Zinc oxide surge arrester. Current sharing consideration. ABB HV Switchgear. Technical Information. Publ. SESWG/A 2364E Edition 1,

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1 Zinc oxide surge arrester Current sharing consideration Technical Information Publ. SESWG/A 2364E Edition 1, Introduction To meet high energy requirements parallel columns of ZnO blocks have to be used in surge arresters or several arresters have to share the current and thus the energy. Typical applications are protection of series capacitors and arresters used in HVDC schemes. ZnO varistors * with as many as 4 parallel columns have been commissioned. It is then of greatest importance to ensure a good current and energy sharing to avoid an uneconomical design. Taken into account the extreme nonlinearity of the material, even rather small differences in residual voltages between parallel columns may prevent a satisfactory current sharing almost completely. * To make a distinction between standard surge arrester applications and special applications like for the protection of series capacitors a ZnO protective device for the latter is designated ZnO varistor. ABB HV Switchgear ASEA BROWN BOVERI

2 General With ZnO arresters the energy capability can be increased to meet any possible energy requirements by simply adding sufficiently many parallel columns provided that no series or parallel spark gaps are used. To make full use of this benefit of ZnO arresters it is however necessary to ensure a good current and hence energy sharing between the parallel columns. The columns can be mounted all in the same housing or in separate housings depending on the necessary number of block columns. For special cases it can also be necessary to ensure that several different arresters share the energy in order to avoid overloading of the arresters. Matched arresters have in such cases to be specially requested, since with standard arresters full current sharing is not necessarily obtained. The energy decisive cases for surge arresters will normally occur for switching surges causing arrester currents of a few hundred of amperes up to one or two ka. Due to production tolerances a typical spread in discharge voltages at these current is about %, which with a factor of nonlinearity in the equation i = k x U α equal to 3 will correspond to a current ratio as high as 1:4. For higher currents as for instance due to lightning surges the factor is smaller, about 11 at 1 ka for a 7 mm diameter ZnO block. Due to the fact that the residual voltage usually is by routine tests checked at this current, the spread in residual voltages is smaller, about.-2% between separate arresters, and thus the current sharing is better even if no special steps are undertaken. With α equal to 11 and a 2% difference in residual voltages a current ratio of 1:1.24 is obtained. ABB procedure to ensure a satisfactory current sharing The ABB routine, which has proved to be successful, to ensure a good current sharing for multicolumn surge arresters, between requested matched arresters as well as for ZnO varistors intended for series capacitors comprises the following steps: 1. Measuring of the discharge voltage of all ZnO blocks at currents of A and 1 ka, waveform 8/2 µs, and storing the blocks in very narrow voltage classes. For series capacitor applications only the A test is of interest. (With a current waveform x/y is understood an impulse with a front time of x µs and half-value y µs.) 2. Selecting the blocks for parallel columns or matched arresters to attain the same residual voltage classification for all block columns. 3. A separate current sharing check of all block sections in a multi-column arrester unit. (With a block section is understood a series part of the arrester. One or several sections in series constitute the interior of the unit). A current impulse with waveform 16/3 µs is applied with an amplitude of approximately A per block column. The currents in all separate columns are simultaneously registered and checked by a computer. Passing criteria is a maximum current deviation of +7% in any column from the. In addition all multi-column arresters are designed for a current deviation of +1% giving a safety margin of 3%. 4. When varistors shall be built or matched arresters are requested, an additional test besides the routine procedure according to point 1 to 3 has to be performed. Each completely assembled unit is tested with an impulse current based on the energy decisive case and with an amplitude, scaled with respect to the number of parallel units. (For series capacitor applications usually 1-1 A). Current waveform is approximately 8/2 µs. Discharge voltage and current are measured and the voltage is by a computer recalculated to a prospected nominal current to make a comparison between different units and/or arresters more accurate. A maximum difference in discharge voltages for the complete arresters corresponding to a specified current deviation is then usually prescribed. When an extremely close matching is required corresponding units for matched arresters or ZnO varistors are subjected to a final test. Corresponding units showing the highest and lowest residual voltages are tested together in order to check that the current deviation is within prescribed limits. As an example: For 6 matched 3-unit arresters with all 3 units slightly different in nominal rating a final test with 3x2 units is performed. Highest and lowest top units are tested together as well as middle and bottom units. Verification of the routine to ensure a good current sharing Representative results from tests to verify the matching routines are presented below: 4 test sections each comprising 4 parallel columns with 3 ZnO blocks in series have been tested. Current sharing have been checked for different current waveforms and amplitudes. The results are shown in Figures 1 and 2. Furthermore test section no. 1 was subjected to the following series of stresses with checking of the current sharing after each series: 2 line discharges with about 6% or rated energy per column in each discharge corresponding to higher stresses than given by line discharge class 4 as per IEC for this type of ZnO blocks. 2 discharges with 1 ka 8/2 µs per column. 2 high current discharges with 6 ka 4/1 µs per column. Test result is shown in Figure 3. 2

