Power Quality Blog

The Dangers of Stray Current Damage in Electrical Systems & Arc Flash Protection

Published by Jim Galloway, International Association of Electrical Inspectors (IAEI) Magazine, Electrical Fundamentals – The Dangers of Stray Current Damage in Electrical Systems & Arc Flash Protection, July 3, 2019

The new extension to the factory is finally finished. The engineers have produced a state-of-the-art design; the qualified electricians have completed a professional installation with quality hardware; and the thorough electrical inspections verifying that everything is up to Code are complete. Now that the power is on, the machinery installation crews are busy getting everything ready for production.

But suddenly there is trouble in the plant. Hundreds of amperes are flowing in an uncontrolled fashion, surging through metallic conduit and panels, melting EMT connectors, arcing the bonding wires in a 3-phase receptacle, and burning through the insulation of the power cord to the brand new, million-dollar machine from Europe. If it continues, soon a live phase conductor will be in contact with a red-hot grounding wire, and an unsuspecting worker could be seriously hurt or even killed when they investigate the problem.

What is going on? Did a something come loose? Could it be a lightning strike? Is the foreign-made machine incompatible with our electrical network? Is the damage being caused by 1^st, 3^rd, 5^th, 7^th, or 9^th level harmonics? Why hasn’t the over-current protection system reacted and tripped a breaker? There’s a very good chance that the problem has something to do with the common arc welding process being used to attach mechanical hardware as part of the new machine installation. But how?

Stray welding current

A stray current is a flow of electric current through unintended conductors such as building structures, electrical grounding or bonding conductors, or other equipment due to electrical supply system imbalances or improper equipment hookup. Often in industrial or construction environments, this trouble occurs due to a very simple error in setup by a welder. In Ontario, Canada, there has recently been a well-documented case of an electrical explosion and at least one fatal electrocution due to damage in a facility’s electrical system where the root cause was identified as stray welding current (SWC).^1-3

Arc welding machines are designed to provide alternating or direct current on the order of hundreds of amperes for industrial, commercial, and residential or hobbyist applications. These specialized power sources are designed to provide a low-resistance electrical circuit (typically less than 0.25 Ohms) on the secondary (or output) side; from an electrical engineer’s perspective, this is essentially a short-circuit condition.

This secondary welding circuit is intended to be completed as an isolated, closed-loop system with two cables (see Image 1). It consists of the following parts:

Electrode lead — is the secondary circuit conductor transmitting energy from the power source to the electrode holder, gun, or torch.

Workpiece lead — is the secondary circuit conductor that is attached to the workpiece by the return current clamp and completes the welding circuit (these devices are commonly and incorrectly referred to as the ground cable and the ground clamp).

The SWC faults can occur with simple setup errors or minor system faults, which can introduce current on the order of hundreds of amperes into building structures, electrical networks, and machinery. Two common examples of SWC are illustrated in Images 2 and 3. However, there are many other possibilities as the welding current will always find the path of least resistance to its source. Often, the SWC fault involves other machinery such as fabricating equipment, power tools, other welding machines, and cranes or hoists. These SWC faults can also originate from portable engine-driven or battery-powered welding equipment that do not even derive power from the grid, and they can damage equipment even when powered off.

Image 2. A stray welding current fault due to an operator error, which involves other electrically powered equipment.

Image 3. A stray welding current fault due to a minor welding equipment malfunction.

Surprisingly, there is currently no foolproof system available to stop these damaging stray current faults. They are not interrupted by the overcurrent protection devices installed even in modern electrical networks, and they are not detected or interrupted in circuits protected by ground-fault circuit interrupter (GFCI) devices. The focus has always been on measuring current flow in the intended conductors, and the assumption is that the grounding or bonding conductors would never see current levels that exceed the level where the overcurrent protection devices would trip.

The scenario described in the introduction should not be happening. However, it is far too common of an occurrence in any facility where welding has been performed to be ignored. An example of the sort of damage that can occur is shown in Image 4. Beyond the electrical systems damage, SWC can cause arcing and fires in unexpected locations in a facility, overheating of lifting chains or slings (leading to arcing or annealing), damage to machinery bearings, and arc strikes leading to undesirable metallurgical transformations in certain alloys. Cases of accelerated corrosion caused by stray direct current electrolysis is also a concern on marine structures and buried metallic infrastructure. Stray current damage, therefore, can also be considered a problem from an economic standpoint since much of the damage may be initially hidden from view—even before it becomes an immediate safety issue. Tens of thousands of dollars of damage can occur in a facility or to machinery and systems from one SWC event, which may not even be immediately detected.

Image 4. Electrical arcing and damage in a 600-V electrical outlet from stray welding current.¹

What can be done?

Stray currents from welding operations can be avoided through the strict adherence to rules of the applicable welding safety standards (e.g., ANSI Z49.1:2012 or CSA W117.2-19). These practices—also spelled out in equipment manuals—include locating the workpiece lead return attachment point as close as practicable to the arc, using well-maintained welding cables of sufficient ampacity, and ensuring that the work return current clamp is firmly attached on an intentionally cleaned spot (free of mill-scale, paint, etc.)⁴ All professional welders should be following these rules; however, it must be noted that anyone can buy a welding machine capable of producing hundreds of amperes and misapply it.

Image 5. A screenshot from the video The Problem of Stray Welding Current.⁵

Conestoga College in Cambridge, Ontario, recently produced educational videos to explain the SWC problem; to show just how easy it is to inadvertently create these damaging fault conditions; and to demonstrate the serious damage that can occur in a machine’s power cord. These videos are now publicly available on the college’s YouTube channel. (See screenshots in Images 5 and 6). The goal of the project is to better inform welding educators, welders, and the welding industry, in general, of the SWC hazard. The electrical engineering and inspection community also needs to be more aware of the SWC problem, what to look for, and how it happens.

Image 6. A screenshot from the video Stray Welding Current Damage to Power Cords.⁶

This work was sponsored by EnerDynamic Systems Inc. (ESI) of Brantford, Ontario. ESI partnered with Conestoga College to assist them in extending a patent-pending design for a stray current interrupter device from renewable energy systems into arc welding applications. The partnership had the goal of developing an engineering control device that can make the damage from SWC a thing of the past.

References

1. D. Hisey, “Stray Current Goes to College,” Canadian Welding Association Journal, vol. 14, pp. 20-25, 2016.
2. Office of the Chief Coroner of Ontario, DOKIS, Kelly (Inquest), Kitchener, Ontario, Canada: Queen’s Printer for Ontario, June 16th–19th, 2003.
3. D. Hisey, “How to Prevent Stray Welding Current Damage in Your Electrical System,” 17 November 2017. [Online]. Available: https://www.ecmweb.com/shock-electrocution/how-prevent-stray-welding-current-damage-your-electrical-system. [Accessed 27 December 2018].
4. Canadian Standards Association, CAN/CSA-W117.2-19 – Safety in Welding, Cutting, and Allied Processes, Toronto: Canadian Standards Association, 2019.
5. Conestoga College & Enerdynamic Systems Inc., “The Problem of Stray Welding Current,” 24 January 2019. [Online]. Available: https://www.youtube.com/watch?v=80ehl2nDXUk. [Accessed 24 January 2019]
6. Conestoga College & Enerdynamic Systems Inc., “Stray Welding Current Damage to Power Cords,” 24 January 2019. [Online]. Available: https://www.youtube.com/watch?v=kIVH5V9ntrY. [Accessed 24 January 2019]

Source URL: https://iaeimagazine.org/2019/2019may/the-dangers-of-stray-current-damage-in-electrical-systems-arc-flash-protection/

Current-Carrying Capacity Parallel Single-Core LV Cable

Published by Lech BOROWIK¹, Artur CYWIŃSKI²
Politechnika Częstochowska, Wydział Elektryczny (1), Pracownia Projektowa omega-projekt (2)

Abstract. The paper presents the problems related to the selection of parallel single-core low voltage cables in terms of their current carrying capacity and accordance with PN-IEC 60364-5-523 national standard, exemplified by electric power transmission from transformer MV/LV 1600kVA, with proximity effects also taken into consideration.

Streszczenie. Przedstawiony poniżej tekst opisuje problemy związane z doborem wielowiązkowych linii kablowych niskiego napięcia pod względem obciążalności długotrwałej zgodnie z normą PN-IEC 60364-5-523 na przykładzie wyprowadzenia mocy z transformatora SN/nN 1600kVA z uwzględnieniem wpływu zjawiska zbliżenia. (Obciążalność długotrwała wielowiązkowych linii kablowych nN).

Keywords: Multi-conductor parallel cables, current carrying capacity, proximity effects.
Słowa kluczowe: wielowiązkowe linie kablowe, obciążalność długotrwała, zjawisko zbliżenia.

Introduction

While designing networks and electrical installation, a problem that often appears in the practice is how to select properly low voltage cables, taking into account their current-carrying capacity. The main document that is used by the designers is the Polish national standard “Low-voltage electrical installations – Current-carrying capacity” PN-IEC 60364-5-523:2001 [1] (translation from IEC 60364- 5 part 52 International Standard). The above-given standard along with other publications [2],[3] refer in detail to the configurations of cables, distances between them and their surroundings. The standard introduces various reduction factors as well as other calculative tools that allow for the optimal selection of wires and cables and protection. It seems, however, that the authors of the above mentioned standard and relative studies, in their calculation or research into the mutual influence of parallel cables have focused principally on thermal phenomena. Thus they have neglected the electromagnetic field generated by the flow of currents, which are significant in particular for parallel core which forms one circuit. This situation requires an attempt to study the effects of uneven load in the parallel line and its effect on current-carrying capacity of the cable. In the present paper the authors focus on the description of the problem on a real-life example of selecting multi-core low voltage cable which is the power lead from 1600kVA transformer. The tests have been conducted in accordance with PN-IEC 60364-5-523:2001 Polish National Standard.

In the example analyzed here, it was necessary to select a low-voltage cable which was connecting a 1600kVA transformer to the main low-voltage switchboard. Due to the location of the transformer station and the building specifications, it was impossible to make bus-ducts connections. Financial limitation made it impossible to build insulated bus-ducts.

A cable line made from single core cables of the YAKXS 1X240 type, laid out on cable tracks, with the temperature kept below 200C has been selected. The distance between the cores of one phase was equal to the diameter of the line, whereas the subsequent phases were laid out analogically at distances considerably exceeding twice of the diameter of a single line.

The nominal transformer current on the low-voltage side, with the power factor cos φ =0,9 (system working with reactive power compensation) for a single phase, under the symmetrical load equals 2576A.

According to the Standard [1], the cables were laid out in accordance with the 52-C12 Table (reference method of installation – G horizontal) . While calculating the current-carrying capacity, the correction factor were taken into consideration – according to 52-D1 Table of coefficients and exponents, taking into account the ambient temperature different from 30^oC (coefficient 1,08) – the increasing coefficient has been skipped, and in accordance with 52-E1 Table (reduction factor 0,79 for a group of more than one circuit). After meeting all the conditions, included in the 523.6 Chapter of Standard [1], the following electrical system has been designed.

Fig.1. Diagram of connecting low-voltage switchboard

For each phase, 6 parallel lines were selected, made from XPLE insulated aluminum cable with the cross-section of 240 mm²of the YAKXS type. The current-carrying capacity of each core, according to 52-C12 Table of norm [1] was 521A. However, if one accepts the values provided by the manufacturer i.e. those from Telefonika Kable catalogue [4], the current-carrying capacity will be 480A. The calculated current value of a single core for the nominal transformer load was 429A. It can be assumed then, that the cables have been selected correctly, with a substantial safety margin up to 10%. The designed system was made in accordance with the specification and put into operation. After a few days of operation, the system suffered a fault, which resulted in a total destruction of the cable line.

