- M. Patsalides is with Dept. of Elec. & Comp. Eng., University of Cyprus, 75 Kallipoleos Avenue, P.O. Box 20537 Nicosia, 1678, Cyprus (e-mail: firstname.lastname@example.org).
- A. Stavrou is with Electricity Authority of Cyprus, Amfipoleos 11, 2025, Nicosia, Cyprus (e-mail: email@example.com).
- G. Makrides is with Dept. of Elec. & Comp. Eng., University of Cyprus, 75 Kallipoleos Avenue, P.O. Box 20537 Nicosia, 1678, Cyprus (e-mail: firstname.lastname@example.org).
- V. Efthimiou is with Electricity Authority of Cyprus, Amfipoleos 11, 2025, Nicosia, Cyprus (e-mail: email@example.com).
- G. E. Georghiou is with Dept. of Elec. & Comp. Eng., University of Cyprus, 75 Kallipoleos Avenue, P.O. Box 20537 Nicosia, 1678, Cyprus (e-mail: firstname.lastname@example.org).
The growth in the use of power electronic devices and the introduction of an increasing number of renewable sources of energy in the distribution network, which produce voltage and current disturbances, can give rise to power quality problems in power systems. With the expected growth of connection densities of PV systems in distribution networks, environmental factors and in particular the fluctuation in solar irradiance can lead to undesirable variations of power and supply quality. In order to be able to predict the harmonic pollution due to PV generation, the simple harmonic model for the grid has been adopted and analyzed. Then a typical distribution system topology has been modeled and the findings of the measurements on PV systems have been applied to the distribution system model. On that basis the solar irradiance dependent level of harmonic distortion due to PV generation has been assessed. The results have been compared with already existing standards in an attempt to evaluate the validity of the use of PQ standards in the modern distribution systems.
Index Terms—harmonic distortion, mini grid, photovoltaic systems, power quality, solar irradiance.
|EPRI||Electric Power Research Institute|
|IEC||International Electrotechnical Commission|
|IEEE||Institute of Electrical and Electronics Engineers|
|THDi||Total Harmonic Current Distortion|
|THDv||Total Harmonic Voltage Distortion|
|T&D||Transmission and Distribution|
With the liberalization of the energy market, power quality aspects are becoming a critical issue for distribution systems and for the development of new strategies in the proper management of electrical energy. The importance of improving power quality performance for power systems can be appreciated in the long term when the economic gains will be apparent. Although the calculation of economic consequences for poor power quality in different sectors of daily life is quite difficult, a power quality survey undertaken in the USA showed roughly that the economic losses attributed to poor power quality range between 119 – 188 billion dollars annually . The main causes of the economic losses are the supply outages and PQ related problems across all business sectors. It is also important to be mentioned that PQ costs in Europe are responsible for serious and avoidable reduction in industrial performance with an economic impact exceeding 150 billion Euros .
Despite the fact that power quality has been a field of study under investigation over the last two decades, no significant steps have been taken in the prediction of power quality problems. PQ related research focused on defining PQ and how to measure it. Great effort has also been devoted in the area of defining appropriate standards to guide utilities in mitigating power quality problems. Unfortunately the structure of the distribution network is altering continuously, due to the evolution of sophisticated, highly sensitive equipment. The daily activities of modern customers rely on variable speed drives, computers, electronic ballasts, and power electronic devices. Such devices not only produce power quality problems, but they also require reliable power of supply to operate correctly. It is therefore obvious that the widespread use of a variety of electronic products for domestic appliances is imposing a burden on utilities to supply good quality of electrical energy .
Recently, the Electric Power Research Institute (EPRI) has created a detailed vision for power quality research for the next years. The target of the specific vision is not only to define the objectives of PQ research for the next 10-20 years and fill in critical gaps, but also to specify the role of PQ in enhancing the economic performance of modern electric suppliers and key organizations and companies . According to EPRI, four different directions must be followed to ensure the successful implementation of the PQ vision:
- The improvement of PQ and reliability with in T&D design, maintenance and planning.
- The integration of PQ monitoring and intelligent applications to maximize system performance.
- The achievement of cost effective PQ compatibility between electrical system and loads.
