Arrester Zno

1168 IEEE Transactions on Power Delivery, Vol. 12, No. 3, July 1997

ult ipulse ightning Currents and Metal-Oxide Arresters

M. Darveniza, L.R. Tumma, B. Richter D.A. Roby University of Queensland ASEA Brown Boveri ASEA Brown Boveri

Australia Switzerland

ABSTRACT

Multiple stroke lightning ground flashes can impose surges of ex- ceptional severity on exposed distribution surge arresters. This paper describes a series of tests on metal-oxide (MO) arresters and varistors to study surface flashover mechanisms. The results show that the more likely causes of surface flashover were - plasma generation, manufacturing defects of the varistor surface coating, dielectric properties of the surface coating and the electrode contact system. For multipulses, plasma enhancement and varistor surface coating were found to play a dominant role in surface flashover.

9. Introd~ction In our earlier study [l], several makes of gapless metal-oxide (MO) arresters were tested using single and multiple impulse currents under the representative service conditions and also

under the conditions specified in the Standards [2,3]. Compar- isons of the effects of multipulse and standard current impulses show important differences. The result of major significance is that multipulse currents can give rise to changes in varistor characteristics (including failure) that are not evident during tests with single impulses [l]. While multipulse currents caused changes in the electrical characteristics of the varistors including some that lead to thermal instability, the most common limit- ing effect was to cause surface flashover of the varistors. This indicates that the effects of multipulses may not be purely a thermal energy problem, rather discrete failure mechanisms (by surface flashover) appear to be involved.

This paper describes tests on MO varistors from one manufacturer; different surface finishes were used to examine surface flashover phenomena. These flashover studies are discussed in section 2 . Based on this study, a multipulse test proposal for distribution arresters is described in section 3.

2. Flashover Studies The probable causes of surface flashover of varistors are - a) plasma created near the edge of metallisation, b) the varistor outer surface (glass) possibly affected by humidity, c) the like- lihood of material defects at the outer edges of the varistor and d) partial discharges produced at the edges and surface possibly affecting the dielectric strength. Studies were conducted to examine these four possible causes of surface flashover and these are reported in this section.

96 SM 398-8 PWRD A paper recommended and approved by the IEEE Surge Protective Devices Committee of the IEEE Power Engineering Society for presentation at the 1996 IEEWPES Summer Meeting, July 28 - August 1, 1996, in Denver, Colorado Manuscript submitted December 27, 1995, made available for printing June 3, 1996

Australia

2.1 Plasma generation

Plasma could be created near the edges of a varistor’s metallisation, accumulating with each successive puls multipulse. The generated plasma would not deionise in the inter-pulse time intervals of about 35 ms, whereas deionisation could take place in the dard lightning impulse test with single pulses separated b cause of plasma production is metallisation and the resulting increase in current density at the edge.

Plasma generation was investigated by placing a metal spacer between two varistors and using sextuple currents. During im- pulsing, the varistor surface and the metal spacer were pho- tographed with a high speed camera (400 frames/second). The photographic method is illustrated in Figure 1. Photographs of pulses 1 to 6 in a given sextuple current reveal that plasma intensity and size increase with each succeeding pulse. It is ob- vious that the accumulated plasma can aid surface flashover on the varistor surface.

2.2 Varistor surface coating

Dielectric behaviour of the varistor surface coating has a major influence on withstand capability towards surface flashovers. Tests on MO surge arresters and varistors with different surface finishes were conducted to examine the surface flashover mechanisms. More details of this study can be found in [4].

Experimental tests

Five types of tests were c ed out to determine the compara- tive performance of five varistor surface finishes. The tests included three applied essentially in accordance with IEC 99-4 [3], namely, (i) single 8 /20ps impulse currents applied to the varistors (initially at room temperature) at one-minute intervals; (ii) single impulse operating duty tests, in which two groups of five 8/20ps impulses were applied at one-minute intervals with the test object pre-heated to 6OoC and energised at rated voltage, thirty minutes separated the two groups; after each current application and for thirty minutes after the final application, 1.05 times the continuous operating voltage was maintained on the test object to check for thermal stability; (iii) two single 4/10ps high lightning impulse currents, the first applied with the test objects at room temperature and the second with the sample pre-heated to 60°C. After the second impulse, thermal stability was checked in the usual way. The other two types of tests used multipulse currents, in particular quintuple (five-pulse) 8/20ps impulse currents, applied in such a way to provide a comparison with the single-impulse tests and the single-impulse operating duty tests; namely, (iv) multipulse currents (inter-pulse intervals of 35 ms) applied to test samples at ambient temperature and pre-heated to 60°C. When more than one quintuple sequence was used, they were applied at intervals of 30 minutes;

