
– the change of the rotation speed, – the control of the duration of each stage of the test – the control of start and stop, – easy start and stop of the drive, – fast emergency stop
Verification tests
The device produced in the Institute for Sustainable Technologies – NRI in Radom (Fig. 10a) was introduced in the Laboratories of High Voltage in the Institute of Power Engineering in Warsaw, where it was tested in work (Fig. 10b).
Fig. 10. Device for rotary testing (a) and the view of the surface discharge on the tested insulator (b)
The basic tests were conducted in the conditions (Table 2) defined by the PN-EN 62217:2007. Rotary tests. The appendix A in the limited range of the duration time of the test (the duration of a full test without pauses in normalised cycle is 1600 hours).
Table 2. Test conditions
Testing voltage
Testing alternating voltage of net frequency in kV is defined by dividing of the real leak path [mm] by 28,6
Allowed content of NaCl in the de-ionised water
1,40 kg/m3 ± 0,06 kg/m3
Ambient temperature
20 °C ± 5 K
Test duration
30 000 cycles
.
The performed tests confirmed the assumptions made in the range of the maintenance parameters and in the range of the construction solutions.
Summary
The introduced device allows performance of the research in the rotary testing of the resistance to the surface discharge and erosion of the polymeric insulators according to PN-EN 62217:2007.
The applied control system of the operation of the device allows controlling of the parameters of testing (rotary speed, duration time) in the wide range.
The performance of tests of resistance to the surface discharge and erosion of the polymeric insulators in the wider range of the parameters than defined by the standard allows more effective detection of the weak points of the construction, which could cause the damage of the insulator during exploitation.
The system might be used in the certification testing and the development works in new products made in companies producing the polymeric insulators.
Authors: dr inż. Tomasz Samborski, dr inż. Andrzej Zbrowski, dr inż. Stanisław Kozioł, Instytut Technologii Eksploatacji – PIB, ul. Pułaskiego 6/10, 26-600 Radom, E-mail: Tomasz.Samborski@itee.radom.pl.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 3a/2012
Published by Liu Liqun, Liu Chunxia, Taiyuan University of Science & Technology
Abstract. This paper gives the feasibility analysis of a wind-PV-battery system for an off-grid power station specially located in remote village of Dongwangsha, Shanghai. The simulation and optimization results indicated the feasibility analyses of the proposed hybrid wind-PV system with storage battery backup. Moreover, the GHG emissions, project costs and savings/income summary, financial viability, and risk analysis are discussed. The proposed hybrid system is more environmental friendly as compared with the diesel only system.
Streszczenie. W artykule przedstawiono analizę wykonalności budowy systemu baterii dla farmy wiatrowo fotowoltaicznej wyłączonej z sieci energetycznej w oddalonej wiosce Dongwangsha, w okolicy Szanghaju. Przeprowadzono kompleksowe badania obejmujące parametry takie jak: emisja GHG, koszty i wpływy projektu, analizę ryzyka. (Analiza wykonalności systemu baterii dla hybrydowej farmy wiatrowo-fotowoltaicznej w Dongwangsha, w okolicy Szanghaju).
Keywords: enewable energy; Feasibility analysis; Hybrid power supply system. Słowa kluczowe: energia odnawialna, analiza wykonalności, hybrydowy system zasilania.
Introduction
The State Grid in China can not supply the total end consumers with enough power due to the large territory and there are so many remote districts, and there is millions of off-grid consumers have to use the stand-along diesel generating system in order to supply the power demand. The diesel only power system consumes a lot of diesel and a mass of greenhouse gases emissions. And with the ever-increasing price of fossil resource, the hybrid generating system is proposed to offer steady and reliable and cheap power supply for the off-grid user as compared with the diesel only generating system.
Many literatures have analyzed the renewable resource generating system by using HOMER or Hybrid2 or RETscreen [1-8]. RETscreen is clean energy project analysis software to analyze the technical and financial viability of possible clean energy projects, which takes advantage of renewable power generating technology to improve conventional electricity grid in the method of replacing sectional or entire traditional electricity grid by renewable energy power generation. This paper presents the feasibility analyses of hybrid wind-PV-battery power system to arouse the regard of the designer and engineers in China. In this context, the present study carries out a feasibility analysis by using RETscreen software of Natural Resources Canada and the data of National Aeronautics and Space Administration (NASA) to analyze a hybrid wind-PV system with storage battery backup for a remote village located in Dongwangsha, Chongming islands, Shanghai, China [9-11].
Site and Electrical load and Meteorological data
Dongwangsha located in Chongming island of Shanghai where apart from the national electrical grid. The diesel only system cannot ensure the continuous electricity supply during breakdown and scheduled shutdown of diesel units. With the change of environmental and social factors, especially the decreases of non-renewable energy resource, the power generated by conventional electrical grid do not meet the demand any more. As a result, a kind of hybrid wind-PV-battery system generating system is compared with the conventional fossil resource generating system, and the proposed system has many advantages superior to others. To be more specific, on one hand, local abundant renewable resource like wind and solar energy is able to supply enough resource for the renewable energy power generation. On the other hand, the use of clean energy reduces the greenhouse gas emissions and environmental damage greatly while generating electrical energy, which can save cost at the same time. The power load of Dongwangsha is shown in Fig.1 [12].
Fig.1. Base case system load characteristics graph.
According to the load characteristics, the maximum monthly power of the load is 35KW. The summer and winter are the periods of peak load. On the basis of the project location and local meteorological data, the scaled annual average value of wind speed is 4.5m/s and the highest values of wind speed are observed during the months of January to February with a maximum of 4.91m/s. The scaled annual average value of monthly average daily total global solar radiation (GSR) is 3.48 KWh / m2 / d, the highest values of GSR are gained during the months of May to August with a maximum of 4.84KWh / m2 / d as can be seen from Fig.2. Compared to the price of PV power system, the wind power system is relatively cheap. Thus, the wind power becomes the technology of base load power system while the PV generator which has more rated power constitute the intermediate load power system in the renewable power generation system. The peak load power system consists of traditional grid electricity. To be mentioned, the peak load power system can not be designed in the renewable system if it was not required so that we can save the cost of diesel and reduce the greenhouse gas (GHG) emissions.
As shown in Fig.3, the base case of power system, which is an off-grid system, its power generation relies on the fuel generator. The fuel rate is 1.5$/L and the total electricity cost is 194191$. In general, it’s not easy for the typical clients to afford to these high fees. They would like to invest in those economical projects. Further more, in the case of the electricity generation efficiency is 31% and the transportation and the distribution losses are 8%, the annual greenhouse gas emissions are 226.2t, which bring much air pollution to the environment.