Measurements and registration of currents in the cable line

After rebuilding the power system, current measurements of each phase in the cable line were made in order to exclude possible asymmetries of the load and system over-current. We have also made check-up measurements of the circuit breaker, equipped with integrated protection system. The check-up of the circuit breaker excluded the possibility of its malfunction while the measurements confirmed that the system load was symmetrical.

The maximum recorded effective value of the current did not exceed the current-carrying capacity in amperes. However, we have noticed a considerable discrepancy between the effective values of current in particular lines being a part of one phase. The courses of effective current values in extremely loaded lines for a selected period of time are presented below.

Fig.2. RMS current extremely loaded core of line

The difference in the load of extremely loaded lines is nearly double. For line L1, the effective current value of a maximum of 528A was recorded, while this value was 280A for line L3 – the proportion of load for extremely loaded lines was 1.9. It is worth recalling that the lines were made from the same material and had the same length whereas differences in the resistance of cable clamps were excluded from the analyses. Due to the significant differences in the load of particular lines as well as the possibility of another fault, we have investigated the causes of the uneven load of the lines. For his purpose, a computational model of the system was developed.

The computational model of the system

The simulations were made in FEMM program which is a suite of programs for solving low frequency electromagnetic problems on two-dimensional planar and axi-symmetric domains. The program currently addresses linear/nonlinear magnetostatic problems, linear/nonlinear time harmonic magnetic problems, linear electrostatic problems.

Six cores with the cross-section of 240mm² and total length of 30m were entered into the model. Then, a sinusoidal current flow (amplitude 3632 A, frequency 50 Hz) was forced as the excitation source, with The cores were assumed to be made of AL 1100 aluminum, with electric conductivity γ = 34,45 MS/m. The cable insulation was not considered in the simulation.

The calculations were made for various types of line configuration – vertically (two variants), spherical layout, parallel layout. For each case, two coefficients k_AS –unbalance and k_PZ – overload were defined

(1) k_AS=I_max/I_min,
(2) k_PZ=6*I_max/I_C,
I_max – current amplitude of the highest load core,
I_min – current amplitude of the lowest load core,
I_C – current amplitude of the circuit (one phase).

Parallel single core in the flat layout In the first variant, the core of the circuit being one phase were laid out as flat – vertically laid, with a distance of 17.5 mm between each one, which means that the gap between each line was equal to the line diameter. The simulation is a model of a real system. The mesh being used for calculations consisted of 44736 nodes and 88748 triangle elements. Table 1 presents the results of the total current calculations for each core, whereas Figure 3 presents the real component of magnetic vector potential A, B = rot A, rot A=0 for this section and the distribution of current density of each core. Fig. 4 presents the eddy and source current density – cross-section of cores (X-Y Plot).

Table 1. Total current of each core

Fig.3 Real component of magnetic vector potential A, distribution of current density

Fig.4 Eddy and source current density – cross-section of cores

The values of the asymmetry and overload coefficients are as follows: k_AS=1.78, k_PZ=1.32 The next variant features the lines of cores that were laid out flatly, with a distance of 35 mm between each one, which is equal to the double diameter of the line. The mesh being used for calculating had 44865 nodes and 89006 triangle elements. Table 2 presents the results of the total current calculations for each core, Fig. 5 presents the real component of magnetic vector potential A for this section and the distribution of current density of the each core. The values of the asymmetry and overload coefficients are as follows: k_AS=1.73, k_PZ=1.33.

Table 2. Total current of each core

Fig.5 Real component of magnetic vector potential A, distribution of current density

Parallel single core in the spherical configuration

The cores were laid out spherically – around a circle. The lines did not come in contact with each other. The mesh being used for calculations had 42953 nodes and 85182 triangle elements. Table 3 presents the results of the total current calculations for each core, Fig. 6 presents the real component of magnetic vector potential A for this section and the distribution of current density of the each core. The values of the asymmetry and overload coefficients are as follows: k_AS=1.03, k_PZ=1.1

Table 3. Total current of each core

Fig.6 Real component of magnetic vector potential A, distribution of current density

Current bus bar in a parallel layout

The lines were laid out parallelly in two layers, with a distance of 17.5 mm between each line and each layer. The mesh being used for calculating had 44701 nodes and 888678 triangle elements. Table 4 presents the results of the total current calculations for each core, Fig. 7 presents the real component of magnetic vector potential A for this section and the distribution of current density of the each core. The values of the asymmetry and overload coefficients are as follows: k_AS=1.39, k_PZ=1.1

Table 4. Total current of each core

Fig.7 Real component of magnetic vector potential A, distribution of current density

Summary and final conclusions

The simulations conducted on the prepared model proved to be compatible with the measurements recorded in the real system. The calculated asymmetry coefficients are, respectively, k_AS=1.9 for the real system, and k_AS=1.73 for the model. The analysis of mutual interaction between parallel lines that formed one circuit showed a sign influence of the proximity effect on the load layout which is also reported in publications [5],[6]. The configuration of core layout shown in PN-IEC 60364-5-523:2001 National Standard as an optimal one (horizontal layout, with the double diameter distance between the lines), seems to be the worst solution if field phenomena are to be taken into consideration. An asymmetry coefficient above 1.7 and overload coefficient above 1.3 practically render it impossible to construct multi-conductor low-voltage parallel cables. An optimal solution from the simulations conducted proved the spherical layout with the coefficient values of k_AS=1.03, k_PZ=1.1. It seems advisable to conduct further studies into the current carrying capacity of multi-conductor low-voltage parallel cables, including three-phase systems, harmonics and thermal phenomena in order to work out an optimal way of constructing multi-conductor cables and make corrections to norm [1]. The authors are planning to make a simulation using another tool (Maxwell from Ansoft), construction of a real-life model and taking measurements in real objects with transformer rated power over 1000KVA that include multi-conductor cable lines. Currently, designing multi-conductor cable lines in accordance with the Standard [1], without considering the proximity effect and performing additional calculations is both erroneous and hazardous.

REFERENCES

[1] PN-IEC 60364-5-523:2001 Instalacje elektryczne w obiektach budowlanych – Dobór i montaż wyposażenia elektrycznego – Obciążalność prądowa długotrwała przewodów
[2] Skibko Z, Obciążalność prądowa przewodów ułożonych wielowarstwowo. Rozprawa Doktorska, Politechnika Białostocka, Maj 2008r
[3] Skibko Z., Analityczne metody wyznaczania obciążalnościprądowej długotrwałej przewodów ułożonych wielowarstwowo, Przegląd Elektrotechniczny 4 (2009), s. 190-194
[4] Telefonika Kable, „Solidna Energia – Katalog kabli i przewodów Elektroenergetycznych”, 2013 r., s. 156,172
[5] Piątek Z., Modelowanie linii, kabli i torów wielkoprądowych, yd. Politechniki Częstochowskiej seria Monografie nr 130, s.33-54
[6] Piątek Z., Jabłoński P., Podstawy teorii pola elektromagnetycznego, Wydawnictwo Naukowo-Techniczne Warszawa, 2010, s. 142, 277-282.

Autorzy: dr hab. inż. Lech Borowik, Politechnika Częstochowska Instytut Telekomunikacji i Kompatybilności Elektromagnetycznej, ul. Armii Krajowej 17, 42-200 Częstochowa, E-mail: borowik@el.pcz.czest.pl;
mgr inż. Artur Cywiński, Pracownia projektowa omega-projekt, ul. Topolowa 1, Tychy, E-mail: artur.cywinski@omega-projekt.pl

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 92 NR 1/2016. doi:10.15199/48.2016.01.16

The Measurement of Input Power of Power Supply in Network Disturbed by Low Frequency Distortions

Published by Stanisław GALLA, Arkadiusz SZEWCZYK,
Gdansk University of Technology Faculty of Electronics, Telecommunications and Informatics

Abstract. In the paper authors present results of observation of input power changes versus harmonics amplitude in supply voltage of low-power power supply device. In the study, the electrical measurements supported with thermal imaging were used. The input circuit elements of studied device responsible for input power increase are pointed.

Streszczenie. W artykule przedstawiono wyniki pomiarów zmian mocy wejściowej pobieranej z sieci zasilającej przez zasilacz małej mocy w funkcji harmonicznych napięcia zasilającego. Wytypowano elementy obwodu wejściowego badanego urządzenia odpowiedzialne za zwiększenie poboru mocy przy zasilaniu napięciem silnie zniekształconym. W badaniach oprócz pomiarów elektrycznych wykorzystano również obrazowanie termograficzne. (Pomiar parametrów mocy wejściowej zasilacza zasilanego przez napięcia odkształcone zaburzeniami niskich częstotliwości).

Keywords: power supply, distortions, measurements.
Słowa kluczowe: zasilacze, zaburzenia, pomiary.

Introduction

Main parameters that describe electronic equipment declared by producer are, among of others, supply voltage, current consumption, working frequency and efficiency. Producer by declaring values of parameters guarantees that in standard working conditions their values will be in specified range. For equipment intended for domestic use, values of those parameters are determined for standard supply conditions, that means the supply voltage is sinusoidal with specified frequency and amplitude (RMS value). However, in real supply nets the voltage is disturbed by low and high frequency components, what was described e.g. in [1, 2, 3, 4]. For low frequency range, the one of parameters that describes the quality of supply voltage is total harmonics distortion (THD). The methodology of THD estimation and its maximum values are briefly described in several international standards [5, 6]. Currently, it is presumed that in public low voltage supply nets the THD value should not exceed 8%. Unfortunately, significantly higher values of THD are registered in several supply subnets and can have harmful influence on electric and electronic equipment that is supplied with such a voltage. The example influence of disturbed supply voltage on electronic apparatus, which is switching power supply is described in this report.

Issue consideration

The considered problem was reported by one of the electronic equipment producer. The user of low-power (<70 W) switching power supply reported significant discrepancy between supply current value he recorded and the value declared in technical note. The user pointed also differences in values of other parameters measured for nominal load. During the work on the problem, it was recognized that even though the supply voltage was 230 V_RMS (+5 V_RMS / -7 V_RMS) it was strongly distorted by harmonics. Conducted measurements showed that THD value reached 11% and both even and odd harmonics was observed. Atypically, the dominant 4^th and 8^th order harmonics were observed. It’s worth to mention here, that for low-power power supply sources the power ratio correction circuit is not required. The mentioned power supply source was then examined in laboratory conditions.

Experiments

In order to examine the influence of each particular harmonic on device behaviour, the device under test was supplied with voltage distorted with harmonic (one at a time) which amplitude exceeded acceptable value specified e.g. in standard [4] and the current consumption was monitored. The device under test was 60 W switching power adapter with following parameters: nominal supply voltage U_n = 230 V, nominal power P = 60 W, input current I_nRMS = 0.6 A, nominal output current, I_o = 3 ADC, nominal output voltage U_o = 18 V_DC. Producer guarantees 20% of parameters accuracy, according to appropriate standards. Figure 1 shows the input circuit of the device.

Fig.1. Input circuit of the device under test

Measurement set-up

Examined device was supplied by CHROMA 61502 programmable AC source. The output RMS voltage of the AC source was U_RMS = 230 V (+/- 1%). The 50 Hz component of the output voltage was U_1RMS = 228,8 V and was constant during the test. The RMS value of each harmonic component was U_nRMS = 23 V, for n = 2 to 40 and the phase shift of the component was 90 degree. Figure 2 shows example test waveform for n = 4 (main component with 4^th harmonic).

During the test, the device was loaded with the constant current I_o = 3 A which gives output power P_o = 54 W by electronic load Array 3721A. Under all test input and output parameters was monitored. Measured input parameters are: root mean square value of input voltage in frequency range 50 Hz – 2 kHz (U_RMS), root mean square value of input current in frequency range 50 Hz – 2 kHz (IRMS), peak current (I_p), real power (P), reactive power (Q), apparent power (S), peak current (I_P), power factor (PF), crest factor (CF).