0885-8977/97/$10.00 0 1996 IEEE

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(v) multipulse operating duty tests carried out in a similar way to the single-impulse operating duty tests, except that a quintuple sequence (time intervals 35 ms), without point-on-wave timing control, was applied in place of each group of five single impulses.

The tests were applied to groups of samples to explore the limits of significant change and failure for single-impulse and multipulse applications. The effects of single impulse currents ranging from 4 to 40kA were compared with quintuples ranging from 5 to 1lkA. For the operating duty tests, the single- and multipulse currents were varied between 6.5 to 1lkA. The high 4/1Qps currents ranged from 40 to 90kA.

T h e Test samples

Each of the twenty five test samples contained a single varistor - diameter 38.2 mm; rated current 5kA, rated voltage 6.25kV and continuous operating voltage 5kV. The samples had five types of surface finishes - three were on varistors alone and two were assembled as arresters. The five types are described and the samples are identified as follows - A) varistor with glass coating (samples L1 to L6) B) varistor with glass coating plus silicone varnish (samples 1 to 6) C) as in B), but assembled as an arrester but without a polymer housing (samples AS1 to 3, AS6) D) varistor with glass coating bonded to a silicone moulding (samples S1 to 3, S6) E) complete silicone moulded arrester containing a glass coated varistor bonded to the silicone housing (samples AF1 to 3). Type C and E samples were tested using the fittings as supplied. Type A, B and D samples required the use of a special fitting, which included disc electrodes (similar to those in the type C and E samples) and a supporting jig comprising two aluminium end plates (80x8Qx11.3mm) and four insulated clamping screws. The varistor samples were placed between the end plates and clamped with suitable pressure.

Before and after diagnostics

The effects of a test on each sample were monitored in two ways. Gross effects such as physical damage or marked changes in current and voltage oscillograms (and derived power and absorbed energy) were easy to identify routinely during the course of the testing. Likewise, thermal stability was easy to check during the operating duty and high current tests, by monitoring the leakage current for up to thirty minutes (after application of the impulses) with the sample energised at 1.05 times the continuous operating voltage.

More subtle changes in the electrical characteristics were identified by before and after diagnostic tests. These included mea- surements of the 1.4 mA DC and AC reference voltages and of the discharge residual voltage at 5kA. All the diagnostic tests were made with the test samples at about the same ambient temperature (within 2OC). The changes were characterised by after/before ratios of the diagnostic test results. It should be noted that most arrester testing standards allow changes of 0.95 to 1.05 (in some cases 0.9 to 1.1) as acceptable.

2.2.1 Test results

The results are under three headings - impulse current tests, operating duty tests and high lightning impulse current tests.

Impulse current tests

The results of the single impulse tests on five samples using currents in the range 4 to 40kA are given in Table 1. Apart from some small burn marks on the block of one sample (2 type B), no gross effects were observed. Also, there were only small changes in the electrical characteristics, as indicated by the after/before ratios which are in the range 0.942 to 1.045. It is considered that the small burn marks on type B sample 2 (near the outer edge of one electrode) were caused by a mechanical defect rather than a flaw in design. It can be concluded therefore that the single impulse tests with currents up to 40kA had no deleterious effects on the samples irrespective of surface finish.

A very different picture emerges on examination of the multipulse test results for nine test samples given in Table 2. The quintuple currents used for the tests varied between 5 and IlkA. Four of the samples failed by surface flashover, namely 1 of 3 type A varistors (at lOkA), both type B varistors (at 6kA) and 1 of the 3 type C varistors (at 11kA). The surface flashovers occurred just on or just below the glass surface of the varistors. The remaining samples showed no evidence of gross damage or degradation in electrical characteristics (with after/before ratios in the range 0.987 to 1.05).