- The PQ technology transfer and knowledge development.
An important part in power quality research-related activities is the better understanding of power quality problems through analyzing sources of poor power quality. The knowledge can be acquired via field measurements and appropriate simulations. As electronic devices are also necessary during the conversion of solar energy into electricity in order for the energy to be supplied to the distribution network, further PQ research is of crucial importance in that area.
In this work systematic measurements of power quality indices made at the output of different PV systems for low, average and high irradiance cases are presented. Furthermore, general conclusions about the impact of high connection densities of PV on the power quality response of a proposed grid topology are extracted through computer simulations. For this purpose a typical mini grid topology is modelled and the findings of the measurements are applied to this model. On this basis the solar irradiance dependent level of harmonic distortion due to PV generation is assessed. Results are presented and analyzed in order to determine how power quality quantities are affected by changes in solar irradiance, and how these changes can affect the adopted grid topology. Special attention has been given to the proper assessment of the amplitude and phase current harmonics obtained from measurements and then used in the model.
III. COMMON POWER QUALITY PROBLEMS AND THEIR EFFECTS
Power quality problems can be divided into different categories according to the characteristics of waveform distortion. Any phenomenon that distorts the common sinusoidal voltage and current waveform used for the transfer of electrical energy in distribution networks can be considered as a power quality problem. The severity of each power quality problem is defined by the already existing standards and guidelines that are mentioned in the next section. Power quality problems include , :
- Short interruptions
- Long interruptions
- Voltage dips and swells
- Surges and transients
- Unbalance, flicker, earthling faults and EMC problems
The occurrence and frequency of PQ disturbances depends on many factors. The topology of the distribution system, the type of customer, the electronic equipments under use, the geographical area and the length of electric lines supplying the user of the distribution network can be considered critical parameters that can affect the way of occurrence of power quality problems. In addition to this, the severity and number of power quality problems varies with load behaviour, climate changes and utility operational practices. A great variety of circumstances can cause power quality problems as follows , :
- Environmental phenomena can lead to voltage disturbances. For example disturbances can exist due to weather variability.
- Operation of large or periodic loads that are connected on the same or adjacent feeder.
- Nonlinear behaviour of sensitive loads that can produce current harmonics. The network impedance in combination with current harmonics produce voltage distortion at customer connection point.
- Large scale integration of inverter connected distributed generators can cause resonance problems and increased harmonic pollution.
- During normal utility operation, capacitor and load switching can cause transients.
In general, power quality phenomena can have technical or financial consequences and can affect in various ways different kind of customers. Large industries, such as semiconductor, pharmaceutical and steel industries experience large financial losses and technical agitation when voltage dips occur at their facility site. The whole operation of the facility might be stopped and has to be restarted. Power quality problems can also cause inconvenience to commercial customers as they might suffer because of business down time, equipment damage and malfunction and data loss. Additionally, regular activities of households can be disturbed with the occurrence of a voltage dip. Utilities cannot be unaffected by the existence of such phenomena. Harmonics flow through the neutral conductor of the power system, thereby inducing excessive heating of various power elements. The existence of harmonics is not desirable in distribution systems because it can cause various problems to the utility and customers. Current distortion can lead to unsafe currents in power-factor correction capacitors, heating and reduction of life in transformers and induction motors, degradation of systems voltage waveforms and malfunctioning of power system protection elements , . The distortion of the voltage waveform due to nonlinear loads changes the power factor of the system, increasing in that way the demand of reactive power. Furthermore, harmonic voltage distortion in the distribution network leads to harmonic currents through linear loads, having as unavoidable result the production of extra losses and the change of load rated characteristics.
The effects of poor power quality on the electrical and electronic equipment vary according to the component as well. Electronic devices can endure specific amounts and intensities of electrical stress before failing. The design of the device plays a vital role in the operation and lifetime of every device , , . Some critical factors that can determine the tolerance and strength of electronic devices are:
- The age of the electronic device.
- The magnitude, duration and nature of the power quality event.
- The frequency of the power quality event.
- The sensitivity of the device to the event that is almost always determined by the specifications of the device design.
- The location of the device within the customer’s installation.