During the measurement, the device under test was also observed by thermographic camera VIGO V50. The camera has spectral range of 8 μm to 14 μm and 384 x 288 pixels resolution. The camera was equipped with 15^o x 11^o lens [7]. Registered thermal images allowed for identification of circuits and components responsible for elevated power consumption by the device under test. First, the thermal image of the DUT was registered for pure, 50 Hz supply, with no harmonics. Next this image was compared with thermal images taken for power supply disturbed by harmonics. Images was taken in thermal equilibrium state, when the temperature of the DUT’s components was stable.

Results and discussion

Figure 3 shows real, reactive and apparent power for each harmonic. The increase in reactive and apparent power is observed for low order harmonics. Moreover, the more significant increase is observed in case of presence of even harmonics.

Fig.3. Real, reactive and apparent input power

The maximum increase, up to 210 VA is observed for 8th harmonic, while for not disturbed voltage the value is about 120 VA. For higher harmonics, above 25^th , the difference in influence of even and odd harmonics is not significant and both reactive and apparent power decrease.

Figure 4 and 5 show input current (RMS and peak value) and crest factor respectively. Both RMS value of current and crest factor corresponds to power consumed by tested circuit.

Fig.4. Root mean square value of input current, I_RMS, for particular harmonics

Fig.5. Changing crest factor under the test

Additionally, the influence of phase shift between fundamental component and harmonic was studied. Figure 6 shows the change in value of real, reactive and apparent power for 3^rd harmonic. Received data shows that the influence of phase shift of the harmonic on power lost in device is less than 20% (+/- 10%) and can be treated as being in tolerance of parameters specified by producer.

Fig.6. Real, reactive and apparent power for 3^rd harmonic versus phase shift

During the test, the temperature of device’s circuit board was monitored using thermal imaging camera VIGO v50. Figure 7 shows thermal image of DUT’s circuit board when supplied with not disturbed supply voltage (with no harmonics added).

Figure 8 shows thermal image of DUT’s circuit board when powered with supply voltage disturbed by 8^th harmonic, while figure 9 shows the one for 11^th harmonic. As is seen in figures 3, 4 and 5, for the 8^th harmonic the maximum input power and for 11^thone the maximum value of crest factor are observed.

Comparing thermal images stored for various supply conditions allows to point out regions and components, where the energy is lost in a form or heat radiation.

In our case the main temperature increase is observed in compensated choke, L (2 x 15mH). Increase of temperature of input filters circuit points, that the energy is lost there and for even harmonics the temperature of filter elements is twice higher than for odd harmonics.

Fig.7. Thermal image of circuit board of tested device supplied with supply voltage with no harmonics (fundamental component only) 1 – elements of input circuit shown in figure 1

Fig.8. Thermal images of circuit board of tested device. Supply voltage disturbed with 8^th harmonic 1 – elements of input circuit shown in figure 1

Fig.9. Thermal images of circuit board of tested device. Supply voltage disturbed with 11^th harmonic tested device. Supply voltage disturbed with 8^thharmonic 1 – elements of input circuit shown in figure 1

Conclusions

Conducted measurements confirmed observation results reported by the device user. In specific supply conditions when the supply voltage is strongly disturbed, device parameters value can be significantly out of range given by producer. It’s worth to mention here, that in standard supply conditions device parameters are within the specified range. In most supply nets, the voltage is distorted mainly by odd harmonics, for which the observed increase in reactive and apparent power is much lower than for even ones. The device user should take appropriate steps to decrease disturbances in his supply net and its adverse influence on various electric devices.

Observed enormous increase in power consumption caused by even harmonics results in increase of input circuit elements temperature. That thermal exposure can lead to decease of device reliability. It should be taken into consideration by producers if it’s worth to evaluate device parameters for maximum negative supply conditions, e.g. for maximum distortion values specified in appropriate standards.

Nevertheless, it should be stated here, that the input filter circuit of examined supply device is not chosen fortunately. It meets restrictions pointed in standard [6] for class B devices in terms of conducted emission, while used inductive element (inductive choke) shows increased thermal emission and cause increase in input power, especially for even harmonics distortions.

REFERENCES

[1] Łuszcz J., Oddziaływanie przekształtników energoelektronicznych dużej mocy na jakość energii elektrycznej, Zeszyty Naukowe Wydziału Elektrotechniki i Automatyki, Nr 31/2012, 211-214
[2] Hanzelka Z., Jakość dostaw energii elektrycznej. Zaburzenia wartości skutecznej napięcia. Akademia Górniczo Hutnicza, 2013
[3] Shafiul I. M, Chowdhury N., Sakil A. K., AtifIqbal K.A., Abu-Rub H., Power Quality Effect of Using Incandescent, Fluorescent, CFL and LED Lamps on Utility Grid, 978-1-4673-6765-3/15
[4] Blackledge J., O’Connell K., Barrett M., Sung A., Cable heating effects due to harmonic distortion in electrical installations, International Association of Engineers: ICEEE12, London, 2012
[5] PN-EN 50160:2010, Parametry napięcia zasilającego w publicznych sieciach elektroenergetycznych
[6] EN 61000-4-30:2015 Electromagnetic compatibility (EMC) – Part 4-30: Testing and measurement techniques – Power quality measurement methods
[7] http://www.vigo.com.pl/pub/File/PRODUKTY/Thermal-imagingsystem/v50.pdf, dostęp z sieci PG: 2016.06.13

Authors: dr inż. Stanisław Galla, Politechnika Gdańska, Wydział Elektroniki i Informatyki, Katedra Metrologii i Optoelektroniki, ul. Narutowicza 11/12, 80-233 Gdańsk, E-mail: galla@eti.pg.gda.pl;
dr inż. Arkadiusz Szewczyk, Politechnika Gdańska, Wydział Elektroniki i Informatyki, Katedra Metrologii i Optoelektroniki, ul. Narutowicza 11/12, 80-233 Gdańsk, E-mail: szewczyk@eti.pg.gda.pl

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 92 NR 11/2016. doi:10.15199/48.2016.11.07

Thermal Problems in HTS Transformer due to Inrush Current

Published by Grzegorz KOMARZYNIEC¹, Tadeusz JANOWSKI², Grzegorz WOJTASIEWICZ², Michał MAJKA², Politechnika Lubelska, Instytut Podstaw Elektrotechniki i Elektrotechnologii (1), Instytut Elektrotechniki w Warszawie (2)

Abstract. With switching on a superconducting transformer to the energetic network, a unidirectional current of high amplitude may appear. Maximal values of the current impulses following shortly one after the other may be several times higher than the critical current values for the superconductor used for making the transformer’s windings. Because of the resistive area propagation overheating of the superconducting tape occurs and that might lead to switching the transformer off working. In the present article the results for transformer HTS of 10 kVA power were presented.

Streszczenie. Włączeniu transformatora nadprzewodnikowego do sieci energetycznej może towarzyszyć prąd jednokierunkowy o dużej amplitudzie. Wartości maksymalne następujących krótko po sobie impulsów prądu mogą przekraczać wielokrotnie prąd krytyczny nadprzewodnika, z którego wykonano uzwojenia transformatora. W skutek propagacji strefy rezystywnej dochodzi do przegrzania taśmy nadprzewodzącej, co może skutkować wyłączeniem transformatora z eksploatacji. W artykule przedstawiono wyniki badań transformatora HTS o mocy 10 kVA. (Problemy cieplne w transformatorze HTS spowodowane przepływem prądu włączania)

Keywords: inrush current, transformer, superconducting, temperature.
Słowa kluczowe: prąd włączania, transformator, nadprzewodnictwo, temperatura.

Introduction

In certain conditions, with switching the transformer onto the energetic network a high value unidirectional current will flow in the windings [1]. Its first impulse may be 40 times higher than the transformer’s rated current. The following impulses are attenuated by the transformer’s windings resistance and the powering circuit’s resistance [2]. Depending on the transformer’s power, this current’s duration may be equal to from several to more than ten thousand periods of the feeding voltage.

Inrush current, in the transformer’s power circuit, causes a series of disadvantageous phenomena [3]. In superconducting transformers (HTS) the flow of this current may cause loss of the superconductive windings state as a result of exceeding critical superconductor values: density of the current, field intensity, temperature.

Due to the heterogeneous structure of the superconductor, the weakest area in the windings becomes the origin of resistive area. In the place of that area’s occurring, the inrush current flows through the stabilizer generating heat by power losses on its resistance, according to the Jouel’s law. The resistive area propagation onto neighboring regions of the superconductor depends on the heat spread speed in the tape, cooling system efficiency and the inrush current wave’s shape.

The windings overheating threatens with the superconductor’s failure and the transformer’s switching off of working [4][5]. Continuing degradation of a second generation superconductor is observed at the temperature of over 600K. Keeping windings at cryogenic temperature is one of the most complex issues in the HTS transformers exploitation [6][7].

Object of research

The trials were executed on a one phase HTS transformer of 10kVA power. The core was done as wound and cut with metal sheet PN ET52-27 of induction B=1.75 T with H=10 A/cm and loss of P=0.8 W/kg at B=1T and f=50 Hz. The transformer’s primary and secondary voltage windings were made of superconducting tape Super Power SCS4050 (RE)BCO of effective value of critical current of 80 A in temperature of 77K in self field. The tape’s bulk was 0.1 mm, the width 4 mm. The windings were isolated by wrapping the superconducting tape with kapton. The windings geometry was shown in the Figure 1. The transformer’s windings are being cooled by liquid nitrogen to reach the temperature of 77K. The transformer’s core works at room temperature. The rated parameters were given in the Table 1.

Fig. 1. The geometry of superconducting transformer

Table 1. The rating of the transformer

Measurement

Measurements were taken in the system presented in Figure 2. The current’s flow was registered indirectly, by measuring the voltage drop on the shunt 60A, 60mV and exactness class 0.5. The data acquisition was realized with measuring card National Instruments USB-6212, using application written at LabVIEW. Switching the transformer on was done with a thyristor system switching the transformer on at the moment of passing of the power network’s voltage the zero value.

Fig. 3. The inrush current of HTS transformer

Fig. 4. The initial pulses of inrush current

In Figure 3 we presented registered inrush current flows i(t) and voltage _upw(t) measured at the HTS transformer’s primary windings. Since the transformer’s switching on, the voltage changes sinusoidally. The highest measured peak value of the TrHTS transformer’s inrush current is 178 A.

Table 2. The parameters inrush currents

HTS transformers inrush current decay time is longer as compared with transformers with copper windings [8][9]. The examined transformer’s registered unidirectional current decay time was 350 ms.

The sufficient condition of going of the superconducting tape into resistive state is exceeding the value of superconductor’s critical current. Exceeding critical current value of tape SCS 4050 (I_c=80 A) occurs after t₀=4 ms (Fig. 4) as measured since switching the transformer onto the power network. The time tg in which the exceeding occurs is 5 ms. In this period it is expected that the tape loses its superconductive properties and its temperature grows.

The circuit with HTS transformer showed rapid attenuation of unidirectional impulses down to current values lower than the superconductor’s critical current. The first impulse’s amplitude exceeds the critical current (80 A) of the superconducting tape SCS4050 (Fig. 4) by 98 A and the transformer’s rated current (44 A) by 134 A. The second impulse coming after 0.02 s is comparable to the critical current value (80 A) and the peak value of the third impulse (50 A) (after 0.04 s) is by 30 A lower than the tape’s critical current value.

Endurance of the SCS4050 tape for thermal failure depends on distribution of the current’s density in each layer at resistive condition. The construction of the tape are shown in Table 3.

Table 3. Layer structure of tape SCS4050

Based on the resistivity of the individual materials, you can calculate the percentage of the current dispersed into individual layers. Assuming the materials are homogeneous the tape’s structure was pictured by parallel connections between resistances representing each layer (Fig. 5) [10].