Operating d u t y tests

The results of single impulse and multipulse operating duty tests on 6 samples are given in Table 3 with currents of 6.5 to 1lkA. The single impulse operating duty tests were only made on one type (E) and no significant effects were evident. Like- wise, no significant effects were caused by multipulse operating duty tests, i.e. no gross effects, no thermal instability and only small changes in the after/before ratios (range 0.989 to 1.05, except for one value of 1.075 for the negative DC reference voltage of type E sample AF2 - this is likely to be an artifact due to the order of testing, as it is known that DC reference voltage is influenced by the order in which tests of different polarity are made).

High lightning impulse current tests

Table 4 shows the test results for currents in the range 40 to 90kA applied to eight test samples of all five types. The two glass coated type A varistors failed by surface flashover at 65kA. One of the type B varistors (glass coating plus silicone varnish) exhibited partial surface flashover damage at 72kA and the other failed by surface flashover at 90kA. The one type C sample showed a burn mark near an electrode after tests at 90kA; its residual voltage was unaffected, but the DC and AC reference voltages were reduced significantly (by about 20 percent). In contrast, the samples with glass coated varistors

effects from the 90kA tests, remained thermally stable, experienced little change in the residual voltage, but had significantly reduced reference voltages.

bonded to silicone mouldings (types D and E) showed no GOSS

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Surface defects

During the fabrication of varistors, an outer insulating surface is provided by spraying glass or insulating material on to the varistor material and fusing it at aboul 500°C. After surface coating, varistor ends are usually metallised with aluminium and bonded at about 50OoC. Because of manufacturing proce- dures and material handling, some varistors may have surface contamination or irregularities on or near surface.

It is likely that surface irregularities or contamination could con- tribute to the initiation of processes leading to surface flashover. An irregularity might take the form of a localised defect in the outer insulating material or at its boundary with the varistor material. Such a defect would produce a localised concentration in the electrical field, which could cause a localised (partial) discharge and begin the process which ultimately resulted in surface flashover. It is of course well known that contamination can degrade the dielectric properties of an insulating surface and this can also lead to surface flashover.

2.4 Electrode contact system

The electrode contact system with the varistors is also an important design feature. If the electrodes are not properly aligned with the varistors, sparking could result during the passage of impulse currents. Consequently, withstood multipulses could produce plasma or partial discharges in the air, which over time could alter the dielectric properties of the varistor surface and produce a condition likely to lead l o surface flashover. Once discharges are initiated, they are sustained by the sizable current flowing through the portion of the material which has not bro- ken down. Flashover is completed when a discharge elongates and develops into a short circuit.

Before discussing the test results of flashover investigations, it is of interest to make an overall comparison with the multipulse current and operating duty tests reported previously [l] for five makes of fully assembled 5kA distribution surge arresters (d in porcelain housings). The previous tests showed that most failures were also by surface flashovers caused by multipulse currents in the range 5 to 9kA. As with the present test results, surface flashovers at relatively low currents (5 to 6kA) were associated with varistors that appeared to have simple or no special finish on the varistor surfaces. However, if there was a special coating on the varistor surfaces, the arresters were able to withstand higher multipulse currents. It is also of interest to note that two of the previously tested arresters failed by thermal instability after appLcation of multipulse currents

Consideration is now given to the results from the current series of tests. Metal-oxide varistors with five different surface finishes were tested with 8/2Ops impulses, multipulses and high current impulses. The varistor material and the processing were the same for all the blocks. The difference was only in the treatment of the surface after the glass coating.

The results have to be seen in two separate dimensions. The first dimension concerns the varistor material itself. In none of the tests did a material failure occur. But changes in the characteristics were observed: e The change in the residual voltage was negligible.

o The change in the reference voltage was significant and de- pended on the stress the was seen after the high c impulse test. This phenomenon is well knowii and is a CO m e b€ both the large magdude and the high steepness of the high current impulse. The second (surface dielectric) dimension showed the failure mode of the tested samples; all failures (independent of the impulse stress) were surface flashovers. Here again two main groups were noticed.

