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Technical information

Guidelines for selection of surge arresters

for shunt capacitor banks

Shunt capacitor banks

Shunt capacitor banks are used to an increasing extent at all voltage levels. There are a variety of reasons for this like the growing need for power trans- fer on existing lines while avoiding transfer of reactive power, better use of existing power systems, improving volt- age stability, right-of-way and cost prob- lems, voltage control and compensation of reactive loads. Thyristor-controlled as well as breaker-switched capacitors are used. Breaker-switched capacitors are installed in distribution, HV and EHV systems. Since detailed studies generally are justified for thyristor-controlled capaci- tors due to the large cost savings which are possible, the general guidelines in this publication deal only with the pro- tection of breaker-switched equipment by ZnO arresters. Three-phase capacitor bank sizes vary from a few tenths of MVAr to several hundreds of MVAr. Both ungrounded wye and grounded wye banks are in use. It is common practice to use ”restrike- free” breakers. However, since many banks are switched on a daily basis, the probability of obtaining high tran- sients associated with capacitor switch- ing increases. Furthermore, the standard- ized procedure to verify that the breaker is restrike-free includes only a limited number of tests.The use of arresters not only gives protection if a restrike occurs but also decreases the prob- ability of multiple restrikes since the trapped charge on the capacitors is reduced. The aim of this brochure is to give guidance for selection of surge arresters for capacitor banks, when such protec- tion is considered to be needed.The protection afforded by different arrester protection levels and positioning (such as phase-ground, phase-phase and phase- neutral) against switching overvoltages is dealt with. Arrester energies related to different protection levels and capacitor MVAr ratings are given and the guidance is summarized in a set of diagrams. Resonance conditions are not dis- cussed since the duty imposed is strongly affected by system conditions and components, grounding etc. It is assumed, therefore, that harmonic and dynamic overvoltages in general are and must be limited by system design and operating procedures. Picture: Shunt capacitor bank in Kolbotten station, Sweden (Swedish Power Grid), 220 kV, 50 Hz, 100MVAr

Computer analysis

A series of computer calculations using the EMTP(DCG/EPRI version) have been carried out to study switching surge pro- tection of capacitor banks installed on systems ranging from distribution levels up to 550 kV. The banks have been assumed to be connected in an ungrounded wye and fed from a relatively strong system with a short-circuit current of 20 kArms. The ratio X/R in the source impedance has been selected to be 10 or more. Sys- tems with neutrals both grounded and ungrounded have been considered. Dif- ferent positioning of arresters such as phase-ground, phase-phase and phase- neutral have been taken into account. The case selected as decisive has been a two-phase restrike with full charge on capacitors due to a previous break. For this case the protection levels and the three-phase MVAr rating of the banks have been varied in order to determine the resulting capacitor volt- age as function of arrester protective level and bank size.The arrester energy stress as function of the same param- eters is also obtained. The three main cases considered with different positioning of the arrest- ers are illustrated in Figures 1 to 3. In the Figures Ug is the phase-ground voltage of the feeding system, R and L resistance and inductance respectively of feeding system, L2 possible series reactor of shunt capacitor, C capaci- tance of shunt capacitor and Zng system grounding impedance which has been set to either zero or infinite. The arrows in the circuit diagrams indicate the modelled fault sequence of a three-phase break followed by a two- phase breaker restrike. Since the expected arrester currents related to capacitor transients are high, the nominal protection levels for the arrester have been taken at 3kA switch- ing surge.These levels are generally given in the catalogues for well-qualified surge arresters. Zng Surge Arresters Ug R (^) L1 L2 C Zng Surge Arresters Ug R L1 (^) L2 C Zng Surge Arresters Ug R^ L1 L2 C Figure 1: Circuit diagram for the case with arresters connected phase- ground. Figure 3: Circuit diagram for the case with arresters connected phase- phase. Figure 2: Circuit diagram for the case with arresters connected phase- neutral.

