Volker Hinrichsen Group 33 Germany, Reg.-No. 382 Pref. Subj. No. 1, Qu. 5.4

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1 Volker Hinrichsen Group 33 Germany, Reg.-No. 382 Pref. Subj. No., Qu. 5.4 Voltage and temperature distribution along high-voltage metal-oxide arresters As further explained in [], depending on the make and diameter of the MO resistors external grading rings are required to control the voltage distribution along high-voltage MO arresters having a length of more than,5 m... 2 m. In case of an ideally linear voltage distribution the maximum non-uniformity factor u max /u mean is u max /u mean = with u max... voltage across the most stressed MO resistor u mean = u total /n with u total... voltage applied to the arrester n... number of MO resistors in series Looking to a voltage distribution along the arrester, which is obtained from an electric field calculation (modelling the arrester exclusively by its capacitances), u max /u mean easily reaches values of two and more. On the other hand, the MO resistors show an increasingly resistive behaviour if u/u mean reaches or exceeds values of u/u mean,2. This results in a certain self grading effect of the arrester. The final voltage distribution is obtained by both the effects of grading rings ("capacitive grading") and making use of the self grading effect ("resistive grading"). A compromise has to be found to balance these two effects: A more effective capacitive grading is obtained by bigger and maybe more than one grading rings. But this may have technical as well as economical drawbacks. Grading rings, which give an optimized capacitive voltage distribution may reduce the withstand voltage of the arrester housing, and they may increase the possible minimum center line distance between the arresters. The impact of an intricate system of grading rings on the arrester cost is obvious. Making more use of the self grading effect leads to higher local electrical power losses and thus elevated operating temperatures of the arrester. According to the background given above it is evident that not only a maximum voltage stress across the MO resistors (i.e. the value of u max /u mean ) may be a criterion for an effective grading but also the arrester temperature, as a certain maximum non-uniformity factor may have been reached by different ratios of capacitive and resistive grading. Both calculations and measurements of the electric field or the voltage distribution respectively incorporate a couple of uncertainties. But while the differences in the resulting non-uniformity values seem to be negligible (e.g. u max /u mean =,9 compared to u max /u mean =,2) the resulting arrester temperatures may differ distinctly. This is the reason why a measurement of the temperature distribution (which is easily to perform and has negligible uncertainties only) within the arrester is a powerful tool to decide on the effectiveness of the grading system []. The question arises about the criterion for a sufficiently effective grading of an arrester. One point may be the electrical field stress with respect to internal partial discharges or dielectric stress of the supporting structure. The changes in the absolute values of local electric field stress with small differences in the maximum non-uniformity factor are negligible, however. Yet a major concern is possible electrical aging of the MO resistors by increased voltage stress as well as due to elevated operating temperatures. The aging performance of the MO resistors and its effect on the arrester performance (mainly with respect to the operating duty test) is subject of the accelerated aging procedure of IEC Test procedures for MO resistors stressed by voltages above their reference voltage are actually under discussion in WG 4 of IEC TC 37. It shall be pointed out here, however, that this is relevant only for those MO resistors, which have an unsufficient aging behaviour (i.e. an increase of power losses with time) even under the standard conditions. It is questionable in general if such a behaviour should be tolerated at all, as today's technology allows to manufacture MO resistors, which do not show this effect. Increased operating voltage as well as elevated temperature act as acceleration factors during the aging test. They will not affect the basic aging performance of the resistors, meaning that MO resistors showing decreasing power losses during the standard accelerated aging procedure will do so at higher temperature and/or voltage either, and vice versa in case of increasing power losses. Just the rate of change in power loss with time increases. This is demonstrated in Figs. and 2 [2], showing some results of investigations with accelerated aging tests at increased voltages and elevated temperatures, respectively. So, in conclusion, there is basically no risk operating high-quality MO resistors at higher voltages or temperatures. Page of 3

2 Volker Hinrichsen Group 33 Germany, Reg.-No. 382 Pref. Subj. No., Qu. 5.4 As another point the operating temperature of the arrester must not reach values, which might affect the performance under operating duty conditions. It is important to point out here that not a hot spot temperature anywhere within the arrester is the relvant parameter but the average temperature. Additionally, non-uniformities of the temperature distribution tend to unify, after the arrester has been heated by impulse energy absorption. It is rather difficult to give a statement on the ultimate limits of permissible maximum non-uniformity factors, operating temperatures and temperature distributions. Nearly twenty years of service experience, however, have shown that maximum non-uniformity factors in the range of,2...,22, operating temperatures up to 5 K above ambient temperature and a temperature spread of K within the arrester (all values obtained in the laboratory under worst case conditions, which are for instance: applied voltage equal to the continuos operating voltage, erection of the arrester with closest distances to earthed parts and directly on the ground) do not have a negative effect on the arrester performance and thus can be tolerated without any risk. P/P,8,6 3 C 5 C 5 C,4, t [ h ] Fig. : Accelerated aging test with U = U c at different temperatures P/Po,8,6 U =.5 Uc U = Uc,4 U =.25 Uc U =.2 Uc, t [ h ] Fig. 2: Accelerated aging test with = 5 C and different voltages Conclusion: The maximum non-uniformity factor is not the only criterion for effectiveness of the grading system. At least in those cases, where the voltage distribution is not only obtained from capacitive but also from resistive grading, also the arrester temperature is of importance. A certain amount of resistive grading is not only acceptable but even desirable, as it leads to a technically and economically optimized external grading system. Increased voltage stress and elevated operating temperatures do not affect the aging performance of the MO resistors, as long as they show decreasing power losses with time in the standard accelerated aging procedure. It is generally questionable if another behaviour should still be tolerated, as MO resistors with decreasing power losses represent the state of art today. Arresters with a maximum non-uniformity factor of,2...,22 have successfully been in service for nearly twenty years. Page 2 of 3