- The path and network impedance between the location of the power quality event and the device.
IV. EXISTING STANDARDS, GUIDELINES AND TRENDS
The most common power quality standards in use nowadays are EN 50160 , IEC 61000 4-30  and IEEE Standard 519-1992. The standard EN 50160 provides recommended levels of different power quality parameters, specifying also the time-based percentage during which levels should not be exceeded. The IEC 61000-4 provides the adequate measurement methods for measuring voltage and current quantities, defines the aggregation periods, describes the measurement formulas and sets the accuracy levels. The main scope of the specific standard was to establish compatibility and common requirements for power quality analyzers to ensure that measurement devices give results of the same accuracy . The IEEE Standard 519-1992 “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems” specifies the limits of harmonic voltage and current at the point of common coupling between end user and distribution utilities. The approach adopted in this standard requires the participation of both end users and utilities.
Nowadays, various research activities revealed the limitation of existing standards, such as Leonardo Energy “Regulation of Power Quality” and KEMA  or ERGEG “Towards Voltage Quality Regulation in Europe” . The most critical considerations about the existing standards and guidelines are:
- Power quality events are being hidden by the time aggregations adopted during the calculation of power quality indices.
- Overall power quality variables are mostly limited to voltage quality only.
- The contribution in maintaining good quality of electricity of utility and user is not well defined.
- The power quality indices meet the limit requirements for only a portion of time.
Limitations generally reflect the existing technology capabilities. As storage and processing limitations of power quality devices are being overcome potentially damaging events would not be missed.
For this work, the main interest lies in the relation between harmonics and photovoltaic systems. Considering that harmonic distortion has probably been the most prolific in the recent years due to the exponential growth of power electronics and generally nonlinear loads, further research is necessary to validate if standards will have the ability to maintain good quality of services provided by distribution networks with the extensive use of photovoltaic systems for power generation. According to the European standard EN50160 (IEC 50160), accommodated by most European Grid Codes, “Voltage characteristics of electricity supplied by public distribution systems”, the limit for total harmonic distortion should not exceed 8 %, including up to the 40th harmonic. The IEEE Standard 519-1992 “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems” specifies the limits of harmonic voltage and current at the point of common coupling between end user and distribution utilities. The approach adopted in this standard requires the participation of both end users and utilities. The limits established by this standard are equal to 5 % for the voltage and current total harmonic distortion that the producer can provide to the customer. The limits for the maximum individual harmonic components are also determined and must be 3 % for voltage lower than 69 kV. The European standard EN 61727 (IEC 61727) “Photovoltaic (PV) systems – Characteristics of utility interface” has established more restrictive limits for voltage and current harmonics. The limits proposed for harmonics are 2 % for total voltage harmonic distortion and 5 % for total current distortion. The maximum for individual voltage harmonics is also limited and must not exceed 1 % , -.
V. MEASUREMENTS AT THE PV PARK OF UNIVERSITY OF CYPRUS – ANALYSIS OF DATA
For the proper modelling of photovoltaic systems, measurements have been undertaken at the Grid Connected Photovoltaic System that is located at the Photovoltaic Park of the University of Cyprus. A power quality analyser was placed at the output of the 15 kWp three phase photovoltaic system to measure the appropriate power quality parameters. The quantities recorded are the power factor, the amplitude and angle of individual current harmonics, the Total Harmonic Distortion (THD) and rms values of voltage and current for a time period of two weeks. A typical example of the solar irradiance measurements for an average day in Cyprus is shown in Fig. 1(a). The solar irradiance profile for a low irradiance day is also shown in Fig. 1(b). By comparing the results for current THD shown in Fig. 2, it is
obvious that solar irradiance plays a significant role in the quality of supplied energy and the distortion of current waveform .