Effect of series reactors Many capacitor banks are supplied with series reactors to either limit the current when switching-in with parallel banks connected or for nearby external faults or to form a filter with the capacitors. It may be common practice, for instance,to tune the reactor-capacitor circuit to a specific harmonic in order to avoid problems with unforeseen har- monics in the system.To cover one such case and check its influence on pro- tection and energy demands on surge arresters, capacitors with series-reactors tuned to the 7th^ harmonic have been considered.It has been found that the main influence is that arrester currents are lower and energies higher. It must also be pointed out that if the primary aim is to protect the capacitors, arresters positioned in front of the reac- tors will not protect the capacitors.The voltage may very well be twice as high at the capacitors than at the arresters. Furthermore, when selecting the con- tinuous operating voltage (Uc) of the arresters, it must be taken into account that the voltage phase-ground will be higher at the capacitor than at the reac- tor. Summary of computer analyses The results of the parametric analyses are summarized in a set of diagrams, one for each method of arrester posi- tioning.To limit the number of diagrams for practical purposes, some approxima- tions have been necessary. However, the resulting capacitor voltages are generally conservative which means that calcula- tions for a specific case may result in 5% to 10% lower values. Note that 1 p.u. is defined as usual i.e. the peak value of the line-to-ground voltage.Therefore, to use the diagrams for a specific arrester protective level, the arrester voltage taken from the catalogues must be recal- culated to p.u. value as defined above. The calculations have been per- formed for a power frequency of 50 Hz. For 60 Hz the diagrams can be used if the actual 3-phase MVAr rating of the bank is multiplied with 0,83 before the diagrams are consulted. (The factor 0, will give the same capacitance as used for the calculations with 50 Hz). Note that for capacitor banks con- nected grounded wye the diagrams for arresters phase-neutral could be used. Also note that cases with ground faults within the capacitor banks pre- ceding a breaker restrike have not been considered. For a capacitor with directly grounded neutral such fault may lead to even higher arrester duties due to a capacitor voltage in excess of 1 p.u. at the instant of the restrike. However, if this case should be considered the arrester energy could be estimated by multiplying the diagram values with the factor (2+U)^2 /9 where U is the expected voltage (in p.u.) on sound phases at ground fault. Arresters phase-ground Ÿ Protection considerations The same capacitor voltage is obtained for grounded as well as ungrounded systems. With a series reactor a few percent lower voltages are obtained due to lower arrester currents.The given values in Figure 4 are for the case with directly grounded neutral without a series reactor and are generally on the safe side (conservative).

500 450 400 350 300 250 200 150 100 50 0 0 10 20 30 40 50 Three-phase capacitor bank size (MVAr) Surge arrester energy (kJ) Protective level 1,8 p.u. 1,9 p.u. 2,0 p.u. 2,2 p.u. 2,35 p.u. 2,5 p.u. 1800 1600 1400 1200 1000 800 600 400 200 0 0 40 80 120 160 200 Surge arrester energy (kJ) Three-phase capacitor bank size (MVAr) Protective level 1,8 p.u. 1,9 p.u. 2,0 p.u. 2,2 p.u. 2,35 p.u. 2,5 p.u. Figure 8/9: Surge arresters installed phase- neutral. Arrester energy as function of capacitor size. Arrester protective level at 3kA. ungrounded the arrester energy will be approximately the same. For a series reactor tuned to the 7th^ har- monic the energy will be slightly higher approximately 5%. Arresters phase-phase Ÿ Protection considerations The same capacitor voltage is obtained for grounded as well as ungrounded systems. With a series reactor up to 5% lower voltages are obtained due to lower arrester cur- rents.The given values in Figure 10 are for the case with directly grounded neutral without a series reactor and generally on the safe side (conservative). Ÿ Energy considerations Figure 11 and 12 give the energy for the case with directly-grounded system neutral. If the system neutral is ungrounded the arrester energy will be approximately the same. For a series reactor tuned to the 7th^ har- monic the energy will be slightly higher, approximately 5%. Figure 10: Surge arresters installed phase- phase. Resulting capacitor voltage as function of capacitor size. Arrester protective level at 3kA. 1000 900 800 700 600 500 400 300 200 100 0 0 10 20 30 40 50 Surge arrester energy (kJ) Three-phase capacitor bank size (MVAr) Protective level 3,1 p.u. 3,3 p.u. 3,6 p.u. 4,0 p.u. 4000 3500 3000 2500 2000 1500 1000 500 0 0 40 80 120 160 200 Three-phase capacitor bank size (MVAr) Surge arrester energy (kJ) Protective level 3,1 p.u. 3,3 p.u. 3,6 p.u. 4,0 p.u. Figure 11/12: Surge arresters installed phase- phase. Arrester energy as function of capacitor size. Arrester protective level at 3kA. 2, 4 2, 2 2, 0 1, 8 0 40 80 120 160 200 Capacitor voltage (p.u.) Three-phase capacitor bank size (MVAr) Protective level 4,0 p.u. 3,6 pu 3,3 p.u. 3,1 p.u.

Selection of arrester parameters

Continuous

operating voltage

Where the capacitor is installed, the maximum permissible operating voltage allowed under normal conditions is usu- ally 5% to 10% above the nominal volt- age level.This must be considered when selecting the Uc of an arrester. If phase-phase arresters are consid- ered, the required Uc is the phase-to- phase maximum operating voltage. In case of harmonics it is important to notice that the Uc of the arrester must be higher than or equal to the expected crest voltage across the arrester divided by √2.