First : only the blocks with glass coating (types A, B and C) failed. Second : the blocks which were completely assembled and moulded in silicone (types D and E) did not fail any of the tests. The pure glass coating is very sensitive to moisture, grease, dust, etc. If severe conditions such as high humidity have to be met, treatment of the glass surface with silicone proves to be effective, although for extreme conditions it was suggested to combine this wit’h a leaching process which removes the sir- face alkali [5]. The best behaviour is shown in a complete surge arrester where the silicone housing is directly moulded onto the blocks thereby providing increased dielectric strength. It should be noted that polymer housings inhibit moisture ingress and surface contamination, but not problems related to material defects, surface defects and electrode contact system. Examples of the importance of surface defects are (i) the failure of one sample L2 (type A) on the first impulse of a lOkA quintuple whereas another sample L1 (of the same type) passed more severe single impulse tests (refer Tables 1 and 2), and (ii) the failure of one polymer housed arrester at 9.5kA quintuple current.

A glass surface may contain a few mono-molecular layers of gel (water) under conditions of high humidity. As a result conductivity of the glass surface increases when comp a glass surface under dry conditions. This surface conductance

re dependence of the ionic conduction (conductivity with increasing temperatures). In the sam ion of multipulses (by

current impulses), could surface. During the

pauses between the pulses of a given multipulse, the erated due to surge absorption could flow from the m material to the glass surface and raises its temperature, lower- ing its surface resistance and so producing surface current flow and leading to surface flashover. During single pulse testing, the time interval betwee ssive impulses is selected to be 60 s . With 60 s time he glass surface has time to cool down because of its th the surrounding air.

A survey of Australian service experience for about 1I years (1980-1991) on gapless oxide surge arresters [6] s major causes of arrest es were faulty varisto electrical storm activity, porcelains and moisture more direct study was adertaken by Darveniza, et. a1 [7]. with the view to determine the conditi apped silicon-carbide surge arresters in service on Aus distribution systems. By chance, twelve MO arresters were included in the hundreds of arresters recovered from service, and these were and examined. Of t arresters, three arr found to be faulty. faulty arresters were disman- tled and the internal components inspected. T w o of the service failed arresters showed heavy carbonisation deposits and surface flashover tracks on the outer surface of the varistors. This may be due to surface flashover by a discharge current, berause. of t,he presence of moisture. The third service failed arrester showed enhancement of surface conductivity on one of the varistors.

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This type of phenomenon was also observed on the tested sample surfaces, which failed during multipulse tests.

Single and multipulse impulse tests on varistors reveal that relatively low magnitude multipulse currents, when compared to single impulse currents, could cause surface flashover. This indicates that a comparable plasma enhancement should have occurred with multipulses. In the case of multipulses, plasma enhancement very likely plays a dominant role in the breakdown initiation.

It may be, especially at higher temperatures, that at the edges of the blocks, where naturally the highest field strength is, the possibility of ionisation is higher when the impulses are very frequent as they are with multipulses. This is possibly due to an increase of free electrons and ions forming a plasma, which does not have time to neutralise between the individual multipulses. These effects need further and more in depth investigation.

The varistor results show how important it is to have a good electrode contact system and a good surface coating around the varistors with high dielectric strength when exposed to extreme conditions such as high humidity. For these reasons, the complete arresters with one varistor seem to perform very well in the conducted tests.

pulses (especially for types A and B coatings). It appears, that the shorter time between the pulses the higher is the failure rate. One expects if peak value and the steepness of the current are kept the same while varying the inter-pulse time, the energy accumulated should lead to failures such as cracking or puncture in the material, but not surface flashovers.

Photographs of tested samples

Colour photographs were made of all the tested samples and these illustrate several features - (i) the occasional presence of minor imperfections on the block prior to any testing, and (ii) minor burn marks and surface flashover damage on the varistors referred to in the multipulse current tests and the high lightning current tests. The reproduction quality in black and white is not of great value and so they are not included in the text of the paper. But they do reproduce well as coloured slides.

3. Multipulse test sequence proposal

The lightning impulse tests specified in Standards, eg. [3], are intended to demonstrate that arresters are stable and discharge lightning surges without failure. During these standard laboratory lightning impulse tests, the time interval between successive impulses is selected to be one minute. However, lightning

is equivalent to a quintuple (five-pulse) t&n of 8/20ps current impulses for the same magnitudes of energy absorbed, charge delivered and current discharged by the surge arrester. The simulation results were also verified with laboratory tests. Limited tests were carried out on a MO arrester with quintuple trains of 8/20ps impulses and equivalent single impulses at 9kA. Com- parisons of before and after reference voltages (at 1 mA) have shown them to be the same. Further studies are required to see if the quintuple 8/20ps currents are more likely to produce failures by surface flashover than j. single (with equivalent energy) 39.4/101.5ps impulse.