Fig.2. Meteorological data of project site.
Fig.3. Base case power system.
Hybrid power system and results and discussion
Compared to base case power system, the proposed power system adds wind turbine (the power capacity per wind turbine with 3KW, as shown in Fig.4) and photovoltaic generator with 25KW and the considered number is 24 and 10, respectively.
If the project uses 20 wind turbines, as shown in Fig.5 to Fig.7, the IRR is only 53.5% while the net annual GHG emission reduction is 198.2t. The amount of electricity delivered to load is just 68.4% [13].
Fig.4. Data of wind turbine of proposed system.
Fig.5. Proposed case power system 1.
Fig.6. GHG emission reduction summary 1.
Fig.7. Financial viability 1.
Fig.8. Proposed case power system 2.
Fig.9. GHG emission reduction summary 2.
Fig.10. Financial viability 2.
If the project uses 30 wind turbines, as shown in Fig.8 to Fig.10, although the amount of electricity delivered to load increases to 97.5%, the net annual GHG emission reduction rises to 214.8t simultaneously. Due to investing much fund in project for the equipments, the IRR drops to only 52.9%.
It can be seen clearly from the table that the IRR (56.7%) of the system which using 24 wind turbines is higher than that (52.9%) of the system using 30 wind turbines as a result of the lower cost of the former system. Further more, the percentage of electricity delivered to load is 82.1%, while the net annual GHG emission reduction rises to 211.6t. In terms of the project economic, the proposed power system comprised by 24 wind turbines tend to be the most economical one. More specifically, in this energy model, the percentage of electricity generated by wind turbines and PV generators are 82.1% and 16.2%, respectively. The remaining electricity supplied by fuel generators accounts for 1.6% of the total. The back-up power system made up with batteries is designed in case of emergency. All in all, the energy structure of this power generating system not only make full use of the local wind energy and solar energy, but also improve local conventional power generating system with low efficiency and high cost as well as meeting the demands eventually. The system design graph is shown in Fig.11 and Fig.12.
Fig.11. proposed case system characteristics.
Fig.12. System design graph.
Fig.13. GHG emission reduction summary.
In terms of GHG emissions, the proposed case power system declines the emissions because of the use of clean energy. If calculated as the emissions of 25t carbon dioxide equal to 1t methane and the emissions of 298t carbon dioxide equal to 1t nitrous oxide, the electricity generation efficiency is 31% and the transportation and the distribution losses are 8%, the amount of GHG emissions of generating 1MKWh electricity declines from 226.2t to 3.5t. At the same time, the gross annual GHG emission reduction is 222.7t. What’s more, the net annual GHG emission reduction is 211.6t and the total emission is 4232t when the GHG credits transaction fee is 5% and the project life is 20 years, as shown in Fig.13.
Fig.14. Project costs and savings/income summary.
In terms of financial analysis, the total cost of renewable energy is lower than that of non-renewable energy considerably. Even though the renewable system invests much in the construction and equipments, the management and operation cost less relatively. It is assumed that fuel cost escalation rate is 2%, inflation rate is 2%, discount rate is 10% and project life is 20 years , the annual savings and income is 194191$, the net present value (NPV) is 1461288$, the annual life cycle savings is 171642$ and the IRR is 56.7%. At the same time, the benefit-cost ratio is 5.33 and the simple payback is 1.9 yr. In a word, as compared to other systems, its project payback peaks to the maximum, as shown in Fig.14. The financial viability and the cumulative cash flows graph are shown in Fig.15 and Fig.16
Fig.15. Financial viability.
In terms of risk analysis, the most critical factor is fuel cost. However, the reduction of fossil fuel and the cost of the system are able to lessen investment risk and sensitivity, as shown in Fig.17.
Fig.16. Cumulative cash flows graph.
Fig.17. Risk analysis.
Conclusion
In this paper, a hybrid wind-PV-battery power system is discussed to explore the possibility of utilizing power of the wind and solar to reduce the dependence on fossil fuel for power generating system to meet the electric requirement of remote village located in Dongwangsha, the seaside of the Chongming islands, Shanghai. In terms of the project economic, the proposed power system comprised by 24 wind turbines tend to be the most economical one. More specifically, in this energy model, the percentage of electricity generated by wind turbines and PV generators are 82.1% and 16.2%, respectively. The proposed system are more environmental friendly as compared with the conventional diesel only system and the greenhouse gases emission is less than the diesel only system. As a conclusion, the feasibility analyse is very important to select the appropriate hybrid renewable power system based on Local meteorological conditions.
Acknowledgments:this work was supported by the Program for the Industrialization of the High and New Technology of Shanxi province (NO: 2010016), Youth Science Foundation of Shanxi province (NO: 2011021014-2), Doctor Fund of Taiyuan University of Science & Technology (NO: 20122018).
REFERENCES
[1] Celik, A.N., The system performance of autonomous photovoltaic-wind hybrid energy systems using synthetically generated weather data, Renewable Energy, 27(2002), No. 1, 107-121. [2] Diaf S., Diaf D., and Belhamel M., et al, A methodology for optimal sizing of autonomous hybrid PV/wind system, Energy Policy, 35(2007), no. 11, 5708-5718. [3] Iqbal M.T., Pre-feasibility study of a wind-diesel system for St. Brendan’s, Newfoundland, Wind Engineering, 27(2003), no.1, 39-51. [4] Khan M.J., Iqbal M.T., Pre-feasibility study of stand-alone hybrid energy systems for applications in Newfoundland, Renewable Energy, 30(2005), no. 6, 835-854. [5] Mills A., Al-Hallaj S., Simulation of hydrogen-based hybrid systems using Hybrid2, International Journal of Hydrogen Energy, 29(2004), no. 10, 991-999. [6] Thompson S., Duggirala B., The feasibility of renewable energies at an off-grid community in Canada, Renewable and Sustainable Energy Reviews, 13(2009), no.9, 2740-2745. [7] Tina G., Gagliano S., and Raiti S., Hybrid solar/wind power system probabilistic modelling for long term performance assessment, Solar Energy, 80(2006), no.5, 578-588. [8] Yang H., Zhou W., and Lu L., et al, Optimal sizing method for stand-alone hybrid solar-wind system with LPSP technology by using genetic algorithm, Solar Energy, 82(2008), no. 4, 354-367. [9] Yang H., Lu L., and Zhou W., A novel optimization sizing model for hybrid solar-wind power generation system, Solar Energy, 81(2007), no. 1, 76-84. [10] National Aeronautics and Space Administration (NASA), http://eosweb.larc.nasa.gov/ [11] Natural Resources Canada, http://www.retscreen.net/ang/ home.php [12] Gu C., Research for simulation and optimization of solar/wind hybrid power station, Shanghai Jiaotong University Master thesis, Shanghai, 55-61, 2004. [13] Guangdong Shangneng wind power equipment ltd., http://www.sunningpower.com/
Authors: Assistant prof. L.Q. Liu, college of electronic and Information engineering, Taiyuan University of Science & Technology, Waliu road 66, Wanbolin district, Taiyuan, China, Email: llqd2004@163.com; Assistant prof. C.X. Liu, College of computer science & technology, Taiyuan University of Science & Technology, Waliu road66, Wanbolin district, Taiyuan, China, Email: lcx456@163.com.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 1a/2013
Published by Electrotek Concepts, Inc., PQSoft Case Study: Evaluation of Harmonic Impacts from Compact Fluorescent Lights On Distribution Systems, Document ID: PQS0322, Date: April 16, 2003.