Fig. 5. Equivalent circuit of SCS4050 tape

In the superconducting state all the current flows through superconductor. Calculations show, that in resistive state of the SCS4050 tape, at temperature of 77K, 89% of current flows through Cu layer, 9.4% through the Ag layer and 1.5% through the Hastelloy layer. With temperature growth, the resistance of layers materials changes. In addition, the reflow of current changes. At 293K we get 88.7%, 9.5%, 1.5% respectively.

The main conductor at the resistive state is copper. At moment tm (Fig. 4) when the inrush current reaches its maximal value, a current of 158 A flows through the copper layer and that gives the momentary current density of 987 A/mm² (Tab. 4). This value is 318 times higher than the maximal acceptable value of current density for copper in the air (3.1 A/mm²). A higher momentary density is observed for the silver layer. At moment t_m it is 1112 A/mm².

Return of the HTS transformer’s winding to its superconducting state happens when three conditions occur simultaneously: (1) intensity of the outer magnetic field is lower than its critical value; (2) the inrush current maximal value is lower than the critical value; (3) superconductor’s temperature is lower than the critical temperature.

The first condition, because of great critical values of the 2G superconductors’ field intensity, is met for the entire inrush current lasting time. The second condition is met for the time t_s=18 ms (Fig. 4) between consequent impulses of the unidirectional component. By this time the cooling process of the superconductor to cryogenic temperature takes place. If the third condition is met depends on the cooling system efficiency in tome ts. The cooling intensity strongly depends on temperatures difference between liquid nitrogen and the cooled surface. In case of the smallest heat currents and the smallest temperatures differences the heat is transferred due to natural convection. With raise in superconducting tape’s temperature vacuolar boiling of the cryogenic liquid occurs until in the peripheral layer the gas form of nitrogen appears. This worsens conditions for transformer’s windings cooling and lowers isolation durability against breakthrough.

Table 4. The instantaneous current density in layers for time t_m (resistive superconducting state)

The third condition is hard to measure by simply measuring the temperature. The tape’s temperature can be estimated by electrical rates measuring. The medial resistivity of the SCS4050 tape in normal state is interesting, estimated for different temperatures. It can be calculated from the equation:

where: p_i, S_i–resistivity and cross sectional area of the i-th material of the superconducting tape, S – total cross-sectional area of the superconducting tape. Knowing the characteristic p=f(T) the superconducting tape’s temperature can be established by measuring electrical rates.

The medial resistivity of the SCS4050 tape at 77K, calculated from the (2) equation is:

The resistivity at 293K known from the direct measurements is:

That gives the conclusion, that change in SCS4050 tape’s temperature by 216K, results in more than 10 times’ raise in medial resistivity. During lasting of inrush current that kind of raise in resistance have not been noted along the whole of the winding (55 cm). Because the current at time t_g exceeds critical current it can be expected, that a hard to spot, local loss of superconductivity occurs.

The examined HTS transformer went through numerous switching on trials. Despite exceeding the critical value by inrush current and a great exceed in acceptable current density for the copper stabilizer no failure of the SCS 4050 tape have been noted.

Conclusions

The methods in designing conventional transformers, with copper or aluminum windings, do not take into account the inrush current phenomenon. This experiment showed that the phenomenon may cause problems in switching on the superconducting transformer. The single unidirectional current impulse of great momentary density observed during switching on a transformer HTS of 10 kVA power, exceeding critical current, may lead to thermal damage in superconducting windings. The experiment showed, that it is difficult to establish the maximal temperature value and spot its appearing in the windings.

The loss of HTS transformer’s superconducting state (with raise in windings temperature in safe limits) may limit negative impact of inrush current. Raise in circuit resistance due to raise in windings resistance lowers the inrush current’s amplitude and its duration [11][12].

HTS transformers should be designed in the way that they stand certain inrush current for a certain period of time without exceeding the acceptable temperature for the transformer’s superconducting windings. There are no respective norms for the time being.

The research was conducted in scope of the project “Analysis of inrush current phenomenon and the phenomena related in superconducting transformers”. The project was financed with means of National Science Center given with the decision no. DEC- 2012/05/D/ST8/02384

The superconducting transformer was constructed in scope of research project no: N510526439, “Elaborating the model design for 1-phase superconducting transformer with windings made of HTS tape of 2nd generation”.

REFERENCES

[1] E. Jezierski, Transformatory, WNT, Warszawa (1983)
[2] M. Jamali, M. Mirzaie, S. Asghar Gholamian, Calculation and analysis of transformer inrush current based on parameters of transformer and operating condictions, Electronics and Electrical Engineering, Electrical Engineering, no. 3(109), (2011)
[3] R. A. Turner, K. S. Smith, Resonance Excited by Transformer Inrush Current in Inter-connected Offshore Power Systems, IEEE Industry Applications Society Annual Meeting, Edmonton, Canada, October (2008)
[4] G. Wojtasiewicz, T. Janowski, S. Kozak, J. Kozak, M. Majka, B. Kondratowicz-Kucewicz, Tests and Performance Analysis of 2G HTS Transformer, IEEE Transactions on Applied Superconductivity, vol. 23, issue: 3, part: 2, (2012)
[5] G. Wojtasiewicz, T. Janowski, S. Kozak, J. Kozak, M. Majka, B. Kondratowicz-Kucewicz, Experimental Investigation of the Model of Superconducting Transformer With the Windings Made of 2G HTS Tape, IEEE Transactions on Applied Superconductivity, vol. 22, issue: 3, (2012)
[6] S. S. Kalsi, Applications of High Temperature Superconductors to Electric Power Equipment, John Wiley and Sons Ltd, April (2011)
[7] J. K. Sykulski, C. Beduz, R.L. Stoll, M.R. Harris, K.F. Goddard, Y. Yang, Prospects for large high-temperature superconducting power transformers: conclusions from a design study, Electric Power Applications, IEE Proceedings, vol. 146, issue: 1, (1999)
[8] G. Wojtasiewicz, G. Komarzyniec, T. Janowski, S. Kozak, J. Kozak, M. Majka, B. Kondratowicz-Kucewicz, Inrush Current of Superconducting Transformer, IEEE Transaction on Applied Superconductivity, vol. 23, issue: 3, June (2013)
[9] G. Komarzyniec, T. Janowski, G. Wojtasiewicz, M. Majka, J. Kozak, S. Kozak, B. Kondratowicz-Kucewicz, Prąd włączania transformatora nadprzewodnikowego, Przegląd Elektrotechniczny, no. 9, (2013)
[10] M. Sjöström, B. Dutoit, and J. Duron, Equivalent Circuit Model for Superconductors, IEEE Transactions on Applied Superconductivity, vol. 13, no. 2, June (2003)
[11] H. Shimizu, K. Mutsuura, Y. Yokomizu, T. Matsumura, Inrush-Current-Limiting with high Tc superconductor, IEEE Transactions on Applied Superconductivity, vol. 15, no. 2, June (2005)
[12] S. Hun-Chul, K. Chul-Hwan, R. Sang-Bong, K. Jae-Chul, H. Ok-Bae, Superconducting Fault Current Limiter Application for Reduction of the Transformer Inrush Current: A Decision Scheme of the Optimal Insertion Resistance, IEEE Transactions on Applied Superconductivity, vol. 20, no. 4, August (2010)

Authors: dr inż. Grzegorz Komarzyniec, E-mail: g.komarzyniec@pollub.pl, Politechnika Lubelska, Instytut Elektrotechniki i Elektrotechnologii, ul. Nadbystrzycka 38a, 20-618 Lublin,
prof. dr hab. inż. Tadeusz Janowski, E-mail: t.janowski@pollub.pl,
dr inż. Grzegorz Wojtasiewicz, E-mail: g.wojtasiewicz@iel.waw.pl,
dr inż. Michał Majka, E-mail: m.majka@iel.waw.pl, Instytut Elektrotechniki, ul. Pożarskiego 28, 04-703 Warszawa

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 90 NR 2/2014. doi:10.12915/pe.2014.02.04

Principle of lightning Protection

Published by Electrical Installation Wiki, Chapter J. Overvoltage protection – Principle of lightning protection

General rules of lightning protection

Procedure to prevent risks of lightning strike

The system for protecting a building against the effects of lightning must include:

• protection of structures against direct lightning strokes;
• protection of electrical installations against direct and indirect lightning strokes.

The basic principle for protection of an installation against the risk of lightning strikes is to prevent the disturbing energy from reaching sensitive equipment. To achieve this, it is necessary to:

• capture the lightning current and channel it to earth via the most direct path (avoiding the vicinity of sensitive equipment);

• perform equipotential bonding of the installation;
This equipotential bonding is implemented by bonding conductors, supplemented by Surge Protection Devices (SPDs) or spark gaps (e.g., antenna mast spark gap).

• minimize induced and indirect effects by installing SPDs and/or filters.

Two protection systems are used to eliminate or limit overvoltages: they are known as the building protection system (for the outside of buildings) and the electrical installation protection system (for the inside of buildings).

Building protection system

The role of the building protection system is to protect it against direct lightning strokes.

The system consists of:

• the capture device: the lightning protection system;
• down-conductors designed to convey the lightning current to earth;
• “crow’s foot” earth leads connected together;
• links between all metallic frames (equipotential bonding) and the earth leads.

When the lightning current flows in a conductor, if potential differences appear between it and the frames connected to earth that are located in the vicinity, the latter can cause destructive flashovers.

The 3 types of lightning protection system

Three types of building protection are used:

The lightning rod (simple rod or with triggering system)

The lightning rod is a metallic capture tip placed at the top of the building. It is earthed by one or more conductors (often copper strips) (see Fig. J12).

**Fig. J12** – Lightning rod (simple rod or with triggering system)

The lightning rod with taut wires

These wires are stretched above the structure to be protected. They are used to protect special structures: rocket launching areas, military applications and protection of high-voltage overhead lines (see Fig. J13).

The lightning conductor with meshed cage (Faraday cage)

This protection involves placing numerous down conductors/tapes symmetrically all around the building. (see Fig. J14).

This type of lightning protection system is used for highly exposed buildings housing very sensitive installations such as computer rooms.

**Fig. J14** – Meshed cage (Faraday cage)

Consequences of building protection for the electrical installation’s equipment

50% of the lightning current discharged by the building protection system rises back into the earthing networks of the electrical installation (see Fig. J15): the potential rise of the frames very frequently exceeds the insulation withstand capability of the conductors in the various networks (LV, telecommunications, video cable, etc.).

Moreover, the flow of current through the down-conductors generates induced overvoltages in the electrical installation.

As a consequence, the building protection system does not protect the electrical installation: it is therefore compulsory to provide for an electrical installation protection system.

**Fig. J15** – Direct lightning back current

Lightning protection – Electrical installation protection system

The main objective of the electrical installation protection system is to limit overvoltages to values that are acceptable for the equipment.

The electrical installation protection system consists of:

• one or more SPDs depending on the building configuration;
• the equipotential bonding: metallic mesh of exposed conductive parts.

Implementation

The procedure to protect the electrical and electronic systems of a building is as follows.

Search for information

• Identify all sensitive loads and their location in the building.
• Identify the electrical and electronic systems and their respective points of entry into the building.
• Check whether a lightning protection system is present on the building or in the vicinity.
• Become acquainted with the regulations applicable to the building’s location.
• Assess the risk of lightning strike according to the geographic location, type of power supply, lightning strike density, etc.

Solution implementation

• Install bonding conductors on frames by a mesh.
• Install a SPD in the LV incoming switchboard.
• Install an additional SPD in each sub-distribution board located in the vicinity of sensitive equipment (see Fig. J16).

**Fig. J16** – Example of protection of a large-scale electrical installation

The Surge Protection Device (SPD)

Surge Protection Devices (SPD) are used for electric power supply networks^[1], telephone networks, and communication and automatic control buses.

The Surge Protection Device (SPD) is a component of the electrical installation protection system.

This device is connected in parallel on the power supply circuit of the loads that it has to protect (see Fig. J17). It can also be used at all levels of the power supply network.

This is the most commonly used and most efficient type of overvoltage protection.