Some of the varistor failures clearly reveal that surface flashover mechanisms were initiated at the cathode electrode; a tongue of discharge (plasma) extending from the cathode end rapidly spread towards the anode. These observations could easily be made from the test samples which were partially damaged.

The elongation mechanism of the discharge column may be due to electric breakdown in the high field region at the d i s charge tip. The presence of these fields is suggested by the tip branching noted at negative polarity. This may be caused by avalanches triggered by photo-ionisation in the pre-tip region. This phenomenon is somewhat similar to streamer breakdown in air, which exhibits pronounced branching. Once injected into the surface material, the electron multiplication is thought to be analogous to that in gas discharge. The electrons entering at the cathode electrode will drift towards the anode under the influence of the field (or the plasma) gaining energy between collisions and losing it on collisions. On occasions the free path may be long enough for the energy gain to exceed the lattice ionisation energy and an additional electron is produced on col- lision. The process is repeated and may lead to the formation of an electron avalanche similar to gases.

Further investigations are needed to explain the occurrence of failures, which appear to depend on the time between the im-

have multiple strokes producing 2 to over 20 (average 3 or 4) with inter-stroke time intervals of 15 to 150ms (average 30 to 40ms). This difference in time intervals is significant in view of this investigation and that of Darveniza and Mercer [SI.

Because of the damaging effects of sextuple currents on 5kA arresters (both of the gapped Sic type [8] and of the MO type), consideration should be given to the introduction of multipulses in lightning impulse current operating duty tests. Current Stan- dards either follow the IEC procedure of applying 4 groups of 5 single impulses applied at 1 minute (min) intervals with 30 min between groups, or the ANSI procedure of applying 20 single impulses at 1 min intervals. The multiple current testing might be implemented by applying a set of four (4) quintuple (five-pulse) trains of 8/20ps currents (with inter-pulse intervals of 40 ms) at 30 minute intervals if following the IEC, or at 5 min intervals if following ANSI [9], with an arrester being energised with power frequency voltage. In either case, the total number of applied 8/20ps impulses remains at 20, and the total elapsed time remains about the same for each (.: 120 min according to IEC and 20 min for ANSI). Quintuples could be applied 1-2 seconds after energisation at the rated voltage of the arrester. The rated voltage could be reduced to the maximum continuous operating voltage (MCOV of the arrester) 10 seconds after each multipulse. After each multipulse application and for thirty minutes after the ha1 application the MCOV could be maintained on the test object to check for thermal stability.

Based on the above guidelines, the authors proposed a multipulse test procedure for inclusion in AS1307-part 2. The adopted multipulse test procedure is slightly different from the procedure generally used for operating duty tests. These changes comprise: (a) The application to the arrester of four groups of quintuple 8/20ps lightning impulse currents in combination with energisation by a specified voltage and frequency (AS1307-Part 2, Clause 7.5.1). The total elapsed time of each quintuple set could be in the range of 0.1 s to 0.25 s. The time interval between the quintuple groups could be 25 to 30 minutes.

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(b) There is no need to time the application of the quintuple impulses with respect to the power frequency voltage waveform. (c) Rated voltage should be applied at least 10 s prior to the application of first lightning impulse current and the arrester should be pre-heated to 60f3”C. The applied voltage should be reduced to 1.05 continuous operating voltage 10 s after each quintuple set of current impulses, and should be maintained for the 25 to 30 minutes between groups of quintuple currents. Rated voltage should be re-applied before the application of the next set of quintuple currents. After the fourth quintuple set, 1.05 times the continuous operating voltage should be maintained for 30 minutes to check thermal stability.

The criteria for assessing the effects of the multipulse tests are - a) thermal stability (evident within thirty minutes), b) sub- sequent changes in residual voltages (limited to 5%) , c) d.c. reference voltages (limited to 10%) and d) power-losses (limited to 5%). This operating duty test protocol is a reflection of field conditions likely to occur on distribution systems in service. In addition, no significant abnormalities should be found in the current and voltage oscillograms recorded during the tests.