Abstract: This case study presents the results of a research investigation into the impact of compact fluorescent lights (CFLs) on an existing distribution system. The analysis uses a combination of EMTP simulations and conventional harmonic analysis simulations (SuperHarm) to predict the distribution system distortion levels as a function of system characteristics and CFL penetration levels. Compact fluorescent light characteristics from measurements and distribution system data collected are utilized to develop simulation models.
This case study is summarized from a paper presented at PQA 1994 by Rory Dwyer and Afroz K. Khan, from Electrotek Concepts, and Robert K. McCluskey, Roger Sung, and Thomas Houy, from Southern California Edison.
INTRODUCTION
Compact fluorescent lights (CFLs) provide significant energy savings over incandescent lighting. As a result, CFLs are being promoted as part of energy conservation programs at many electric utilities. A recent study performed by Worcester Polytechnic Institute (Emanuel, 1992) estimated the impact of these lights on distribution system voltage distortion levels. The results indicated that relatively low CFL penetration levels could cause the feeder voltage distortion to exceed 5%.
A number of questions associated with the models used in the Worcester Polytechnic Institute study need to be addressed. For instance, only the feeder of interest was modeled while other feeders supplied from the same bus, with their loads and capacitors, were not included in the model. The effect of the load characteristics on system resonance conditions is also a question. These concerns are explored in the following research investigation.
COMPACT FLUORESCENT CHARACTERISTICS
Measurements were performed for a wide variety of CFLs. These tests evaluated the harmonic characteristics of each lamp. This resulted in three general categories of CFLs:
1. High distortion electronic ballast CFLs. These typically have current distortion levels exceeding 100%.
2. Low distortion electronic ballast CFLs. These have distortion levels typically below 30% based on using some type of harmonic control technology in the ballast, such as series filters.
3. Magnetic ballast CFLs. These have distortion levels below 20% based on the characteristics of the lamp in series with a ballast inductance.
DESCRIPTION OF DISTRIBUTION SYSTEM STUDIED
A simplified one line representation of the system studied can be seen in Figure 1. This system consists of six parallel feeders serviced by a 44.8 MVA substation transformer with a substation capacitor bank of 6000 kVAr. The primary feeder modeled for this study is the Feeder 1 12/6.9kV circuit. Approximately 31% of the loads on this feeder are commercial. These commercial loads are serviced line to line. The residential loads are serviced line-to-line overhead and either line-to-line or line-to-neutral underground.
Figure 1: Feeder System One-Line Diagram
FREQUENCY RESPONSE CHARACTERISTICS
The distribution system frequency response characteristics determine the impact of the harmonic currents on the system voltage distortion. The frequency response is determined by the system strength, the capacitors in service, and the amount of load (damping). Several key configurations were investigated. Figure 2 illustrates the frequency response characteristics with the substation capacitor bank incorporated along with some of the banks on the parallel feeders. The substation capacitor bank moves the primary system resonant peak to reach the 5th harmonic. In relation to the other scans performed, this case appears to be the worst situation regarding undesirable high 5th harmonic impedances. This scenario will be utilized as the basis for evaluating CFL penetration impacts.
Figure 2: Effect of Nonlinear Load Representation
One important effect that needs to be considered is the load characteristics, especially when the load includes a significant amount of power electronics equipment (power supplies). The load characteristics utilized in the previous section consist of basically resistive load to include the damping effect on system resonances. A resistive representation is not accurate for most nonlinear loads.
EMTP Simulations
Simulations from the Electromagnetic Transients Program (EMTP) are used to develop approximate models that can be used in the frequency domain simulations to evaluate the impact of these loads on the overall system frequency response characteristics. The EMTP model was developed to obtain a benchmark comparison for a residential circuit example. The circuit shown in Figure 3 was used to represent a single distribution transformer supplying four residences. Each residence is assumed to have six 25 W CFLs. Typical impedances are used to represent the household wiring and the cabling between the residences. Other loads are also included for the residences: 100 W electronic load (TV), 150 W incandescent lighting, and 100 W refrigerator load.
Figure 3: Simulated System
Effect on System Frequency Response
To obtain the frequency response characteristics for the impact of non-linear loads on the system, the parameters obtained from the EMTP simulations for load characterization were incorporated into the existing frequency scan model. Figure 4 illustrates the resulting characteristics at mid-feeder.
Figure 4: Frequency Response Comparison
The figure above provides a comparison of the light loading frequency response characteristics with linear loads and with both linear and non-linear loads. As expected, the frequency scan with the addition of non-linear loads shifts the resonance points to the right with a slight additional harmonic resonance around the 2nd harmonic. The addition of the non-linear load characteristics, derived from EMTP simulations, results in a filter effect at the lower frequencies. It can be seen that the high impedance, previously observed at the 5th harmonic, has been radically reduced. This effect results in the increase of the impedance at the 7th harmonic.
EVALUATION OF EXPECTED HARMONIC DISTORTION LEVELS
Base Case Distortion Levels
The least favorable feeder capacitor configuration was used to calculate harmonic levels due to nonlinear loads currently existing in the system with the exclusion of CFLs. The impedance scan of Figure 2 indicates that this configuration results in a peak resonance at or near the fifth harmonic. Voltage THD levels resulting from this “unfavorable” capacitor configuration are shown in Figure 5 and subsequent figures as horizontal lines.