Solar Irradiance Cases vs Time
Fig. 1(a). Solar Irradiance observed for an average day
Fig. 1(b). Solar Irradiance observed for a low solar irradiance day
Power Quality Quantities vs Time
Fig. 2(a). THDi measured for an average day
Fig. 2(b). THDi measured for a low solar irradiance day
Power Quality Quantities vs Solar Irradiance
Fig. 3(a). THDv vs Solar Irradiance
Fig. 3(b). THDi vs Solar Irradiance
In order to formulate a clearer picture for the effect of solar irradiance on voltage and current waveforms, power quality quantities were correlated with instantaneous solar irradiance measured during a two week period and the results are shown in Fig. 3. The Voltage and Current THD are shown in Fig. 3(a) and Fig. 3(b) respectively, and the results confirm the high harmonic content in the current waveform. The total voltage harmonic distortion measured at the output of the system is not strongly dependent on the fluctuations of solar irradiance, but the current harmonics, on the other hand, are very sensitive to changes of incident radiation. The total voltage harmonic distortion ranges from 1.5 % to 2.2 %, as shown in Fig. 3(b). The current total harmonic distortion, on the other hand, has a larger range of values, from 6% to 65%.
Large scale integration of photovoltaic systems and their effect on distribution networks is also of great interest. For this purpose, measurements were applied to a proposed mini grid topology and simulation was done for three different solar irradiance conditions. The proper manipulation of measurements was necessary to extract the appropriate cases for the simulation.
After the analysis of the data, three cases were extracted from measurements and the average values of each case for phase L1 are shown in Fig. 4. The data were subdivided into four categories using Matlab software as follows:
- High Solar Irradiance Case (750 W/m2 and above)
- Average Solar Irradiance Case (between 350 and 650 W/m2 )
- Low Solar Irradiance Case (between 25 and 250 W/m2 )
- Extremely Low Solar Irradiance Case (below 25 W/m2 )
The last case has not been considered as the quantities measured have a wide range of variation during extremely low solar irradiance conditions.
Average Individual Current Harmonics during different Solar Irradiance Conditions
Fig. 4(a). Average Current Harmonic Amplitude- Low Irradiance Case
Fig. 4(b). Average Current Harmonic Amplitude- Average Irradiance Case
Fig. 4(c). Average Current Harmonic Amplitude- High Irradiance Case
VI. TOPOLOGY OF THE SYSTEM UNDER TEST
The grid configuration proposed for simulation is shown in Fig. 5. The topology is composed of linear loads, grid connected photovoltaic systems and a step down transformer. The external grid supplies the mini grid at 11 kV. The voltage is stepped down to low voltage in a distribution substation to supply the energy needs of linear loads. Two types of distribution substations were considered, having transformers of a rated value of 500 kVA and 1000 kVA respectively. Grid connected photovoltaic systems were added to the mini grid topology to satisfy a part of the energy demand. The installed photovoltaic systems have a rated value of 15 kW each. Linear loads vary from 4 to 35 kVA and have a power factor that lies in the range of 0.8 to 0.99, according to the diagram shown below.
Fig. 5. Topology of the System under Test
VII. PROCEDURE AND FINDINGS
The power quality analyser used for the measurements has the ability to sample the current waveform and decompose it into individual harmonics. This procedure is the most adequate for the modelling of photovoltaic systems as the nonsinusoidal output current of a photovoltaic system can be represented by an AC power source that can produce the fundamental current and harmonics of desirable order and amplitude , . The analysis of measured power quality parameters was based on the solar irradiance profile obtained during the measurement period. The solar irradiance profile was normalized to the sampling time period of the power quality analyser in order for the data to be correlated correctly. The normalized data were subdivided into four categories using Matlab software as mentioned earlier.
Finally, the proposed mini grid topology was simulated with the DigSilent PowerFactory Software. The harmonic angles were adjusted using the bus voltage and current angles according to  before inserted into the software. The models of the transmission lines and transformers used by DigSilent PowerFactory Software are described in  and the calculation of power indices is defined in . The equipment data used for the modelling of distribution lines, cables and transformers are those used for the analysis and simulation of the distribution network at the Electricity Authority of Cyprus.