Rated voltage

The rated voltage (Ur) is a measure of the overvoltage capability of the arrester and is selected based on tempo- rary overvoltages,TOV. A recommended selection procedure is found in Ref. (11) for phase-ground arresters. Special attention should be paid to overvoltages caused by backfeed for arresters on a distribution system with ungrounded wye-delta transformers. In case of earth fault, phase-neutral arresters will be subjected to lower TOV than phase-ground arresters.The same selection procedure as for phase-ground arresters thus results in a safety margin. Only if it is problematic to find an arrester with sufficiently low protective level connected phase-neutral and with a rated voltage based on this selection procedure, it will be necessary to con- sider more carefully the possible TOV for phase-neutral applications. Phase-phase arresters are not sub- jected to TOV during earth faults. For any ABB high voltage arrester and for a normal choice of Uc of less than or equal to 0,8 times rated voltage the resulting TOV capability will be, as a minimum, as follows: Ÿ 1,42 * Um for 1 s or shorter times Ÿ 1,35 * Um for 10 s or shorter times Ÿ 1,27 * Um for 100 s or shorter times The Uc of the arrester shall be equal to or higher than Um which is the high- est system voltage. Such selection usu- ally will be sufficient to cover possible phase-phase TOV.

Energy capability

The energy capability of ZnO varistors may be somewhat dependent on the dis- charge current amplitude i.e. for shorter durations of an impulse, the energy capability is less. Usually the duration of arrester discharge currents resulting from capacitor transients is much shorter and the currents higher than obtained in the long-duration current withstand test as per IEC, which is the most relevant standardized test to define the energy capability. Some cau- tion, therefore, is recommended when selecting an arrester type which will meet the energy requirements. A derat- ing regarding energy capability may be necessary if given rated energy is based on existing IEC standard tests or even much longer current impulses. The derating factor is not generally mentioned in manufacturers’ standard catalogues and should be requested. If the probability of a breaker

Example 2 Ÿ System voltage: 245 kV Ÿ Temporary overvoltages (duration 1s or less) phase-ground: 1,55 p.u. phase-phase: 2,45 p.u. Ÿ System grounding: Directly grounded Ÿ Capacitor bank connection: Ungrounded wye Ÿ Rated 3-phase power: 80 MVAr Ÿ Desired protective level: 2,2 p.u. For arresters installed phase-ground: Uc of the arrester must be higher than or equal to 245√3=142 kV Rated voltage, following the proce- dure in Ref. (11) and for a particular arrester, must be higher than or equal to 193 kV. Next higher ”standard” value is 198 kV. For 198 kV rated voltage, the Uc for this particular arrester is 156 kV which is sufficient even if the operating voltage at the capacitor is 5-10% above the given 245 kV. Figure 4 shows that an arrester pro- tective level at 3 kA of 1,8 p.u.(360 kV) or less is needed to ensure a capacitor voltage less than or equal to 2,2 p.u. According to Figure 6 80 MVAr and 1,8 p.u. protective level result in an arrester energy of 820 kJ which means 820/198 = 4.1 kJ/kV rated voltage. Summary of required arrester data for connection phase-ground: Ÿ Rated voltage: 198 kV or more Ÿ Protective level at 3kA: 360 kV or less (switching surge) Ÿ Energy capability for capacitor dis- charges: 4,1 kJ per kV rated voltage or more A protective level of 360 kV at 3 kA for a rated voltage of 198 kV is extremely low even for 20kA arresters of line dis- For arresters installed phase-neutral: Uc of the arrester must be the same as for arresters phase-ground. At ground fault the voltage will be less than for the arresters connected phase-ground. Here is a possibility there- fore to decrease the rated voltage. How- ever this means higher energy and less safety margins.Therefore, first check what the same rated voltage will result in. Figure 7 shows that an arrester pro- tective level of 2,35 p.u. is sufficient and the Figure 8 shows that the energy will be 105 kJ for an arrester with a protec- tive level of 2,35 p.u. or higher at 3 kA which means 3,2 kJ/kV rated voltage. Summary of required arrester data for connection phase-neutral: Ÿ Rated voltage: 33 kV or more Ÿ Protective level at 3kA: 69 kV or less (switching surge) Ÿ Energy capability for capacitor dis- charges: 3,2 kJ per kV rated voltage or more If an arrester with protective level of 64,7 kV is selected, the same as for the phase-ground case, the required energy capability as per Figure 8 would be about 120kJ or 3,6 kJ per kV rated volt- age.