Figure 5: Voltage THD due to Existing Harmonic Loads
THD levels are shown in Figure 5 for three locations along Feeder 1: near the substation, mid-feeder, and at the feeder end. A range of values is shown for each location to illustrate the effect of feeder loading. As load approaches its minimum (which corresponds to 50% of the peak load reported by SCE), linear load tends to decrease more rapidly than harmonic load. The result is that voltage THD is highest at minimum load. Assuming a maximum allowable voltage THD of 5%, as suggested by IEEE Standard 519-1992, Figure 5 shows that harmonic distortion should not be a problem on the system, regardless of feeder loading or capacitor configuration.
Effect of CFL Penetration vs. Voltage Distortion
Distortion levels due to CFL penetrations of 50 W, 100 W, and 150 W per household are shown in Figure 6. Voltage THD values can be very high – up to 16% for the case of maximum CFL penetration, unfavorable capacitor configuration, and minimum feeder loading. This is a very conservative estimate, for it assumes that all CFLs in the system use the high current THD electronic ballasts mentioned earlier. Another factor contributing to the high distortion levels simulated for this case is that damping due to CFLs and other electronic loads is represented in this model by simple parallel RL branches, rather than by the more accurate RLC representation developed. This was done because past research efforts on the effects of CFLs have assumed simple RL damping. The WPI study made this assumption and concluded that CFL penetrations of as little as 200 kW per feeder could cause unacceptable voltage distortion. Relying upon the same assumption for the Feeder 1 study resulted in a similar conclusion, as Figure 6 indicates. Figure 8 shows how much the RL damping model overestimates harmonic levels on the feeder.
Figure 6: Effect of CFL Penetration on Voltage Distortion Along Feeder 1
Figure 6 also illustrates the dramatic effect that feeder capacitor configuration can have on harmonic distortion. The vertical line portion of the bar graph indicates a “favorable” capacitor configuration – all capacitors in the circuit in service. With all banks switched on, the parallel resonance is shifted upward away from the fifth harmonic. The resulting voltage THD levels are approximately 50% of the corresponding levels with a fifth harmonic resonance (horizontal lines).
Effect of CFL Type
CFL harmonic injection characteristics are an important effect to consider. System distortion levels are dramatically reduced when low current THD electronic ballast or magnetic ballast CFLs are employed. Figure 7 shows that if these types of CFLs are used throughout the system, feeder voltage THD levels can be expected to remain at or below 5%, regardless of system loading, capacitor configuration, and CFL penetration (150 W per household is assumed in the figure).
Figure 7: Effect of CFL Type on Voltage THD
Effect of Electronic Power Supply Damping Model
The final case considered is the effect of the damping model assumed for electronic loads. Figure 8 compares the worst case of Figure 6 (150 W CFL per household) with the same case using the RLC damping model. The result is a 20-30% reduction in THD levels. Significant as it is, the drop in distortion is understated, because it assumed that all residential transformers are connected line-to-neutral. However, because the majority of transformers are in fact line-to-line, the amount of ninth harmonic current injected into the system is overstated in this case.
Figure 8: Effect of Damping Model on Voltage THD
CONCLUSIONS
The conclusions from these simulations are generally in agreement with the conclusions of previous studies performed by WPI. However, the new work, illustrating the important effect of the load models, indicates that the actual distortion levels that can be expected are not as severe as predicted in previous studies. SCE is not presently experiencing any significant harmonic problems with CFLs in use by customers; however, results of this modeling study indicate that further investigation into high- distortion-level CFL effects may be warranted. The distortion levels that can be expected are very dependent on system conditions. Capacitor configurations will cause system resonances that can magnify specific harmonic components. Because similar modeling results could apply to penetration of ASDs in heat pumps and electric vehicle battery chargers, these loads may also warrant further investigation and should be modeled separately due to their different operating characteristics.
REFERENCES
[1] A. E. Emanuel, T. J. Gentile, D. J. Pileggi, E. M. Gulachenski, C. E. Root, “The Effect of Modern Compact Fluorescent Lights on Voltage Distortion”, IEEE/PES 1992 Summer Meeting. [2] R. Zavadil, M.F. McGranaghan, G. Hensley, K. Johnson, “Analysis of Harmonic Distortion levels in Commercial Buildings”, First International Conference on Power Quality: End-Use Applications and Perspectives, 1991, B-24, pp. 87-92. [3] W. M. Grady, A. H. Chaudry, A. Mansoor, and M.J. Samotyj “An Investigation of Harmonics Attenuation and Diversity among Distributed Single-Phase Power Electronic Load”, Transmission and Distribution Conference 1994, pp. 110-116.
RELATED STANDARDS IEEE Standard 519
GLOSSARY AND ACRONYMS CFL: Compact Fluorescent Light EMTP: Electromagnetic Transients Program THD: Total Harmonic Distortion
Published by Yanbo CHE, Jian CHEN, School of Electrical Engineering & Automation, Tianjin University, Tianjin China.
Abstract. Microgrid system has received more and more attention internationally in recent years. As the most effective form of distributed generations, microgrid system has also found wide applications in many areas. In this paper, the optimal configuration issue of microgrid system is described briefly first. And then the monitoring system of microgrid system is discussed in details. Different control methods of microgrid system and their advantages and shortcomings are analyzed later. The comparative analysis of different control methods is carried out. Finally, a laboratory-scale microgrid system is proposed as an example to verify the microgrid control strategy. The operation experimental results show that the laboratory-scale microgrid system can operate in grid-connected or islanded mode, with a seamless transfer from one mode to the other, and hence increase the reliability of energy supplies. The study can be used to guide the research of microgrid systems.
Streszczenie. Zaprezentowano różne rozwiązania sieci typu microgrid. Zaproponowano metody sterowania i kontroli systemem, metody optymalizacji konfiguracji oraz systemy monitoringu. (Analiza projektowania i sterowania sieciami typu microgrid)
Keywords: microgrid, optimal configuration, monitoring system, control strategy Słowa kluczowe: microgrid, systemy monitoring, konfiguracja sieci.
Introduction
In recent years, distributed generation (DG) technologies such as photovoltaic (PV), wind turbine (WT), combined cooling heating and power (CCHP), and fuel cells have received wide interest due to the benefits such as high reliability, good quality power supply, environmental preservation, and energy cost reduction [1]. However, increasing amounts of individual distributed generators will also cause as many problems as it may solve [2, 3]. A large number of distributed generators will probably introduce difficulties of control and manage. A better way to realize the emerging potential of distributed generation is to treat generation and associated loads as a system [4]. In order to solve the contradiction between power system and distributed generation and improve the operation performance of power system, microgrid technology comes into being. Microgrid can operate in grid-connected mode or islanded mode and hence increase the reliability of energy supplies by disconnecting from the grid in the case of network faults. Microgrid is the most effective form of distributed generations.