THD Response of Minigrid with PV installations and 500kVA/1000 kVA Distribution Transformer for different Solar Irradiance Cases
Fig. 6(a). THDv vs Bus ID – Low Irradiance Case
Fig. 6(b). THDv vs Bus ID – Average Irradiance Case
Fig. 6(c). THDv vs Bus ID – High Irradiance Case
The results obtained after the simulation of the two proposed topologies for a 500 kVA and 1000 kVA distribution transformer are shown in Fig. 6. In both cases, it is obvious that the THD of all buses increases as the solar irradiance gets higher values. On the other hand, the average current harmonic amplitudes in the high irradiance are much lower than in the medium and low irradiance cases as can be seen in Fig. 4. Despite this fact, the effect of the individual current harmonics on the voltage THD (Fig. 6) is more pronounced in the high irradiance case since the power contribution from the photovoltaic system is much higher during this period. Buses that are located at the end of the radial network and have photovoltaic systems connected are also experiencing problems with harmonics due to the fact that the mini grid network is getting weaker at the edges. The THD obtains the highest value at the point of the network where the highest concentration of photovoltaic systems exists (Bus ID 6). In addition to this, the network topology with the lower power rated value of distribution transformer has less immunity to voltage changes caused by harmonics. According to the EN 50160 standard the limit for voltage THD is 8% including up to the 40th harmonic. As can be seen in Fig. 6(c), the THD on Bus ID 6 is approaching 6.3%. A significant observation made is that the limits described in IEEE Standard 519-1992 “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems” are exceeded .
Harmonics are considered to be the main power quality problem in any kind of distribution network due to the widespread use of power electronics. The quest for increasing the energy produced by renewable sources will make the situation worse if appropriate protective actions are not taken in the near future. Further research is necessary to understand the behaviour of harmonic sources and establish effective guidelines for the proper installation of grid connected photovoltaic systems in mini grids and distribution networks.
The authors would like to acknowledge the generous and continuous support of the Electricity Authority of Cyprus.
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Minas Patsalides received his BSc degree from University of Cyprus and currently he is a PhD student at the Department of Electrical and Computer Engineering, University of Cyprus. Minas has obtained the top mark of his year from the Department of Electrical and Computer Engineering, University of Cyprus. His research interests include measurements and analysis of power quality events, renewable sources of energy and applications of ArcGIS Systems in the evaluation of measurements of electromagnetic fields.
Andreas Stavrou received his BSc and MSc degrees from Leningrad State Technical University, USSR in 1988 and 1990 respectively and his PhD degree from Aberdeen University, Scotland in 1995. He joined the Electricity Authority of Cyprus, in 1996. He is currently in the Transmission Substations Construction and Maintenance department in the South East Area. His research interests lie in the condition monitoring of electrical equipment (cables, electrical machines) power quality and power system evolution to accommodate renewable energy sources.
George Makrides received the BEng First Class Honours degree in Electrical and Electronic Engineering from Queen Mary University of London in 2003. He continued his studies obtaining the MPhil degree in Engineering from the University of Cambridge and graduated in 2004. He worked for two years as a radio network engineer in a private telecommunication operator of Cyprus and he is currently a PhD student at the University of Cyprus, Department of Electrical and Computer Engineering. His research interests include renewable sources of energy and specifically photovoltaic systems.
Venizelos Efthimiou received his BSc in Electrical Engineering & Electronics, MSc in Power Systems, and PhD in Transmission Lines & Transformers degrees from the University of Manchester Institute of Science and Technology, in 1975, 1976 and 1979 respectively. He joined the Electricity Authority of Cyprus in 1979 where he is currently employed. He has been involved in major projects in the EAC and he has several publications in refereed journals and conferences in the field of power transmission.
George E. Georghiou is currently an Assistant Professor at the Department of Electrical and Computer Engineering, University of Cyprus. Prior to this, he was the undergraduate course leader in Electrical Engineering at the University of Southampton, Department of Electronics and Computer Science and a Research Advisor for the Energy Utilisation, University of Cambridge. Having graduated from the University of Cambridge with a BA (1995 – First Class), MEng (1996 – Distinction) and PhD (1999), Dr Georghiou continued his work at the University of Cambridge in the capacity of a Fellow at Emmanuel College for a further three years (1999-2002). His research interests lie predominantly in the area of renewable sources of energy and in the utilization of electromagnetic fields and plasma processes for environmental, food processing and biomedical applications, BioMEMS, Nanotechnology and Power Systems.