SPD connected in parallel has a high impedance. Once the transient overvoltage appears in the system, the impedance of the device decreases so surge current is driven through the SPD, bypassing the sensitive equipment.

Principle

SPD is designed to limit transient overvoltages of atmospheric origin and divert current waves to earth, so as to limit the amplitude of this overvoltage to a value that is not hazardous for the electrical installation and electric switchgear and controlgear.

SPD eliminates overvoltages

• in common mode, between phase and neutral or earth;
• in differential mode, between phase and neutral.

In the event of an overvoltage exceeding the operating threshold, the SPD

• conducts the energy to earth, in common mode;
• distributes the energy to the other live conductors, in differential mode.

The three types of SPD

Type 1 SPD

The Type 1 SPD is recommended in the specific case of service-sector and industrial buildings, protected by a lightning protection system or a meshed cage.

It protects electrical installations against direct lightning strokes. It can discharge the back-current from lightning spreading from the earth conductor to the network conductors.

Type 1 SPD is characterized by a 10/350 µs current wave.

Type 2 SPD

The Type 2 SPD is the main protection system for all low voltage electrical installations. Installed in each electrical switchboard, it prevents the spread of overvoltages in the electrical installations and protects the loads.

Type 2 SPD is characterized by an 8/20 µs current wave.

Type 3 SPD

These SPDs have a low discharge capacity. They must therefore mandatorily be installed as a supplement to Type 2 SPD and in the vicinity of sensitive loads.

Type 3 SPD is characterized by a combination of voltage waves (1.2/50 μs) and current waves (8/20 μs).

SPD normative definition

Note 1: There exist [T1] + [T2]SPD (or Type 1 + 2 SPD) combining protection of loads against direct and indirect lightning strokes.
Note 2: some [T2] SPD can also be declared as [T3].

Characteristics of SPD

International standard IEC 61643-11 Edition 1.0 (03/2011) defines the characteristics and tests for SPD connected to low voltage distribution systems (see Fig. J19).

**Fig. J19** – Time/current characteristic of a SPD with varistor

Common characteristics

• U_c: Maximum continuous operating voltage
This is the A.C. or D.C. voltage above which the SPD becomes active. This value is chosen according to the rated voltage and the system earthing arrangement.

• U_p: Voltage protection level (at In)
This is the maximum voltage across the terminals of the SPD when it is active. This voltage is reached when the current flowing in the SPD is equal to In. The voltage protection level chosen must be below the overvoltage withstand capability of the loads. In the event of lightning strokes, the voltage across the terminals of the SPD generally remains less than Up.

• I_n: Nominal discharge current
This is the peak value of a current of 8/20 µs waveform that the SPD is capable of discharging minimum 19 times^[2].

Why is I_n important?
I_n corresponds to a nominal discharge current that a SPD can withstand at least 19 times^[2]: a higher value of I_n means a longer life for the SPD, so it is strongly recommended to chose higher values than the minimum imposed value of 5 kA.

Type 1 SPD

• I_imp: Impulse current
This is the peak value of a current of 10/350 µs waveform that the SPD is capable of discharging of discharging at least one time[3].

Why is I_imp important?
IEC 62305 standard requires a maximum impulse current value of 25 kA per pole for three-phase system. This means that for a 3P+N network the SPD should be able to withstand a total maximum impulse current of 100kA coming from the earth bonding.

• I_fi: Autoextinguish follow current
Applicable only to the spark gap technology. This is the current (50 Hz) that the SPD is capable of interrupting by itself after flashover. This current must always be greater than the prospective short-circuit current at the point of installation.

Type 2 SPD

• I_max: Maximum discharge current
This is the peak value of a current of 8/20 µs waveform that the SPD is capable of discharging once.

Why is I_max important?
If you compare 2 SPDs with the same I_n, but with different I_max: the SPD with higher Imax value has a higher “safety margin” and can withstand higher surge current without being damaged.

Type 3 SPD

• U_oc: Open-circuit voltage applied during class III (Type 3) tests.

Main applications

• Low Voltage SPD

Very different devices, from both a technological and usage viewpoint, are designated by this term. Low voltage SPDs are modular to be easily installed inside LV switchboards.

There are also SPDs adaptable to power sockets, but these devices have a low discharge capacity.

• SPD for communication networks

These devices protect telephone networks, switched networks and automatic control networks (bus) against overvoltages coming from outside (lightning) and those internal to the power supply network (polluting equipment, switchgear operation, etc.).

Such SPDs are also installed in RJ11, RJ45, … connectors or integrated into loads.

Notes

1. find out more in application examples such as SPD for EV charging application, SPD for photovoltaic applications and SPD application example in Supermarket
2. Test sequence according to standard IEC 61643-11 for SPD based on MOV (varistor). A total of 19 impulses at In:
• One positive impulse
• One negative impulse
• 15 impulses synchronised at every 30°on the 50 Hz voltage
• One positive impulse
• One negative impulse
3. for type 1 SPD, after the 15 impulses at In (see previous note):
• One impulse at 0.1 x Imp
• One impulse at 0.25 x Imp
• One impulse at 0.5 x Imp
• One impulse at 0.75 x Imp
• One impulse at Imp

Source URL: https://www.electrical-installation.org/enwiki/Principle_of_lightning_protection

How to Select the Right Current Transformer for Your Application

Published by Accuenergy, November 4, 2021.

Help me choose the Right CT

If you have a power measurement project coming up, chances are you have narrowed down your search for a power meter to a few choices. Whether it’s a multi-circuit application or a high-precision metering in an industrial setting, the next step in project preparation is selecting the right current transformer to maximize the performance of your power meter. When going through the selection process, it can be helpful to answer a few, basic application questions to reach a decision and consider several parameters including current transformer output, conductor size, amperage range, and accuracy. If you need help deciding, reach out to your power meter manufacturer so they can help guide you to the CT that best meets your project’s measurement objectives and budget.

CT Output:

What current transformer output is your power meter compatible with?

Current transformers are available with several output options, some of the most popular of which include 333mV, 5A, or 80mA. A critical question in the current transformer selection process, it is important to note which output your metering equipment is compatible with. While it is possible that the meter may work with multiple output options, it may not be possible to make in-field adjustments to this setting or it may need to be configured by the factory.

Unlike typical split-core or solid-core current transformers, Rogowski coils have a unique output that is generally rated at a low AC voltage (e.g., 150mV or less) per 1000A. In addition, there is an inherent 90-degree phase shift. Many meters and other measurement devices require a higher signal than what a Rogowski can provide on its own and are not configured to compensate for the phase shift, so it is important to work with your meter manufacturer to determine whether this specialized CT is directly compatible with your device.

Conductor Size:

Are you measuring around large busbars/conductors or small branch circuits?

The dimensions of the conductor are a critical consideration and can be one of the leading deciding factors in CT selection. Any CT that is used needs to be able to physically fit around the conductor you plan to measure. At the same time, oversizing a CT to accommodate a small conductor may not make sense in terms of both cost and the space required in the electrical panel, which may not have enough room to accommodate a large, rigid current transformer. In this situation, a flexible Rogowski coil can make it easier to measure in crowded electrical panels or switchgear because they can easily slip around oversized bus bars in tight spaces, making them an ideal compromise between large window size and flexible functionality.

Load Size:

How many amps will you be measuring?

Like the physical dimensions, the size of the load under measurement is a key consideration. All current transformers have a current input range, or amperage range, specification which indicates the size of the load they can effectively measure. If the load fluctuates throughout the day—for example, when occupancy is low during evening hours—it can be helpful to choose a current transformer with a broad current sensing range, such as a flexible Rogowski coil. It is also important to note that, if a load goes outside the sensor’s range, the meter may not be able to measure the load accurately, so it is important to always choose a sensor with a range that matches what you intend to measure.

Accuracy Rating:

Does the project involve billing tenants for their consumption?

When it comes to tenant billing, selecting equipment with the highest accuracy is of the utmost importance. In fact, in any application where “money changes hands,” power monitoring equipment must meet certain accuracy requirements and is often labeled “revenue grade” to indicate its conformance to accuracy standards. What does revenue grade accuracy mean? It is generally understood to be better than 1% accuracy and, more often, in the range of 0.5% accuracy or better. Before selecting a revenue grade sensor, be sure to check which industry accuracy standards they meet to ensure the accuracy class matches your project requirements. A common revenue grade accuracy standard is IEC 60044-1 0.5 Class.

On the other hand, if you’re simply collecting overall consumption trend data for a facility, a 1% accuracy sensor may be sufficient, and you may not need to upgrade to a revenue grade model.

Form Factor:

Will the project be new construction or a retrofit application?

This question may also be framed as, “Will a split-core or solid-core current transformer be a better for my application?” Although either sensor type may be used for any job, it is almost always easier to use a split-core, or a Rogowski coil, current transformer for a retrofit application because it can easily open to fit around a conductor and does not require wire disconnection as part of the installation process. Alternatively, while a facility is still under construction, installing a solid-core CT does not require much additional work since facility shutdowns or wire disconnection are not yet disruptive. Another consideration is cost: Although the up-front price of a solid core CT is lower, the initial savings is negligible when compared to the largely uncalculated installation cost which must include shutdowns and disconnections, adding time and labor to the overall project.

Regulatory Requirements:

Does your application require a sensor that meets UL or other regulatory certifications?

A UL Listed current transformer has undergone rigorous testing to ensure that it complies with nationally recognized safety standards. Unlike a current sensor that is a UL Recognized Component, which means it is intended to be a component within a complete system or product, a UL Listed sensor can be sold as an end-user product and is designed to minimize installation hazards such as shock or fire. It may be that your application mandates a UL Listed current sensor to meet safety code requirements. If this is the case, be sure to look for CTs with a UL Listed marking that indicates they comply with XOBA UL2808 and CSA C22.2 61010-1.

Another key regulatory requirement is a CE mark. This mark is required for products used in the European Economic Area (EEA) which includes countries like Germany, France, Spain, Italy, and others. Unlike other quality marks, such as UL, the CE mark on a product means that it conforms with European safety, health, and environmental standards. The CE mark should be visible on product labeling and documentation.

A third regulatory requirement you may encounter concerns Measurement Canada approval. Tenant billing applications in Canada may require both a Measurement Canada approved meter and current transformers, each of which must meet rating, design, accuracy, testing, and other requirements. For example, a few characteristics of Measurement Canada approved CTs include that they must be solid core, meet an accuracy class of 0.6% or better, and be either 5A, 80mA, or 100mA output devices. The nature, scope, and location of your project will dictate whether Measurement Canada approval is required. Check the product labeling and documentation to determine whether a sensor meets the regulatory requirements.

Using Rogowski coils:

My power meter does not work directly with Rogowski coils. Is there a way that I can still use a rope CT?

Nearly any project can benefit from Rogowski coil current transformers which offer many advantages including a large window size, broad amperage range, lightweight flexibility, and no saturation point. However, if your power meter only accepts 333mV, 5A, 1A, or another standard output, it will not directly work with a Rogowski coil. Fortunately, there is a simple solution to this challenge and that is to use an integrator. An integrator is an electronic device that makes it possible to change the output of a Rogowski coil to a commonly accepted output, such as 333mV or 5A, so that it can work with a host a power meters, protection relays, or other devices. By adjusting the input ranges to match nearly any system, an integrator is a simple solution to solve a common compatibility dilemma and bridges the divide between Rogowski coils and industrial metering equipment.

Source URL: https://www.accuenergy.com/articles/current-transformers/how-to-select-the-right-current-transformer/

Electrical Shock and its Effects

Published by Alex Roderick, EE Power – Technical Articles: Electrical Shock and its Effects, August 17, 2021.

Anyone working on electrical equipment should have respect for all voltages, have knowledge of the principles of electricity, and follow safe work procedures.