Nowadays, a considerable research has been undertaken on the microgrid technology. As part of the research, a series of microgrid test facilities, such as CERTS microgrid test bed, GE microgrid in America [5, 6], Aichi, Kyoto, Sendai microgrid in Japan [7, 8], Labein, Kythnos, CESI microgrid in Europe [9, 10], have been built for possible demonstrations of advanced distributed generation system. But as a newly-emerged thing, the practical applications of the microgrid is still in the initial stage, and further research is still necessary. Design and control issues are the key points which decide the performance of microgrid system. How to design the configuration and control method of microgirdr systems rationally and effectively is the most important one to be solved among relevant issues.
This paper discusses optimal configuration, monitoring system and introduces control strategy of microgrid system. A case study is given to introduce and discuss. Conclusion is stated in the last part.
Microgrid design
1 Optimal configuration
As shown in fig. 1, microgrid system typically includes PV, WT, energy storage systems, diesel generator, loads, and other distributed generations. In order to fulfill the target of economical and optimal operation of systems, the optimal capacity configuration of components is very important in system design. In related researches, scholars have made great efforts to study the problems of optimal configuration of systems. The optimal capacity configuration is summarized in [11] briefly. The authors of [12] made the optimal design adopting the method of genetic algorithm. In [13] the simulation software HOMER is used to analyze the systems.
Fig .1 Typical structure of microgrid system
HOMER is designed to analyze the configuration of renewable power systems. And it can simulate grid-connected and off-grid microgrid systems. HOMER compares different designs based on technical and economic characteristics in research of the optimal solution. At the same time, it can help analyze the effects brought by uncertainty and changes of input data, such as renewable energy generation installed capacity, diesel generator capacity, energy storage system capacity and resource data. The microgrid system model in HOMER is shown in fig. 2.
Fig. 2 The microgrid system model in HOMER
2 Monitoring system
Microgrid system consists of renewable energy generation systems (such as wind, solar, biomass, and so on), clean energy generation systems (fuel cells, micro gas turbines, and so on), energy storage systems, and a variety of loads. It is characterized by containing a variety of distributed energy. In view of the above characteristics, microgrid monitoring system must match the following main tasks and requirements:
(1) In order to monitor each generation unit simultaneously and improve the reliability and operational efficiency of microgrid system, monitoring system should use distributed structure so that it can monitor the operating parameter comprehensively.
(2) It should support a variety of communication protocols so that it can communicate with distribution terminal device, includes receiving and handling parameters of analog or digital in different formats.
(3) The system configuration, subsystem configuration, the device configuration, the task configuration can all be defined and modified online.
(4) The operation of the whole system, each subsystem and each device in system can all be monitored and remote controlled.
(5) The historical operation data of microgrid system is the most significant data which can be used to make accurate prediction of system state and promote the operation and control of the microgrid system. Therefore, the monitor system must be able to record and long-term storage the operating data of microgrid system. In addition, the data can also be used for accident analysis, statistical analysis, calculation and future planning [14].
Fig. 3 Monitoring subsystem topology
Fig. 4 The structure of monitoring system
The monitoring system of micro-network system makes the distributed generation as sub-unit monitoring, the equipment of subsystem as the monitoring device. Using the communication between serial port and various devices to achieve the acquisition of real-time operational data, the detection of switch status, and remote monitoring equipment. Monitoring subsystem topology is showed in fig.3 The inverter is the core of monitoring subsystem because grid-connected inverter is the heart of all distributed generation units and stores almost all of the running data. For data acquisition and remote monitoring are achieved through the controller and communication interface. The structure of monitoring system is shown in fig. 4. Specific programs and control process are set by the controlled system.
Microgrid control
Compared with traditional large-capacity thermal power, there are volatility in primary energy (such as wind and solar) and bidirectional flow in dynamic allocation of secondary energy (such as bidirectional energy flow between microgrid and large power grid, bidirectional energy flow in the bus of energy storage units) in microgrid. In addition, the load following reaction speed of each DG unit is very different. All these features add the complexity of microgrid automatic management, especially the optimal scheduling [15].
Fig. 5 Hierarchical control structure of microgrid
The microgrid system often has a hierarchical control structure, as shown in fig.5. There are two control layers: MMS and local controller (LC).The MMS is a centralized controller that deals with management functions, such as disconnection and resynchronization of the microgrid and the load-shedding process. In addition to this function, the MMS is responsible for the supervisory control of microsources and the ESS. Using collected local information, the MMS generates a power output set point and provides it to the LCs. An LC is a local controller that is located at each microsource and controls the power output according to the power output set point from the MMS [16]. The hierarchical control structure contributes to the control performance of microgrid.
Besides the control structure, control methods are the research core of microgrid. The volatility of distributed generation and the differences of distributed generation units have increased the complexity and difficulty of microgrid control. Distributed generations in microgrid are connected with the busbar through power electronic devices that are very different from the generators that connected directly with the grid. In addition, energy storage systems are usually equipped to increase the system inertia. So the traditional control methods are no longer applied properly in microgrid operation control.
At present, there are mainly two control methods, master-slave control and peer-to-peer control [17]. There is a main control unit in master-slave control to maintain the constant voltage and frequency. The main control unit adopts V/F control while other distributed generations adopt PQ control to output certain active and reactive power. Each unit is equal in peer-to-peer control. Peer-to-peer control is based on the method of external characteristics of declining. It associates frequency versus active power, voltage versus reactive power respectively. Through a certain control algorithm, the voltage and frequency will be adjusted automatically without the help of communication.
The control method based on drooping characteristics is widely used in peer-to-peer control. One is f-P and V-Q control which produce reference active and reactive power of distributed generations by measuring the system frequency and amplitude of output voltage of distributed generations. As shown in fig.6. The other is P-F and Q-V control which produce the frequency and amplitude of output voltage by measuring the active and reactive power of distributed generations. As shown in fig.7.
Fig. 6 F-P and V-Q control
Fig. 7 P-F and Q-V control
Master-slave control and peer-to-peer control each has advantages and disadvantages. They are suitable for different operation. At present, the built experimental systems commonly adopt master-slave control in China. Compared with master-slave control, peer-to-peer control has certain advantages but its application needs further study.
Case study
Fig. 8 Schematic diagram of a laboratory scale microgrid
The structure of the microgrid (MG) system is shown in fig. 8. It is a single phase system, with 230V, 50Hz, comprising PV simulator, wind simulator and battery storage [18]. Both of them are connected to the AC grid via flexible power electronic interface [19]. Also there is a Microgrid Central Controller (MGCC) which is responsible for the optimization of the microgrid operation.