charge class 4 and 5. It certainly will require multiple column arresters if the TOV amplitude could not be reconsid- ered to a lower value which makes it possible to select a lower rated voltage for the arrester. However, if the given TOV’s are realistic, connection of the arresters phase-neutral could be an alter- native or even installation of phase- phase arresters. For arresters installed phase-neutral: Figure 7 shows that for a capacitor volt- age of 2,2 p.u. an arrester protective level of 2 p.u. (400 kV) is sufficient. Figure 9 shows that 2 p.u. will give 600 kJ in the arrester or 3 kJ per kV rated voltage. Summary of required arrester data for connection phase-neutral: Ÿ Rated voltage: 198 kV or more Ÿ Protective level at 3kA: 400 kV or less (switching surge) Ÿ Energy capability for capacitor dis- charges: 3,0 kJ per kV rated voltage or more For arresters installed phase-phase: Uc of the arrester must be higher than or equal to 245 kV. Rated voltage for any ABB arrester as per the discussion above (page 8, Selec- tion of Ur) must be higher than or equal to (2,55/√3) / (1,25/1,42) * 245 kV = 318 kV. Next higher ”standard” value is 330 kV. An arrester rated 330 kV is usually applied on 420 kV systems and the Uc for that type of arrester is usually 264 kV which is sufficient even if the operating voltage at the capacitor is 8 % above the given 245 kV. Figure 10 shows that for a capacitor voltage of 2,2 p.u. an arrester protective level of 3,6 p.u. (720 kV) is sufficient. Figure 12 shows that 3,6 p.u. will give 1250 kJ in the arrester or 3,8 kJ per kV rated voltage. Summary of required arrester data for connection phase-phase: Ÿ Rated voltage: 330 kV or more Ÿ Protective level at 3kA: 720 kV or less (switching surge) Ÿ Energy capability for capacitor dis- charges: 3,8 kJ per kV rated voltage or more The protective level for the minimum required rated voltage is not particularly low. With arresters phase-phase, there- fore, it is possible to improve the protec- tion further, which on the other hand leads to higher requirements on arrester energy capability. For an arrester pro- tective level of 670 kV which could be met by 20 kA arresters of line discharge class 4 and 5 the voltage across the capacitors , phase-neutral, will be as low as 2,05 p.u.

References

set of arresters or existing phase-ground arresters in the station if found neces- sary due to the exposure to lightning surges. Note! The guidelines do not discuss resonances which can result in even higher duties than listed. On the other hand it is difficult in practice to foresee resonances and, if they do occur, an arrester failure may be acceptable instead of trying to design for every pos- sible case. It must be noted that the main aim with provision of arresters, i.e. the protection of the capacitors, is still fulfilled even if the arrester fails. [1] High Voltage Surge Arresters Buyers Guide, ABB publication SEHVP/A 2200en. [2] McGranaghan M.F.,Reid W.E.,Law S.W. and Gresham D.W., ”Overvoltage protec- tion of shunt-capacitor banks using MOV arresters”. IEEE trans,PAS-103,No.8,August 1984. [3] Harder J.E.,Hutchinson D.E. and Rust W.B. ”138 kV shunt capacitor research bank-design & experience”. IEEE paper 89 SM 610-7 PWRD [4] ”Surge protection of high voltage shunt capacitor banks on ac power systems- survey results and application considerations” Report by working group 3.4.17 of IEEE, Surge Protective Device Committee. IEEE WM 1991. [5] Even. ”M.O. arresters as lightning and switching overvoltage protection of capacitor banks. Stresses on arresters”. Colloquium paper SC 33-1987 (Tokyo) 03.01. [6] Lima Allen J. ”Ungrounded wye capacitor bank overvoltage protection”. Col- loquium paper SC 33-1987 (Tokyo) 03.02. [7] Nigol and Snider. ”Metal-oxide arresters (MOAs) as lightning and switching overvoltage protection of capacitor banks. Stresses on the arresters.” Colloquium paper SC 33-1987 (Tokyo) 03.03. [8] Esmeraldo P.C.V. and Nora Dias L.E. ”Metal-oxide arresters (MOAs) as switching overvoltage protection of capacitor banks at Adrianopolis substation”. Collo- quium paper SC 33-1987 (Tokyo) 03.04. [9] Bélanger J. and Menemenlis N. ” Metal-oxide arresters as switching overvoltage protection of capacitor bank circuit breakers” Colloquium paper SC 33-1987 (Tokyo) 03.05. [10] McGranaghan and Wilson. ”Surge arrester protection of capacitor banks”. Colloquium paper SC 33-1987 (Tokyo) 03.06. [11] Metal Oxide Surge Arresters in AC systems, CIGRE Technical Brochure No 60 by Working Group 06 of Study Committee 33. [12] Stenström L. and Mobedjina M., Guidelines for selection of surge arresters for shunt capacitor banks, Electra, Vol. 159, April 1995.

ABB Power Technology Products AB High Voltage Products Surge Arresters S-771 80 Ludvika, Sweden Tel: +46 (0)240 78 20 00 Fax: +46 (0)240 179 83 www.abb.com/arrestersonline 2312en / 2001-