An electrical shock occurs when a body becomes a part of an electrical circuit. The effects of an electrical shock vary from a moderate sensation to paralysis to death. Also, severe burns may occur internally and where the current enters and exits the body. The quantity of electric current flowing through the body in milliamps (mA), the amount of time the body is exposed to the electric current, the route the current takes through the body, and the physical condition of the body through which the current flows; all influence the severity of an electrical shock.

Table 1. Electrical shock results in any time a body becomes part of an electrical circuit.

* in mA
**effects vary depending on time, path, amount of exposure, and condition of the body

Prevention is the best medicine for electrical shock. Anyone working on electrical equipment should have respect for all voltages, have knowledge of the principles of electricity, and follow safe work procedures. All technicians should be encouraged to take a basic course in cardiopulmonary resuscitation (CPR), so they can aid a coworker in emergency situations.

A person’s body becomes a part of an electrical circuit during an electrical shock. The body of a person offers varied resistance to the flow of current. Sweaty hands have less resistance than dry hands. The resistance of a wet floor is less than that of a dry floor. The lower the resistance, the higher the current flow. As the current flow increases, the severity of the electrical shock increases.

If a person is receiving an electrical shock, power should be removed as quickly as possible. If power cannot be removed quickly, the victim must be removed from contact with live parts. Action must be taken quickly and cautiously. Delay may be fatal. Individuals must also avoid being a casualty while attempting to rescue another person. If the equipment circuit disconnect switch is nearby and can be operated safely, shut OFF the power. Excessive time should not be spent searching for the circuit disconnect. In order to remove the energized part, insulated protective equipment such as a hot stick, rubber gloves, blankets, wood poles, plastic pipes, etc., can be used if such items are accessible.

After the victim is freed from the electrical hazard, help should be called, and first aid (CPR, etc.) begun as needed. The injured individual should not be transported unless there is no other option and the injuries require immediate professional attention.

Grounding

Grounding is the connection of portions of the distribution system to earth in order to establish a common electrical reference and a low impedance fault path to facilitate the operation of overcurrent protective devices. Grounding provides an electrically conductive path designed and intended to carry current under fault conditions from the point of a fault on a wiring system to the electrical supply source. Grounding facilitates the operation of overcurrent protection devices.

For systems that are solidly grounded, grounding provides a means to limit the voltage to the ground during normal operation and to prevent excessive voltages due to lightning, line surges, or unintentional contact with higher-voltage lines and to stabilize the voltage to the ground during normal operation.

The non-current-carrying metal parts of a transformer installation are required by the NEC^® to be effectively grounded. Conductive materials enclosing conductors or equipment are grounded to prevent a voltage or difference of potential on these materials. Circuits and enclosures are grounded to allow overcurrent devices to operate in the event of insulation damage or ground faults.

The electrical distribution system is grounded by connecting it to a metal underground water pipe, a building’s metal frame, a concrete-encased electrode, or a ground ring. To prevent problems, a grounding path must be as short as possible and of sufficient ampacity, never be fused or switched, be a permanent part of the electrical circuit, and be continuous and uninterrupted from the electrical circuit to the ground.

The ground is provided at the main service equipment or at the source of a separately derived system (SDS). A separately derived system (SDS) is a system that supplies electrical power derived or taken from transformers, storage batteries, solar photovoltaic systems, or generators. See Figure 2. The majority of separately derived systems are produced by the secondary of a distribution transformer.

Figure 1. A separately derived system (SDS) is a system that supplies electrical power derived or taken from transformers, storage batteries, solar photovoltaic systems, or generators. Image Courtesy of Sask Power

The neutral ground connection must be made at the transformer or at the main service panel only. The neutral ground connection is made by connecting the neutral bus to the ground bus with a main bonding jumper. The main bonding jumper (MBJ) is a connection at the service equipment that connects the equipment grounding conductor, the grounding electrode conductor, and the grounded conductor (neutral conductor). The purpose of the main bonding jumper is to bond the neutral and equipment grounding conductor together with the enclosure to create a common reference potential.

An equipment grounding conductor (EGC) is an electrical conductor that provides a low-impedance ground path between electrical equipment enclosures within the distribution system and takes current back to the source. A grounding electrode conductor (GEC) is a conductor that connects grounded parts of a power distribution system (equipment grounding conductors, grounded conductors, and all metal parts) to the grounding system.

A grounded conductor is one that has been intentionally grounded. The grounded conductor is commonly a neutral conductor. However, not all electrical distribution systems use the grounded conductor as a neutral. For example, corner-grounded delta systems contain a grounded conductor that is not a neutral conductor. Therefore, it is not correct to refer to all grounded conductors as neutral conductors, although that is the case in the majority of electrical distribution systems.

Ground Fault Circuit Interrupters

A ground fault circuit interrupter (GFCI) detects an imbalance of current in the normal conductor routes and opens the circuit to safeguard against electrical shock. A GFCI opens the circuit when the current in two conductors of an electrical circuit differ by more than 5 mA. A GFCI is designed to trip quick enough (1/40 of a second) to avoid electrocution (1/4 of a second).

A potentially dangerous ground fault is any quantity of current above the level that might cause a harmful shock. Any current more than 8 mA is regarded as potentially harmful — depending on the path the current follows, the physical state of an individual receiving the shock, and the length of time the individual is exposed to the shock. GFCIs are therefore necessary for places like homes, hotels, resorts, industrial sites, receptacles around swimming pools, and other places where a person may encounter a ground fault.

A GFCI compares the current flowing through the ungrounded (hot) conductor with the current flowing through the neutral conductor. A ground fault occurs if the current in the neutral conductor falls below the current in the hot conductor. The missing current is returned to the source via some path other than the intended one (fault current).

GFCI protection can be installed at various points throughout a circuit. Ground fault protection is provided at the point of installation with direct-wired GFCI receptacles. GFCI receptacles can also be used to protect all other receptacles installed downstream along the same circuit. When implemented in a load center or panel board, GFCI circuit breakers offer GFCI protection as well as conventional circuit overcurrent protection for all branch-circuit elements connected to the circuit breaker.

Plug-in GFCIs

Plug-in GFCIs protect against ground faults for devices that are plugged into them. These plug-in devices are frequently used by personnel working with power tools in areas without GFCI receptacles.

Portable GFCIs are designed to be easily moved from one location to another (see Figure 2). Portable GFCIs commonly contain more than one receptacle outlet protected by an electronic circuit module. Portable GFCIs should be inspected and tested before each use. GFCIs have a built-in test circuit to ensure that the ground fault protection is operational.

Figure 2. A portable GFCI can be used on a job site to protect workers. *Image Courtesy of Cable Organizer*

A ground fault circuit interrupter (GFCI) safeguards against the most common type of electrical shock hazard, the ground fault. Line-to-line contact hazards, such as a technician holding two hot wires in each hand, are not protected by GFCIs. GFCI protection is mandatory in addition to NFPA grounding requirements.

Author: Alex earned a master’s degree in electrical engineering with major emphasis in Power Systems from California State University, Sacramento, USA, with distinction. He is a seasoned Power Systems expert specializing in system protection, wide-area monitoring, and system stability. Currently, he is working as a Senior Electrical Engineer at a leading power transmission company.

Source URL: https://eepower.com/technical-articles/electrical-shock-and-its-effects/

Method of Selecting the Most Important Power Lines in a Transmission and Distribution Network

Published by Dalibor VALEK, Radomir SCUREK,
Technical University of Ostrava, Faculty of Safety Engineering

Abstract: This article describes the method used to graph theory in security analysis. For the pursposes of this study, an environment is referred to here as a network of power lines and devices and a power grid is considered as a system of vertices which combine to make a network. Exploration of Fiedler´s theory will be applied herein to select the most important power lines for the entire network. Components related to these lines are logicly ordered and considered by the author´s modified analysis. This method has been improved and optimalized for risks related with illegal acts. Each power grid component has been connected with a variety of possible attacks and this device was gradually evaluated by five coefficients which takes values from 1 to 10. the level of risk was also assessed on the coefficient basis. In the last phase the most risky electricity network devices have been selected. Security measures have been proposed on the selected devices.

Streszczenie. W artykule opisano wykorzystanie teori grafów w analizie zabezpieczeń. Opracowano model sieciowy z wykorzystaniem teorii Fiedlera. Każdy z elementów sieci połaczono z różnymi formami ataku i opisano odpowiednimi wspolczynnikami. Na tej podstawie wybrano najbardziej ryzykowny wariant. Metoda wyboru najważniejszych linii w sieciach rozdzielczych i przesyłowych.

Key words: Security, matrix, power grid, graph partitioning, analysis.
Słowa kluczowe: bezpiecześtwo, sieci rozdzielcze i przesyłowe, teoria grafów

Introduction

Significant diversification of technical infrastructure from telegraph to internet has been realized within the last fifty years. From a wide point of view, the electric network, traffic network and communications network makeup the foundation of all prosperous companies. Usually these networks are based on a large amount of heterogeneous components characterized by complex dependencies and relationships between them. To ensure the structural integrity, effectivity and overall reliability of the network, illegal acts, security and terrorist protection must be considered.

There has been extreme growth in the consumption of electricity within last two decades. It was caused mainly by the social-economic changes in eastern Europe. Directly proportional to energy consumption has been the rise in newly-built and expanded nuclear power plants and power plants producing electricity from renewable sources.

After the disaster at Fukushima nuclear power plant, discussions started in Europe about the safety of nuclear power plants and the consequences of radioactive substance leaks. In the event of a radioactive leak, for instance, in Europe; there would be reduced production of electricity from nuclear power plants and significant reduction in annual electric energy production. There would, as well, be a very serious impact to energy security. This shortfall would be covered mainly by electricity produced from renewable energy sources and fossil fuel plants. If there would be a massive production of electricity from renewable energy sources, there would be many more opportunities to one-off the impacts to the transmission system.

These hard predictable impacts are caused mainly by renewable sources, mainly caused by wind power plants. Wind power plants work on the opposite principle than conventional power plants because they supply electricity only in case of wind blowing.

During these large fluctuations of electricity transfer, the important thing is to have the robust electricity system, such as electrical wiring, transformers or power plants, be resistant to component damage and to ensure these components are secured against the attacks of different crime or terrorist organizations. These attacks could cause blackout which could mean very severe consequences to human life and country‘s economy. Therefore it will be increasingly important to deal with security measures on devices across our electricity system. With selection of the most important components helps the graph theory which is described in this article.

Power grid

All devices that provide electricity from the production to the final customer are components where failure can result in a threat of the electricity supply to the final consumer. Among the main devices in electricity system is the electrical grid, electrical stations and electrical wiring.

Devices which are considered in the analysis are parts of the electrical grid and the power plants. The Electrical grid is the system of interconnected devices which is used for electricity transmission, transformation, and distribution. Further devices are used for metering, controlling while other devices are used for passive or active protection.

The transmission system is a system of devices which is used to transmit electrical energy with a voltage of 400 kV and 220 kV (depends on the country) from the manufacturers to the power nodes. The Distribution system transmits electrical energy with a voltage of 110 kV and 22 kV (depends on the country) from the transmission system to the customers. Customers in this context are the cities, the factories and the households.

Voltage is transmitted by the different types of wiring. Usage of a specific wiring type which depends on many factors such as quantity of the transmitted voltage, quantity of the transmitted electric current, voltage drop and so on. Wiring is most often outdoors but can also be cabled on the ground or under the ground, the latter option being the more expensive solution. Outdoor wiring has to be resistant to weather changes, extreme weather conditions, humidity and it must have sufficient mechanical solidity to the intention damage. Cable wiring is used in residential areas, industrial areas and in commercial buildings.

Outdoor wiring is carried by electrical pylons. Construction of electrical pylons can be made of wood, reinforced concrete, steel or aluminum alloy. Many types of electrical pylons exist and the difference is mainly it their design or construction.

Electrical pylons are designed from the construction point of view to resist extreme weather conditions and wind power. Instead of main cantilevered pylons, the grid consists of reinforced pylons to ensure stability in case wires break. Without this reinforcement, the main cantilevered pylons would not stay in the right position. These reinforcement pylons are made of special steel which can resists the climatic exposure [1].