The MGCC functions range from monitoring the actual active and reactive power of the distributed resources, voltage and frequency of the AC bus. Also it is responsible for the maximization of the microgrid’s value and the optimization of its operation by sending control signal settings to the distributed resources and controllable loads via communication lines [20]. In this paper, RS485 communication lines are used to realize this function.
For the laboratory-scale MG, based on the control strategies of the micro sources and the battery energy storage, a series of experiments were carried out, the power output of the distributed generators and battery, voltage and frequency of the AC bus were real-time measured and analyzed by the Power Quality Analyzer.
A. Transfer from islanded mode to grid-connected mode While switching from islanded mode to grid-connected mode, the voltage and frequency should be maintained within acceptable limits. The dynamic response process is shown in fig. 9. At t3, the MG is synchronized to the grid and its voltage and frequency become equal to the values of the network. The PV simulator and wind simulator maintain constant power output and the battery will be charged by the grid. As shown in fig.9, the voltage and frequency will fluctuate accordingly with the grid.
Fig. 9 Transition from islanded to grid-connected mode
B. Transfer from grid-connected mode to islanded mode Also the voltage and frequency should be maintained within acceptable limits when switching from grid-connected mode to Islanded mode. The dynamic response process is shown in Fig. 10.
At t4, the MG is disconnected from the grid and returns to Islanded mode operation. During this transition, the voltage and frequency will decrease slightly. For the safe operation, the distributed generators will disconnect from the MG (the output power become zero), the battery will increase its power output accordingly for the power balance of the system. Due to the voltage/frequency control of the battery inverter, the voltage and frequency of the MG are restored to the nominal value. Then at t5, the distributed generators will connect to the MG again and return to its initial operating state. From the voltage and frequency behaviors in Fig. 9 and Fig. 10, it may be observed that MG stability is not lost when facing transition between Grid-connected mode and Islanded mode.
Fig. 10 Transition from grid-connected to islanded mode
Conclusion
In this paper, discussion and analysis of microgrid design and control are carried out. And the design method of HOMER is introduced briefly. The monitoring system design of microgrid is described in details. The control methods of microgrid system are mainly demonstrated. The master-slave control and peer-to-peer control are analyzed in detail. A laboratory-scale MG system has been set up. The operation experimental results show that the laboratory-scale MG system can operate in grid-connected mode or islanded mode and hence increase the reliability of energy supplies, with a seamless transfer from the one mode to the other.
Acknowledgment
This work is supported by National Key Foundational Research Project ‘Distributed Generation Supply System Pertinent Foundational Research’ (No.2009CB219700)
REFERENCES
[1] Jian W., Xing-yuan L., Xiao-yan Q., Power System Research on Distributed Generation Penetration, Automation of Electric Power Systems, vol. 29(24), pp. 90-97, 2005. [2] A.M. Azmy and I. Erlich, “Impact of distributed generation on the stability of electrical power system,” Power Engineering Society General Meeting, vol. 2, pp.1056-1063, 2005 [3] J.G. Slootweg and W.L. Kling, “Impacts of distributed generation on power system transient stability,” Power Engineering Society Summer Meeting, vol.2, pp.862-867, 2002 [4] R. Lasseter, A. Akhil, C. Marnay and J. Stephens et al, “White Paper on Integration of Distributed Energy Resources. The CERTS Microgrid Concept,” Consortium for Electric Reliability Technology Solutions (CERTS), CA, Tech. Rep. LBNL-50 829, 2002. [5] R.H. Lasseter and P. Piagi, “Control and Design of Microgrid Components, Final project report,” PSERC publication 06-03, [Online]. Available: http://certs.aeptechlab.com/ [6] US Department of Energy Electricity Distribution Programme, Advanced Distribution Technologies and Operating Concepts – Microgrids, [Online]. Available: http:// http://www.electricdistribution. ctc .com/Microgrids .htm [7] Toshihisa Funabashi and Ryuichi Yokoyama, “Microgrid Field Test Experiences in Japan,” Power Engineering Society General Meeting, pp. 1-2, 2006 [8] S. Morozumi, “Micro-grid demonstration projects in Japan,”IEEE Power Conversion Conference, pp.635 642, April, 2007. [9] Oleg Osika, Aris Dimeas and Mike Barnes et al, “DH1_Description of the laboratory micro grids,” Tech. Rep. Deliverable DH1, 2005 [10] European Research Project More Microgrids. [Online]. Available: http://Microgrids.power.ece.ntua.gr [11] J. Chen, Y. B. Che, and J. J. Zhang, “Optimal configuration and analysis of isolated renewable power systems.” Power Electronics Systems and Applications (PESA), 2011 4th International Conference on pp. 1284-1292. [12] Eftichios Koutroulis, Dionissia Kolokotsa,Antonis Potirakis, Kostas Kalaitzakis. Methodology for optimal sizing of standalone PV–Wind. Solar Energy,2006(80): 1072-1088. [13] S.M. Shaahid, M.A. Elhadidy. Economic analysis of hybrid PV-diesel-battery power systems for residential loads in hot regions. Renewable and Sustainable Energy Reviews,2008(12): 488-503. [14] J. D. Ren, Y. B. Che, and L. H. Zhao, “Discussion on monitoring scheme of distributed generation and micro-grid system,” Power Electronics Systems and Applications (PESA), 2011 4th International Conference on, pp. 1-6. [15] Y. B. Che, and J. Chen, ”Control research on microgrid systems based on distributed generation,” Applied Mechanics and Materials, vols. 58-60 (2011), pp. 417-422. [16] Jong-Yul Kim, Jin-Hong Jeon, Seul-Ki Kim, Changhee Cho, June Ho Park, Hak-Man Kim, and Kee-Young Nam: Cooperative Control Strategy of Energy Storage System and Microsources for Stabilizing the Microgrid during Islanded Operation. IEEE Trans. Power Electron. Vol. 25 (2010), p. 3037-3048 [17] Yang Zhangang, Wang Chengshan, Che Yanbo: A Small-scale Microgrid System with Flexible Modes of Operation. Automation of Electric Power Systems. Vol. 33 (2009), p. 89-92 [18] Y. B. Che, Z. G. Yang, and K. W. E Cheng, “D. Construction, operation and control of a laboratory-scale microgrid,” Power Electronics Systems and Applications, PESA 2009. 3rd International Conference on, pp. 1-5. [19] D. Georgakis, S. Papathanassiou and N. Hatziargyriou, “Operation of a prototype Microgrid system based on microsources,” Power Electronics Specialists Conference, pp. 2521- 2526, 2004 [20] D. Nikos Hatziargyriou et al, “DC1_Microgrid Central Controller strategies and algorithms,” Tech. Rep. Deliverable DC1, 2004
Authors: Yanbo CHE: Associate professor in school of electrical engineering and automatic, Tianjin University, E-mail: ybche@tju.edu.cn Jian CHEN: PHD student in school of electrical engineering and automatic, Tianjin University, E-mail: happy_chenjian@163.com.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 5b/2012
Published by Petre Lucian OGRUTAN1, Lia Elena ACIU1, Dan LOZNEANU1, Ioan ROSCA2, Transilvania University of Brasov, Romania (1), Transilvania Sud Electricity Distribution (2)
Abstract. Protection devices against over-voltage and over-currents for low voltage systems are equipped with varistors and gas-discharge tubes around a fuse, having a disconnection time about 200ms. This paper proposes an intelligent protection system that includes a real-time energy distribution monitoring system for the consumer, providing rapid load disconnection/reconnection without any human intervention, which is considered as an important improvement.