3 % deviation from 1 column No.1 column No.2 column No.3 column No.4 Test section No Test section No Test section No Test section No ma d.c..2 ka 8/2 µs. ka 8/2 µs 3 ka 8/2 µs 1 ka 8/2 µs per column and wave form Figure 1. Current sharing as function of current amplitude 3

4 % deviation from 1 column No.1 column No.2 column No.3 column No.4 Test section No Test section No Test section No Test section No ka 8/2 µs. ka 3/6 µs. ka *) 16/3 µs *) Routine test. ka Line discharge 3 ms per column and wave form Figure 2. Current sharing as a function of current waveform. s A per parallel block column. 4

5 % deviation from 1 column No.1 column No.2 column No.3 column No ka 1 ka. ka 1 ka. ka 1 ka. ka 1 ka 8/2 µs 8/2 µs 8/2 µs 8/2 µs per column and wave form 1) 2) 3) 4) 1) Before application of current impulses 2) After 2 line discharges with 6 % of rated energy in each discharge 3) After 2 discharges with 1 ka 8/2 µs per column 4) After 2 high current impulses 6 ka 4/1 µs per column Figure 3. Changes in current sharing caused by possible arrester operating duties % deviation from (ka) per column wave shape 8/2 µs 1) 2) 1) Current sharing before accelerated ageing test 2) Current sharing after an accelerated ageing test at 1. COV and 11 C during 6 weeks Figure 4. Changes in current sharing caused by accelerated ageing test.

6 Possible changes in current sharing due the continously applied voltage In order to check that the long time exposure to continuous operating voltage will not affect the current sharing, two test sections, each comprising two ZnO blocks in series, were subjected to an accelerated ageing test. The continuously applied Hz voltage was 84% of the reference voltage corresponding to a higher or equal voltage stress than 1. x COV (Continuous Operating Voltage) for any EXLIM surge arrester. The test was run at an elevated temperature of 11 C during 1 hours (6 weeks). This test was in accordance with the proposed IEC standard for ZnO surge arresters and intended to correspond to a minimum life-time of 1 years at 4 C. Before and after the accelerated ageing test the current sharing was measured at.2,., 3 and 1 ka with a current waveform of 8/2 µs. Test results are presented i Figure 4. Handling of spares High energy and current discharges may cause a small change in the protective characteristics of ZnO arresters. Typically, as verified in type tests according to international standards, around 1% which is much less than what was obtained and allowed for the old type of SiC surge arresters with series gaps. This small change will not affect the normal performance of the arrester. However, due to the extreme nonlinearity of the ZnO material the current sharing between two from the beginning matched arresters may be affected if only one of the arresters is subjected to energy and current discharges and the other not even energized. In reality this has to be considered only for some special cases when parallel arresters are used to meet very high energy requirements or in applications as protection of series capacitors and for arresters in HVDC schemes. For such cases therefore, it is strongly recommended that spare arresters and/or arrester units are energized right from the commissioning date among the other arresters and/or units. This will ensure uniform ageing of all arresters and/or arrester units including the spare units and that the current sharing tolerances established in the routine tests will be maintained. Summary and conclusions Following conclusions can be drawn from the presented test results. 1. The routine test described earlier with a current wave 16/ 3 µs and amplitude A per column will ensure a good current sharing for all possible severe switching surges for a multicolumn ZnO arrester. 2. Current sharing is independent of current front time i.e. the same current sharing will be obtained for the same amplitude even if the arrester is subjected to lightning strokes with front times of about 8 µs or transmission line discharges with durations of several ms. 3. Current sharing will neither be seriously affected by operating duties as lightning strokes and switching surges nor the continuously applied voltage. 4. It is preferable to perform a current sharing test with a current amplitude close to the one for the energy decisive case. For ZnO varistors in series capacitor application this usually means power frequency currents of 1-1 A and for surge arrester application switching surges from a few hundreds of amperes to one or two ka. To perform this test for instance with 1 ka 8/2 µs may or may not ensure a good current sharing for the decisive cases. Because of a smaller factor or nonlinearity, for higher currents it will always be easier to ensure good current sharing for higher than lower currents i.e. lightning currents compared with switching surges or normally expected power frequency currents. ABB HV Switchgear AB Surge Arresters Division Dept. SESWG/AF Box 71 S Ludvika, Sweden Tel Telex 747 abblu s Telefax SESWG/A 2364E Printed in Sweden. Production: ReklamCenter AB, Västerås. 199-