Further work is needed to correlate laboratory studies using multipulse tests with service performance of MO arresters in the field. This work is in progress.

ents Support for this work (which was carried out at the University of Queensland) has been provided by the Australian Research Council, the Australian Electricity Industry Research Board and by ASEA Brown Boveri Power Transmission (Australia).

5. References 1.

2.

3.

4.

5.

6.

7.

8.

M. Darveniza, D. Roby and L.R. Tumma, ”Laboratory and Analytical Studies of the Effects of Multipulse Light- ning Current on Metal-Oxide Arresters”, IEEE Transac- tions on Power Delivery, Vol. 9, April 1994, pp. 764-71

AS. 1307-1986, Surge Arresters (Diverters) Part 2 - Metal Oxide Type for A.C. Systems, Standards Australia, North Sydney, Australia 2060.

IEC Standard 99-4, Surge Arresters Part 4, Metal-oxide Surge Arresters without Gaps for A.C. Systems, 1991-11, First Edition.

M. Darveniza, L.R. Tumma, B. Richter and D. Roby, ”The Effects of Multipulse Currents and other Lightning Parameters on the Performance of Surge Arresters”, 22nd International Conference on Lightning Protection, 19-23 September 1994, Budapest, Hungary

J.B. Birks, Ed., Modern dielectric materials:, Heywood & Company, London, 1960.

J. Diesendorf, ”Over-voltage protective devices”, Short course program on Insulation Coordination in High Volt- age AC Systems, The University of Queensland, Brisbane, 23-26 November, 1993.

M. Darveniza and D.R. Mercer, ”The Reliability of Distri- bution Surge Arresters’, Proc. lEEE/KTH Power Tech. Cord., Stockholm, June 1995, Paper No. SPT HV 11-01-

Darveniza and D.R. Mercer, ”Laboratory Studies of the Effects of Multipulse Lightning Currents on Distribution

0010, pp. 327-31.

Surge Arresters”, IEEE Trans. on Power Delivery, Vol.

9. ANSI/IEEE Standard C62.11, Standard for Metal-Oxide 8, July 1993, pp. 1035-44.

Surge Arresters for AC Power Circuits, 1987.

BIOGRAPHY

Mat Darveniza, born in Australia in 1932, is a g University of Queensland (BE, DEng) and of London (PhD). He is Professor (Personal Chair) in Electrical Engineering at the University of Quee . His research interests are in lightning, high voltage and cal insulation. He is a Fellow of the IEEE, the Institution of Engineers Australia and the Academy of Technological Sciences and Engineering. He is past-chairman of the IEEE Australian Council and of the PE Chapter, Queens- land.

L.Reddy Tumma, born in India in 1960, obtained his BE from the Osmania University, India in 1982, ME from Indian Insti- tute of Science, Bangalore, India in 1984 and PhD from the University Queensland, Australia in 1995. He worked in industry for over four years in the field of switchgear protectio is a member of the IEEE.

Bernhard Richter, born in Germany in 1950, obtained the Dip1.- Ing (FH) in 1973 from the Technical High School (Beuth) and the Dip1.-Ing. in 1979 from the Technical University of Berlin. After five years as a scientific assistant at the Institute of High- voltage of the University of Berlin he joined BBC, now ABB, in 1985, where he works now in the development of surge arresters. Besides other international activities he is member of the JEC working groups 06 of TC 81 arid 05 of SC 37 A.

David A. Roby, born in India in 1951, is an electrical engineering graduate from the Indian Institute of Technology, Madras (B.Tech.). He is the Business Unit Manager - Surge Arresters with ABB Power Transmission Pty Limited and a member of subcommittee EL/7/3 - Surge Arresters. His research interests are in gapless metal-oxide technology, system protection and low energy self blast circuit breakers.

Figure 1: Plasma enhancement measuring method using a high speed camera

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I I and 40kA I I I I 1 1 1

Table 1: Summary of after/before ratios of the single impulse test results Sinele impulses applied I D.C. reference I A.C. reference I Discharge I Sample

No.