Streszczenie. Urządzenia chroniące przed przepięciami i przetężeniami w instalacji elektrycznej zawierają warystory, iskierniki oraz bezpieczniki o czasach działania około 200 ms. W artykule przedstawiono inteligentny rozwiązanie ochrony zawierające system monitorowania użytkownika energii w czasie rzeczywistym oraz zapewniające szybkie wyłączenie/załączenie bez ingerencji człowieka (Zastosowanie urządzeń wykorzystujących mikrokontrolery do ochrony i monitorowania instalacji elektrycznej)
Keywords: safety, microcontroller, monitoring, low voltage, distribution systems Słowa kluczowe: bezpieczeństwo, mikrokontroler, monitorowanie, instalacja elektryczne
Introduction
A recent study achieved for the European Community [1] was aimed at investigating the causes of accidental fires in order to establish effective safety provisions for low voltage consumers against such hazards. The study mentions, for instance, a percentage of 31% of fires caused by the electric section of the equipments. Within this context, ascertains that most of the fires were caused by the defects in the electrical system – shortcircuits and overvoltages. Consequently there is a worldwide concern for identifying and applying new methods of protection.
The levels of the overvoltages occurring in electrical supply systems are lower than those arising in electrical power transport and distribution systems yet under unfavorable conditions the former can also cause significant equipment damage. A situation specific to low voltage systems is the occurrence of hazardous accidental voltages at the grounding connections of the equipments due to the effects of the carried overcurrents. Under the present conditions of operation of the energy market and due to the increased requirements in terms of the quality of services imposed on electrical energy providers, the need to determine the level of overvoltages becomes increasingly important in distribution and supply systems as well. This provides the opportunity to identify the areas of increased electrocution risk, as well as other potentially hazardous situations to consumers in the investigated low voltage systems. Also, the possibility of hazards due to damages produced in low voltage cable systems cannot be neglected.
The computer-assisted simulation of the transient electromagnetic regimes and sequences of such regimes is the only feasible method for performing parametric analyses at the level of accidental voltages, overvoltages and overcurrents in the system. Such parametric analyses allow for determining the level of voltages and currents circulation in any point of the system on a given range of its operating parameters. This is virtually the only solution to anticipate the sensitive areas of potential risk.
Knowing the current values in any point of the system allows an adequate planning of the preventive maintenance of grounding connections, of protection systems and points with damaged insulation, especially in cable systems.
Knowing the voltage values in any point of the system allows to avoid operation at voltage values that are unacceptable not only to consumers but also to certain system components for a prolonged operation regime, knowing the touch and step voltages in all ground connections of the analyzed system for any operating mode including anomalous situations like broken neutral wires or potential equalization conductors.
The measures that can be implemented as following to the performed analyses will produce immediate effects in terms of avoiding unwanted events with respect to electrical safety, reduction of frequency and duration of electrical power outages with the associated technological and economic advantages. Adequate planning of maintenance activities, which follows as a consequence to such analyses, results in enhanced services quality of the provided electrical power. The EMTP-ATP simulation program allows to determine the voltage and current variations occurring in different points of an electrical network, highlighting those locations where an intervention appears necessary by using types of protection devices as proposed in this paper [2]. A single electronic device including embedded systems for overcurrent, overvoltage and user protection against electrocution represents a necessity at this level of modernization [3]. The data exchange with the exterior moves the fuse system into the 21st century by ensuring the facilities for remote control [4]. E.g. a quick and safe fuse which operates at a precise value of the short-circuit current could improve the quality of electrical power by reducing the voltage sags in the system. The paper presents a system for consumer protection against short-circuits and overvoltages that can occur in the low voltage power supply grid. The proposed system was subjected to practical tests for various power factors.
Presentation of the protection device
The protection device proposed in this paper operates as an electronic circuit breaker based on a microcontroller which provides the possibility of transferring distant events into an event log. Its structure is shown in Fig. 1. The proposed device has the following advantages:
-High cut-off speed (maximum 10ms) which is faster than with conventional methods; -Does not require human intervention since after power cutoff in the case of short-circuit or overvoltage, the power supply is automatically restored (for short-circuits only after eliminating the cause);
-Due to its two-stage action for the case of occurrence of low-value overvoltages or overcurrents, the average voltage value is subtracted thus preventing the load to be cut-off from the supply. This advantage can be obtained for the triac equipped version only;
-The load is cut-off at voltage/current zero-crossing which reduces the perturbations;
-The communication interface to the Internet enables online transfer of electrical power consumer information.
Fig. 1. Schematic for the consumer protection device
The acquired data including voltage and current values can be transmitted over distance and made available to the interested parties (consumer or electrical power distributor). The necessary condition for data communication is bidirectionality, namely to provide the possibility to remotely control the device by either the client (consumer) or the electrical power distributor. This feature could be useful if the distributor schedules electrical works that could involve potential risks to the consumer (client), by providing the possibility of disconnecting the client over the repair period. The data transmission can be achieved:
-using Bluetooth if there is no GSM coverage or cable Internet is unavailable. The consumer (client) can view the event report on a computer with Bluetooth interface while the representative from the distribution company has direct access with a notebook from the exterior;
-Over GSM modem, while communication is assured over SMS or GPRS if there is GSM coverage, which is the preferred solution in most cases;
-Over the INTERNET, with a web server-like device which collects the data from the microcontroller and posts them on a website.