Electrical stations [1] belong to electrical grid and are divided into transformations, switch stations and substations. Most of these devices are created by the substations which can be a single building or a bounded area. These substations take care of the input and output electricity flow and they are consisted of conductors, insulators and switch, safety or control devices [4]. In the buildings are mainly situated substations with voltage up to 35 kV and in the outdoor areas are situated substations with very high voltage over 52 kV.

Utilization of graph theory

Electrical wiring, telecommunication, transport infrastructure or others engineering network form system of nodes which make together graphs of different shapes with many degrees of complexity. Many networks are designed on the basis of landscape relief.

Graph characteristics

By the graphs we can represent a set of objects which we illustrate the interdependence of the various elements. Objects are assigned as vertices (power plants, transformers etc.) and their connections are called edges (transmission network, distribution network). Graph can be basically represented by simple model of a real network which emphasizes topological properties of objects and neglects their geometric properties. Graphs can be divided as a directed and undirected. Undirected graph is defined as G = (V,E) where V indicates vertices and E indicates edges. In case of undirected graph we do not consider the order of vertices which is used in case electrical network analysis [5].

Graph theory

Given a connected graph G it is possible possible to divide it into two smaller graphs having roughly the same number of edges and vertices and one shared edge. There are many methods to achieve this. One is the Spectral graph Partitioning. This method involves the usage of Laplacian matrix and divide vertices of a connected graph G into two subgraphs by usage of Laplacian matrix eigenvectors [3].

Adjacency matrix

Graph can be represented by the adjacency matrix. It is defined as G, A(G) = (a_i,_j)

This means that the adjacency matrix A represented by the graph G has for i-th row and j-th column value equal to 1 in case there is an edge between node i and j. Otherwise there is assigned value 0 for this position.

Degree matrix

Other matrix defined by graph G is degree matrix D(G) = (d_i,_j)

Degree matrix is a diagonal matrix, which provides us information about degrees of each vertex – number of edges entering or exiting from concrete vertex. This matrix is used together with the adjacency matrix to create a Laplacian matrix.

Laplacian matrix

Laplacian matrix is another way how to represent graph.

Matrix L(G) defined by graph G: L(G) = D(G) – A(G)

Laplacian matrix is difference between the degree matrix and adjacency matrix.

Spectral graph partitioning

Spectral graph partitioning is based on simple principle. For the graph G defined by vertices and edges G = (V,E) which has to be dividend is calculated Laplacian matrix L(G). By the spectral partitioning of Laplacian matrix are calculated eigenvalues and eigenvectors. Such an eigenvector which is related with the second smallest eigenvalue provides us required graph partitioning. This vector is named as a Fiedler´s vector.

Algorithm for finding the most important electrical wirings

Input parameter of the algorithm to calculate the most important electrical wirings is adjacency matrix. Values of single rows and columns of the adjacency matrix are the inputs given by user according to the power lines map. These values are loaded into Excel and then exported to computer program.

Computer program then calculate from adjacency matrix the degree matrix and Laplacian matrix followed by spectral graph partitioning and finding the required Fiedler´s vector. Distribution of final graph is determined by the sign related to Fiedler´s vector. Vertices with positive numbers are assigned to first part of graph and vertices with negative numbers are assigned to second part of graph. As a most important power line is then selected line which connects both parts of graph.

Example of spectral graph partitioning

To illustrate the above theory, a very simple example of spectral graph partitioning is shown.

Given a simple network of ten vertices and eleven edges. To the single vertices are assigned coordinates to display them in program according to its potential deployment in the territory.

According to picture 1 is designed adjacency matrix where single rows and columns are valued on the basis of existence the edges between single vertices.

From adjacency matrix, we can get according to defined equation the Laplacian matrix and Fiedler´s vector related to the second smallest eigenvalue.

Table 1. Adjacency matrix

Table 2. Laplacian matrix and related Fiedler´s vector

Fiedler´s vector divides the network into two approximately equal parts. Positive values of vertices are part of the first portion and negative values of vertices are part of the second portion. Red marked edges displayed on the picture number 2 represent the smallest amount of edges between both network parts. If we bring this situation into the electrical grid environment so we talk about electrical lines in which its failure mean blackout on the largest possible area.

Fig. 2 Selection of the smallest number of edges

Graph theory and its spectral graph partitioning is able to select the most important power lines. Then we can use conventional risk analysis only on the selected components of transmission and distribution network.

Analysis of attack on the electricity system

For consideration of vulnerability of single objects in the electricity system can be used modified method FMEA. This method was modified by the author to be more suitable for consideration of illegal acts. Further in the article, represented analysis named by shortcut FMEAIA (Failure Model and Effect Analysis of Illegal Acts) will be explored.

Classic FMEA method [2] is most commonly used in automotive industry to search and evaluation of potential defects in processes and products. This analysis stands on the subjective evaluation of its author. It means that analysis is highly dependent on the author’s experiences. Level of risk is determined by the multiplying of subjectively evaluated coefficients which are Occurrence, “O“, Severity, “S“, Detection, “D” and in additional in FMEAIA analysis is Appeal, “A” and Accessibility, “AA“.

Level of risk is calculated according to formula:

R = (O · S · D · A) / AA

Values of each coefficient may vary from 1 to 10 according to table 3.

In the evaluation process of the electricity system we should start with the selection of the most important places given from Spectral graph partitioning method described above. Selected devices placed on calculated territory should be considered in the analysis according to above described method of FMEAIA.

Example of analysis procedure according to FMEAIA method can be used as follows:

1. Distribution of system into the devices in terms of production and in term of the electricity wiring.

2. Distribution of devices into the buildings, pylons, wiring and further devices which are fixed with the ground (for example wind power plants, photovoltaic power plant etc.).

3. Distribution into the further smaller technical devices. Example of evaluation of several selected devices is shown in the table 4.

There are many types of attacks which could be considered but for purpose of this this article were used only few attacks to better illustration of FMEAIA method. On the basis of above analysis was found out that the highest value of risk has following attacks:

1. Incision of reinforced and steel pylon in transmission system.

2. Placing the bomb on the reinforced and steel pylon in terms of transmission system.

3. Pylon fusion by the welding machine.

Proposal of security measures

The procedure of risk evaluation is followed by the phase of security measures proposed on the highest risk evaluated devices. Security measures should be effective, economical and easily feasible. In general the principle ALARA should be considered in this case because this principle takes into account value of protected object and value of devices which protect this object.

For the minimalization of risk represented by the incision and fusion of reinforced and steel pylon in transmission system it is possible to apply measures which physically prevent contact with single pylon or make activity of attacker more uncomfortable and time prolonged. Among the suitable security measures we can classify:

• Usage of hardened steel for lower part of pylon construction.
• Concreting of pylon foundations up to height 2 meters above the ground level.
• The definition of perimeter around the pylon by the fence or barbed wire.

Proposed security measures are suitable to be realized mainly in terms of reinforced pylons which are included in grid of cantilevered pylons. Distance between cantilevered pylons of Donau type can be in suitable terrain 500 meters. For the type of attack with bomb usage can be applied similar security measures like in case of the incision and fusion the pylon but in case of bomb attack it depends on the level of energy which will be released within the explosion.

Table 3. Classification of each coefficient

Table 4.

Conclusions

This article describes components which are included in the electricity system and components used for the transmission and distribution of electric energy which is connected to the network. This network can be understood as vertices connected together with edges. In this case was used the graph spectral partitioning method to select the most important power lines. After this selection we are able to consider components included in calculated territory with conventional risk analysis. Applied risk analysis was modified by author from FMEA analysis to have more suitable analysis for the illegal acts. Two new coefficients were applied to this modified analysis – appeal and accessibility. Due to this modification this FMEAIA analysis is more suitable for illegal acts consideration. The result from the analysis is that the most risky devices in terms of illegal acts are mainly devices for transmission of electric energy which have much better accessibility then the devices for electricity production. Due to this conclusion, it is necessary to select the most important devices in specific terrain. Security measures to reduce the risk of attack to an acceptable level must be applied to these devices.

REFERENCES

[1] Internet encyclopedia of energy. Elektrizační soustavy [online].[cit. 2014-03-20]. Available from WWW
[2] Internet source Process Quality Management. FMEA – Failure Mode and Effect Analysis [online]. [cit.2014-03-20]. Available from WWW <URL: http://www.pqm.cz/NVCSS/fmeacs.html>
[3] SLININGER, B.: Fiedler´s Theory of Spectral Graph Partitioning, University of California, March 2013. Available from pdf format on WWW <URL:http://www.cs.ucdavis.edu/~bai/ECS231/returnsfinal/Slininger.pdf>
[5] OCHODKOVÁ, E.: Graph algorithms, Technical university of Ostrava, faculty of electrotechnics and informatics, Ostrava,

Authors: Dalibor VALEK, Radomir SCUREK , Technical university of Ostrava, Faculty of safety engineering, Lumírova 13/630, Ostrava – Výškovice, 70030 , dalikk@email.cz, radomir.scurek@vsb.cz

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 90 NR 11/2014. doi:10.12915/pe.2014.11.41

Overvoltage of Atmospheric Origin

Published by Electrical Installation Wiki, Chapter J. Overvoltage protection – Overvoltage of atmospheric origin

Overvoltage definitions

Overvoltage (in a system) any voltage between one phase conductor and earth or between phase conductors having a peak value exceeding the corresponding peak of the highest voltage for equipment.

Definition from the International Electrotechnical Vocabulary (IEV 604-03-09); available on [1]

Various types of overvoltage

An overvoltage is a voltage pulse or wave which is superimposed on the rated voltage of the network (see Fig. J1).

This type of overvoltage is characterized by (see Fig. J2):

• the rise time tf (in μs);
• the gradient S (in kV/μs).

An overvoltage disturbs equipment and produces electromagnetic radiation. Moreover, the duration of the overvoltage (T) causes an energy peak in the electric circuits which could destroy equipment.

**Fig. J2** – Main characteristics of an overvoltage

Four types of overvoltage can disturb electrical installations and loads:

• Switching surges: high-frequency overvoltages or burst disturbance (see Fig. J1) caused by a change in the steady state in an electrical network (during operation of switchgear).

• Power-frequency overvoltages: overvoltages of the same frequency as the network (50, 60 or 400 Hz) caused by a permanent change of state in the network (following a fault: insulation fault, breakdown of neutral conductor, etc.).

• Overvoltages caused by electrostatic discharge: very short overvoltages (a few nanoseconds) of very high frequency caused by the discharge of accumulated electric charges (for example, a person walking on a carpet with insulating soles is electrically charged with a voltage of several kilovolts).

• Overvoltages of atmospheric origin.

Overvoltage characteristics of atmospheric origin

Lightning strokes in a few figures: Lightning flashes produce an extremely large quantity of pulsed electrical energy (see Figure J4)

• of several thousand amperes (and several thousand volts),
• of high frequency (approximately 1 megahertz),
• of short duration (from a microsecond to a millisecond).

Between 2000 and 5000 storms are constantly undergoing formation throughout the world. These storms are accompanied by lightning strokes which represent a serious hazard for persons and equipment. Lightning flashes hit the ground at an average of 30 to 100 strokes per second, i.e. 3 billion lightning strokes each year.

The table in Figure J3 shows some lightning strike values with their related probability. As can be seen, 50% of lightning strokes have a current exceeding 35 kA and 5% a current exceeding 100 kA. The energy conveyed by the lightning stroke is therefore very high.

**Fig. J3** – Examples of lightning discharge values given by the IEC 62305-1 standard (2010 – Table A.3)

**Fig. J4** – Example of lightning current

Lightning also causes a large number of fires, mostly in agricultural areas (destroying houses or making them unfit for use). High-rise buildings are especially prone to lightning strokes.