Lt

2

3 AS1

s2

Type of surface coating A

B

B C

D

_ _ v -

30kA, 40kA and five single

voltages 1.4mA voltages 1.4mAp residual voltage +ive 1 -ive +ive 1 -ive +ive I -ive 0.959 I 1.008 0.992 I 1.029 1,019 I 1.026

pulses of 20kA at an interval of 1 min. 4, 7.5, 10, 15, 23, 32 and 0.955 1.045 ,995 1.032 1.015 1.031 38kA 20 kA and 40kA 2 cycles of 5 single pulses

0.971 1.022 0.992 1.035 1.026 1.022 0.954 1.043 0.991 1.017 1.005 1.001

38kA 20 kA and 40kA 2 cycles of 5 single pulses

0.971 1.022 0.992 1.035 1.026 1.022 0.954 1.043 0.991 1.017 1.005 1.001

t results

with peak values of 6kA 5 single pulses of 25kA, and sinEle pulses of 26

I

6 I B I90kA I - I - 1 - I - I - I - I failed AS3 I C I 90kA I 0.795 I 0.884 I 0.818 I 0.868 I 0.991 I 1.00 I Burn mark near

0.942 1.006 0.995 1.031 1.03 1.016

I I HV electrode I I I I I I I I I

with peak values of 6kA 5 single pulses of 25kA, and sinEle pulses of 26

I I L

s3 I D I 90kA I 0.791 I 0.909 I 0.788 I 0.855 I 0.992 I 1.009 I AF1 I E I 90kA 1 0.734 I 0.859 I 0.78 I 0.824 I 1.00 I 1.021 1

0.942 1.006 0.995 1.031 1.03 1.016

Sample Type of Stress applied No. surface

coating AF3 E 2 groups of five l l k A single

D.C. reference A.C. reference Discharge voltages 1.4mA voltages 1.4mAP residual voltage Remarks +ive I -ive +ive I -ive +ive I -ive 1.01 1 1.03 1.03 I 1.02 1.03 1 1.02

pulses 2 quintuples of l l k A with test object at 60°C quintuples of 5.5, 6.5 & lOkA with test object at 60°C

1.024 1.029 1.03 1.048 1.021 1.05

1.006 1.023 1.012 1.026 1.016 1.022

AS3 C 2 quintuples of l l k A 1.W 1.05 1.017 S6 D 2 quintuples of l l k A 0.989 1.016 1.032

AF2 E 2 quintuples of l l k A 1.006 1.075 1.018 with test object at 60°C

with test object at 60°C

1.03 1.014 1.018 1.05 1.03 1.021

1.032 1.014 1.00

z

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ISCUSSlON

JOHN B. F'OSEY (Westfield Centei, OH) The authors have reported failures of unhoused blocks caused by multipulse discharges Also reported is "Blocks which were completely assembled and moulded in silicone (types D and E) did not fail any of the tests" and "showed no evidence of gross damage or degradation in electrical characteristics "

Even though no housed arresters failed, a proposal 1s advanced that consideration be given to the introduction of multipulse in the operating duty tests

Would the authors please clarify how data from the testing of assembled arresters supports the suggested change in standards? The published test data does not seem to support the proposal, but instead shows unassembled blocks fail but assembled arresters are immune to the proposed multipulse test procedure.

Manuscript received August 14, 1996.

H. s. Brewer and M. G. Comber (Hubbell/Ohio Brass Company, Wadsworth, OH): We read this paper with great interest, especially with regard to the proposal for a new standard arrester test. We would like to make a few observations and pose a few questions to the authors.

1. Our understanding of the utility industry experience with metal-oxide distribution arresters is that the total failure rate is very small and only a small percentage of those arresters which fail do so as a result of iightning. While the existing high-current short-duration tests prescribed in both ANSI and IEC standards may not represent real-world lightning, they have apparently served as an excellent means for demonstrating the * adequacy of metal-oxide arresters for real-world service. It is possible that existing designs of metal-oxide arresters would not meet the multikulse requirements proposed by the authors, perhaps requiring redesign to use larger metal-oxide elements. Could the authors comment on the value of anticipated improvement in system reliability resulting from the proposed multipulse test in relation to possible product cost increases?