The relay-based command is more straightforward but the microcontroller must nevertheless detect the zero voltage/current crossings since coupling and decoupling the load must occur during zero crossing in order to protect the relay contacts. Another problem associated to this solution is the relatively high cost involved by a relay capable to withstand high voltages and currents. While it is a simple task to command a load using a relay, the command with solid-state devices is more complicated. Next, is given a brief description of consumer command using triacs and thyristors, which is the usual command method. Mains voltage control applications involve some particular difficulties:
-High currents and voltages, which requires a special attention and safety measures such as optical decoupling, grounding potential management, galvanic isolation of the oscilloscope, etc;
-Especially for transient regimes, the controlled process is usually more complex;
-The need for developing industry applications leads to cost restrictions, requires higher reliability, compliance to EMC standards, etc.
For the microcontroller command variant it is recommendable to use galvanic decoupling between the digital control circuit and the triac, that is, from the mains supply section [3]. The isolation can be achieved through transformers, optocouplers or opto-triacs that assures a turn-on current flowing in the same direction as the main current to provide an optimum command of the triac. The current through the opto-triac is obtained by dividing the main current through the triac, thus assuring that the triac’s turn-on current has the same direction as the main current. The analysis of the voltage and drawn current is achieved by the microcontroller through the embedded A/D converter. The current is analyzed using a dedicated current transducer (Hall) while voltage analysis is performed through rectification, filtering and division. Whenever a short-circuit or a voltage surge occurs, the microcontroller stops sending turn-on pulses to the triac or cuts off the relay command current so that the consumer’s power supply is cut off as well.
For the triac command variant the protection method was devised as a two-stage process. If overvoltages or overcurrents are detected these are can be classified into:
-Non-hazardous (+/- 3%), requiring no countermeasures;
-Less hazardous (+/-10%), which requires modification of the triac’s firing angle. Depending on the magnitude of the over-voltage and over-current, a timer is programmed which determines the turn-on delay of the triac (the control angle). This command reduces the average load voltage and so compensates the voltage or current increase;
-Hazardous (over10%), blocks the gating pulses to the triac thus disconnecting the load from the mains supply. The voltage is continuously measured and after returning to normal values the load is reconnected to the supply system. When short-circuit is detected, the supply is re connected after a delay period and if the short-circuit persists the triac is blocked for another interval of time.
The condition of over-voltage or over-current is evaluated over one period of the supply voltage by verifying whether the average value of the current and voltage samples are within normal limits. In order to increase the operating safety, the evaluation over two periods of the voltage, that is 20ms, was software-implemented as well. This will increase precision but the cut-off time increases as well. For the relay variant where the two-stage protection is not possible, the software must assure power cut-off at voltage zero crossing. Considering the delayed response of the relay, the cut-off command must precede voltage zero crossing by a time interval depending on the type of the used relay.
Simulation of the proposed protection device
Several Simulink simulations of the operation of constructional versions, relay and triac, of the protection device were performed.
Fig. 2. The simulation results of RLC load cut off at voltage increase
Figure 2 and Figure 3 show the results of the performed simulations. As can be seen on the presented simulations, the two-stage action assures that, in case of low over-voltages or over-currents, the average voltage value decreases and the load is not cut off from the mains supply ensuring uninterrupted operation.
Fig. 3. The simulation results of RLC load cut off at current increase
Experimental results
The tests were conducted with a resistive load made of nickeline conductor, a capacitive load consisting of a capacitor bank and a movable iron core inductor. The test schematic is presented in Figure 4.
Fig. 4. Laboratory experimental tests
Several measurements were conducted on both constructional variants using resistive and RLC loads for different inductivities by moving the variable core. Some significant waveforms (for several load combinations with limiting values) are presented in Figures 5 and Figure 6. Each graph presents the load voltage (the sinusoid with larger amplitude) and the load current (the lower amplitude sinusoid).
Fig. 5. Relay actuator, RL load, cos φ=0.58 inductive
Load re-coupling and decoupling at current zero crossing can be observed, along with diminished perturbations. The relay variant generates short duration voltage surges both for load re-coupling and decoupling. For mainly inductive loads, current zero crossing produces in both variants a voltage fluctuation which is similar to those obtained by simulation.
Fig. 6. Triac actuator, RL load, cos φ=0.62 inductive, respectively RLC load, cos φ=0.30 inductive.
Conclusions
The simulations show the advantages of a triac used as actuator element. This can be achieved due to the possibility of the two-stage command when detecting overcurrents or over-voltages and stepwise re-coupling of the load. However the triac command variant presents some disadvantages:
-A limiting resistor for the short-circuit current at its maximum admissible non-repetitive value of the triac is permanently connected in series with the load, for power dissipation;
-In a supply system characterized by frequent over-voltages and major load variations the lifetime of the triac decreases significantly [5];
-The behavior of loads with a variable inductive component is incompletely known.
The relay version does not allow a stepwise protection but if the consumer is decoupled at load current zero crossing, the reliability will increase. The relay should be able to operate at high switching speed and high load current (20A), which means that this version will involve higher costs. After several laboratory tests conducted with different types of loads and various power factors, the results demonstrated that the protection device operates in accordance with its design characteristics for inductive power factors (that are specific to electric transport lines) even for very low ones, as results from the presented graphs.
REFERENCES
[1] Kobes M., Groenewegen K., Duyvis M.G.: Consumer fire safety: European statistics and potential fire safety measuresReport, Netherlands Institute for Safety Nibra, 2009. [2] Istrate M., Gusa M., Assessment Of Two Single-End Fault Location Algorithms In An ATP Approach, Revue Roumaine de Sciences Techniques – Électrotechique et Énergétique, Tome 54, 4, 2009, pp. 345 – 354. [3] Romanca M., Ogrutan P.: Embedded Systems. Microcontroller Applications, Transilvania University, Brasov, 2011. [4] Williams, C.: “Electronic Fuse Overcurrent Protection” in Transmission and Distribution Conference and Exhibition, 2005/2006 IEEE PES, Dallas, TX, May 2006, pp. 1226 – 1228. [5] Chiste, A. ; Funke, J.: “Electronic systems protection via advanced surge protective devices” Proc. Telecommunications Energy Conference, Montreal, Canada,Sept. 30 – Oct. 3, 2002, pp. 22 – 26.
Authors: Prof. PhD eng. Petre Ogrutan, Transilvania University of Braşov, Eroilor Av. 29, 500036, Braşov, E-mail: petre.ogrutan@unitbv.ro; Assoc. prof. PhD eng. Elena Aciu, E-mail: lia_aciu@unitbv.ro
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 6/2012
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