Effects on electrical installations

Lightning damages electrical and electronic systems in particular: transformers, electricity meters and electrical appliances on both residential and industrial premises.

The cost of repairing the damage caused by lightning is very high. But it is very hard to assess the consequences of:

• disturbances caused to computers and telecommunication networks;
• faults generated in the running of programmable logic controller programs and control systems.

Moreover, the cost of operating losses may be far higher than the value of the equipment destroyed.

Lightning stroke impacts

Lightning is a high-frequency electrical phenomenon which causes overvoltages on all conductive items, especially on electrical cabling and equipment.

Lightning strokes can affect the electrical (and/or electronic) systems of a building in two ways:

• by direct impact of the lightning stroke on the building (see Fig. J5 a);
• by indirect impact of the lightning stroke on the building:

– A lightning stroke can fall on an overhead electric power line supplying a building (see Fig. J5 b). The overcurrent and overvoltage can spread several kilometres from the point of impact.

– A lightning stroke can fall near an electric power line (see Fig. J5 c). It is the electromagnetic radiation of the lightning current that produces a high current and an overvoltage on the electric power supply network. In the latter two cases, the hazardous currents and voltages are transmitted by the power supply network.

– A lightning stroke can fall near a building (see Fig. J5 d). The earth potential around the point of impact rises dangerously.

**Fig. J5** – Various types of lightning impact

In all cases, the consequences for electrical installations and loads can be dramatic.

**Fig. J6** – Consequence of a lightning stroke impact

The various modes of propagation

Common mode

Common-mode overvoltages appear between live conductors and earth: phase-to-earth or neutral-to-earth (see Fig. J7 ). They are dangerous especially for appliances whose frame is connected to earth due to risks of dielectric breakdown.

Differential mode

Differential-mode overvoltages appear between live conductors:

phase-to-phase or phase-to-neutral (see Fig. J8). They are especially dangerous for electronic equipment, sensitive hardware such as computer systems, etc.

Characterization of the lightning wave

Analysis of the phenomena allows definition of the types of lightning current and voltage waves.

• 2 types of current wave are considered by the IEC standards:
– 10/350 µs wave: to characterize the current waves from a direct lightning stroke (see Fig. J9);

• 8/20 µs wave: to characterize the current waves from an indirect lightning stroke (see Fig. J10).

These two types of lightning current wave are used to define tests on SPDs (IEC standard 61643-11) and equipment immunity to lightning currents.

The peak value of the current wave characterizes the intensity of the lightning stroke.
• The overvoltages created by lightning strokes are characterized by a 1.2/50 µs voltage wave (see Fig. J11).

This type of voltage wave is used to verify equipment’s withstand to overvoltages of atmospheric origin (impulse voltage as per IEC 61000-4-5).

Source URL: https://www.electrical-installation.org/enwiki/Overvoltage_of_atmospheric_origin

The Basics of Grounding Electrical Systems

Published by Lorenzo Mari, EE Power – Technical Articles: The Basics of Grounding Electrical Systems, April 10, 2020.

This article breaks down the complexities found in the fundamental field of grounding for the correct, faultless operation of electrical systems.

Grounding, or earthing, is a fundamental topic for the correct operation of electrical systems and devices. However, few people understand this matter or the reason it is used.

Grounding is a huge topic full of standards, practical rules, misconceptions, surprises, and some magic. The rules for grounding are quite difficult, and at times appear unclear.

This introductory article discusses the basic principles of grounding, provides an overview of the main grounding applications, and lays the basis for examining these applications from first to last.

What is Grounding?

In analyses of electrical installations, you will frequently see the terms ground, grounded and grounding. There are several formal definitions of these terms in different standards and codes. However, as its name suggests, grounding is a connection of the electrical system, electrical devices, and metal enclosures to the ground. It is also known as earthing, i.e., connection to the earth.

Even though non-grounded electrical systems do exist — either because they are excepted from grounding by codes or by operational reasons — most arrays are grounded in one way or another.

Is the Ground a Conductor of Electricity?

Although not the best, yes, the ground is an electrical conductor. It is used to carry fault currents, signals, and radio waves.

Groundwave propagation is particularly important on the low- and medium-frequency portion of the radio spectrum. There are underground low-frequency radio antennas that were developed during the early days of the 20th century. This electrical property becomes visible when lightning travels to and from the earth.

Ground connection. *Image courtesy of Pixabay.*

It is also important to know that sometimes the earth, as a conductor, is assumed to have a potential of zero and is taken as reference in many voltage measurements.

Power system grounding is very important since most faults involve ground. Then, it has a basic role in the protection of its components as well as safety for the operator. There are a variety of grounding techniques utilized for mooring an electrical system to the ground. Let’s look at each type next.

System Grounding

System grounding refers to the limit of the defined values the voltage has to the ground in every part of the electrical system. It connects the current-carrying point of the electrical system to the ground, i.e., the neutral of transformers and rotating equipment as well as lines.

Neutral Grounding

The art and science of neutral grounding are of paramount importance in this analysis. A choice of methods for grounding the neutral in transformers and rotating equipment has emerged to control the fault rate and transient disturbances, improving the continuity of service. The main types of neutral grounding are:

• Ungrounded: Grounding is not done on purpose, but the system is grounded because of its natural capacitance to the ground
• Through impedance
* Resistance — high-resistance, low resistance
* Reactance — high-reactance, resonant (high-reactance, as well), low reactance
• Solid (effective)

Most neutral grounding is solid. In this method the neutral is kept at ground potential, with the following advantages:

• Limits the voltage that will be applied to the equipment insulation. Recall that the materials used in the insulation have to be able to withstand the applied voltage;
• Limits system voltage to ground or equipment enclosures, under normal and fault conditions, increasing personnel safety;
• Minimizes potential transient overvoltages;
• Provides for a source of ground-fault current relaying, allowing fast fault clearing.

Other Grounding Methods

Other grounding methods are sometimes employed in systems 600V and below.

• Line grounding
* Zig-zag grounding transformer
* Corner-of-the-delta
• Mid-phase grounding

Equipment and Safety Grounding

People must be safeguarded because a small quantity of current circulating through the body may cause big damage or death.

Equipment grounding connects all non-current-carrying metal parts of the wiring system or apparatus to the ground. Examples include the cabinet of the service equipment, the frames of transformers and motors, the metal conduit and boxes, the metal shield of shielded cables, poles, towers, and more.

Equipment grounding limits voltage between non-current-carrying parts and between these parts and earth to a safe value, boosting protection. It also enables fast fault clearing.

Furthermore, to protect people and animals in the vicinity, power plants and substations are built on grounding mats. This practice minimizes electric shock potentials.

Equipment grounding. *Image courtesy of Pixabay.*

Bonding Equipment for Meeting Safety Standards

Bonding consists of the interconnection of all non-current-carrying metal parts of the installation to assure electrical continuity and conductivity. In this way, metal pieces are at a common and minimum potential above the ground. Codes require bonding in grounded and ungrounded arrays.

This interconnection behaves as a low impedance path that conducts ground-fault current safely and helps the swift operation of overcurrent protective devices on a grounded system as well as the operation of ground fault detectors on high-impedance grounded and on ungrounded systems.

Codes also deal with the bonding of metal building parts (non-electrical) that may be energized accidentally.

Protecting Against Static Electricity With Static Grounding

The purpose of controlling static charges is to protect people and property.

The friction between two surfaces of insulating materials can cause electrons to be transferred from one surface to the other, creating a potential difference of thousands of volts. This potential difference can cause static sparks, which are a source of fires and explosions.

Electronic components and equipment are incapable of withstanding the instantaneous power produced by static. There are several methods of safeguarding from the hazards of static electricity, grounding and bonding are two of them.

Static grounding provides a low-resistance ground connection, mitigating the static electricity generation. This practice prevents sparking between the bodies.

Hazardous locations are particularly important to grounding because they may have flammable or ignitable materials and sparks caused by static could ignite the atmosphere.

Electrostatic induction may also be the origin of transient conditions that triggers unintentional events in adjacent circuits, producing false relay operations, tripping of circuit breakers, or false signals in control circuits, to name a few.

Lightning Protection Grounding

Lightning protection plays a key role in the design and operation of electric power systems. In areas with frequent storms, lightning is the most common cause of outages and damage.

A lightning protection system intercepts or diverts lightning and provides a certain path for conducting the surges safely to the ground by adequate down conductors to grounding electrodes. Thus, it helps prevent disastrous events like fires, injuries, and deaths.

Lightning protection plays a key role in the design and operation of electric power systems. *Image courtesy of Pixabay.*

In addition to electric power systems, tall structures like smokestacks, tanks, towers, and buildings may require lightning protection systems, although not all objects or structures at a given site will need them. Again, hazardous locations are important because lightning produces sparks and the risk of fires and explosions is high.

Keep in mind that it is impossible to protect 100% of a structure from direct impacts, except by completely encapsulating it with metal.

Regarding transmission systems, a well-devised ground wire system can substantially reduce the outage rate because it will shield the phase conductors by receiving the direct impact of lightning strikes.

Protecting Against Lightning-Induced Overvoltages

Transient overvoltages are daily events in electric power systems. Switching is their main initiator, but switching surges are relatively easy to handle. However, lightning surges are the most severe and difficult to manage. They may increase the system voltage to many times the rated voltage. If the equipment in the power system is not protected against lightning surges, considerable damage will occur.

Over-running ground wires, besides shielding against direct lightning strokes, lessens the effects of induced surges.

Likewise, surge arresters are connected in shunt across the pieces of electrical equipment to divert transients to the ground.

Grounding Techniques for Protecting Electronic Equipment

Computers, communication systems, instrumentation, and control equipment require proper grounding for correct operation. More often than not, the safety grounding of equipment is the same for electronic equipment as it is for any other kind of apparatus.

Control room. *Image courtesy of Unsplash.*

Occasionally special grounding techniques, different from conventional safe grounding practices, are applied to electronic equipment, but care must be taken to avoid these techniques leading to unsafe practices.

Some electrical distribution systems for electronic equipment have been installed mistakenly in an intent to minimize the amount of electrical noise seen in the grounding system. But these installations do not comply with the rules of the National Electrical Code (NEC), jeopardizing personnel safety.

Protection of data circuits from disturbances or damage does not always involve grounding, although good grounding makes this protection easier.

A Review of Grounding Techniques and Uses

One of the most important but least understood considerations in the design of electrical systems is grounding.

Grounding consists of a low impedance connection to the earth. The ground is a poor conductor but good enough for this purpose.

Grounding has a key role in the correct operation of the electrical systems, either power or electronics, as well as protecting people.

• System grounding helps detect and clear ground faults.
• Equipment grounding provides a return path for ground-fault current.
• Bonding keeps electrical continuity and conductivity.
• Static grounding prevents the build-up of static electricity reducing the chance of fires or explosions where hazardous materials are handled.
• Lightning protection grounding helps protect structures and equipment from direct strikes.
• Overhead ground wires and surge arresters, connected to ground, can limit dangerous system overvoltages to safe values.

Fundamentally, grounding an electronic system is the same as grounding any electrical system. However, care must be taken to prevent special grounding techniques from generating hazardous conditions.

Author: Lorenzo Mari holds a Master of Science degree in Electric Power Engineering from Rensselaer Polytechnic Institute (RPI). He has been a university professor since 1982, teaching topics as electric circuit analysis, electric machinery, power system analysis, and power system grounding. As such, he has written many articles to be used by students as learning tools. He also created five courses to be taught to electrical engineers in career development programs, i.e., Electrical Installations in Hazardous Locations, National Electrical Code, Electric Machinery, Power and Electronic Grounding Systems and Electric Power Substations Design. As a professional engineer, Mari has written dozens of technical specifications and other documents regarding electrical equipment and installations for major oil, gas and petrochemical capital projects. He has been EPCC Project Manager for some large oil, gas & petrochemical capital projects where he wrote many managerial documents commonly used in this kind of works.

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