2 . Investigations by Dr. Uman (University of Florida) and Dr. Krlder (University of Arizona) have shown that multiple strokes within a lightning fiash may be spatially separated by distances corresponding to several typical span lengths of a distribution line. This would suggest that a single arrester is not likely to be exposed to multiple strokes. This may account for the l o w rate of lightning-caused arrester failures and would obviate the need for a multipulse test. What are the authors thoughts on this?

3. The reported study was performed on metal-oxide elements of only one manufacturer. The elements were directly coated with a glass collar. Other manufacturers use other collar materials, both inorganic and organic. Have the authors conducted simlar multipulse tests on

continuing work?

4 . The authors previous work (and the work of others) indicates that a major cause of porcelain-housed distribution arrester failures is moisture ingress. While most polymer-housed arrester designs contain little free air space and moisture ingress should be less of a concern, it is still necessary to have some means of detentuning that arrester performance is not degraded by moisture ingress. Many polymer-housed designs do not permit simple disassembly to inspect for moisture and therefore some electrical test evaluation means are usually employed. Do the authors have an opinion on the merits of a multipulse test as such an evaluation means?

Manuscript received August 30, 1996.

M. Darveniza, L.R. Tumm B. Richter and D.A. Roby. We are pleased to respond to the interesting points raised by the discussers. It is a common comment from manufacturers and suppliers of metal-oxide distribution arresters that there total failure rate is small and only a small percentage of arrester failures can be attributed to lightning. This is in accord with arrester performance surveys carried out by Utility Association (eg. by AP 33 in Australia). Yet individual electricity utilities speak of arrester failures as a cause for concern. And most people would agree that metal-oxide arresters have not been in service long-enough to establish their long term reliability. general view is that laboratory tests with multipulses hav wn that failures can occur for current magnitudes and pulse multiplicities that are realistic simulation of those caused by natural lightning in the field. Further, it is clear that the capacity of an arrester to withstand the effects of multipulses is a function of the design, in particular of the dielectri varistor surfaces. So we multipulse test procedure (as an optional test) in the Standards will be of value and should result in arrester designs with an adequate capacity to withstand impulse currents from multiple-stroke lightning flashes.

Mr. Posey asks why we proposed change (multipulse test as an optional test) in the Standards. In some arrester designs varistor blocks are housed in hollow insulators (porcelain or in other polymer housing) where they are in contact with air or other material. These varistors in the arresters could undergo similar electrical stress as the blocks under test. Furthermore surface coatings of the blocks, or interface between the blocks and surrounding material are different due to the different design e various manufacturers. So this test proposal should p a method to test different designs and materials in order to detect weak designs.

The above proposal shouId also be see fact that some porcelain housed arresters designed and tested to pass a single 65kA impulse would fail on application of 5 to 9kA multipulses. In some cases failure occurred before all 5 impulses were complet

Both discussers note that eported in this paper refer to varistor blocks from one manufacturer. However, in the preceding paper [I], we described tests on varistors of several makes and all experienced multipulse failures with 8/20 ps currents in the range 5 to 9kA.

Reference was also made to reports of multipulse termination points for multiple-strok ing flashes to ground. Several groups have reporteJ thi g (as well as Uman and his colleagues) - but all hav ed that the proportion of lightnings with multiple termination points does not exceed 35 to 40%. So multiple currents are a common experience for arresters.

A common theme amongst most comments we have received (at various forums) ab0

erties of the coating on the that the proposal to include a

ulse studies is that there

seems to be no compelling reason for them from field experience with metal-oxide arrester failures. We are not convinced that this is correct, eg. we agree that problems due to moisture ingress should be less for polymer housed arresters than for those with porcelain housings. But only time in service will prove if this is really so. In fact, our continuing research project is to correlate failure modes produced in the laboratory with failure modes encountered by arresters in the field. This work is being conducted now with the co-operation of about fifteen utilities, who will send us their failed and sumect arresters for examination and test. Manuscript received November 20, 1996.

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On a general note, reference is also made to a paper [Cl] by one of the authors describing failure modes in various types of surge protection.

[Cl] M. Darveniza, “Failure Modes of Surge Protective Devices”, 23rd. Int. Conf. On Lightning Protection (ICLP), Florence, Sept. 1996, Paper 6.6, Vol. 11, pp. 640-645.

Arrester Zno

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