3 Volker Hinrichsen Group 33 Germany, Reg.-No. 382 Pref. Subj. No., Qu. 5.4 References [] Hinrichsen, Göhler, Lipken, Breilmann Economical overvoltage protection by metal-oxide surge arresters integrated in 42-kV centre-break disconnectors - Substation integration, design and test experience CIGRÉ Conference Paris 2, paper 33-4 [2] Hinrichsen Monitoring of High Voltage Metal Oxide Surge Arresters VI Jornadas Internacionales de Aislamiento Eléctrico Bilbao, 22./ , Paper 6.4 Page 3 of 3

4 General H [mm] H= 2 mm H= 465 mm H= 85 mm 2,7,8,9,,2,3,4,5,6 U/Umittel Grading rings necessary for arrester heights >,5 m... 2 m EV HBA2 Hin 8. CIGRÉ 2/333/

5 "Capacitive" vs. "Resistive" Grading resistive capacitive resistive capacitive h / h max h / h max u / u mean u / u mean Same values of u max /u mean despite different amounts of resistive grading Making partly use of resistive grading leads to more compact grading systems Due to the effect of resistive grading not only the non-uniformity factor of voltage but also the temperature distribution is important EV HBA2 Hin 8. CIGRÉ 2/333/2

6 Measurement of Temperature Distribution Position of the full ring Position of the half ring 25 Position of the full ring Position of the half ring Height / mm 2 5 Position of the intermediate flange Height / mm 2 5 Position of the intermediate flange ,4,6,8,2,4,6,8 2 U/U mean Fig. 7: Calculated voltage distribution along the arrester (U r = 336 kv) at applied maximum line-to-earth voltage of the system (U m = 42 kv) dt / K Fig. 9: Measured temperature distribution along the arrester (U r = 336 kv) at applied maximum line-toearth voltage of the system (U m = 42 kv) EV HBA2 Hin 8. CIGRÉ 2/333/3

7 Criteria for Effective Grading - Electric Field Stress Possible effects: risk of internal partial discharges dielectric stress of supporting structure Only small changes of absolute values of the electric field stress with changes of the maximum non-uniformity u max /u mean Absolute values of electric field stress not decisive EV HBA2 Hin 8. CIGRÉ 2/333/4

8 Criteria for Effective Grading - Aging of MO resistors Accelerated aging test:: Part of operating duty test acc. to IEC Quality assurance for running MO production Standard test conditions: = 5 ºC, U =,5 U c, t = h (6 weeks) Accelerated Aging Test acc. to IEC ,8,6,4 Normal Behaviour Aging Resistor,2 P/Po,8,6,4, sqrt (t) [sqrt (h)] EV HBA2 Hin 8. CIGRÉ 2/333/5

9 Criteria for Effective Grading - Aging of MO resistors Accelerated aging test under non-standard conditions: P/P,8,6 3 C 5 C 5 C,4, t [ h ] Influence of elevated temperature P/Po,8,6 U =.5 Uc U = Uc,4 U =.25 Uc U =.2 Uc, t [ h ] Influence of increased voltage EV HBA2 Hin 8. CIGRÉ 2/333/6

10 Criteria for Effective Grading - Aging of MO resistors Increased operating voltage as well as elevated temperature act as acceleration factors during the aging test. They will not affect the basic aging performance of the resistors, meaning that MO resistors showing decreasing power losses during the standard accelerated aging procedure will do so also at higher temperature and/or voltage, and vice versa in case of increasing power losses. Just the rate of change in power loss with time increases. Aging performance not affected by higher voltages and temperatures EV HBA2 Hin 8. CIGRÉ 2/333/7

11 Conclusions The maximum non-uniformity factor is not the only criterion for effectiveness of the grading system. At least in those cases, where the voltage distribution is not only obtained from capacitive but also from resistive grading, also the arrester temperature is of importance. A certain amount of resistive grading is not only acceptable but even desirable, as it leads to a technically and economically optimized external grading system. Increased voltage stress and elevated operating temperatures do not affect the aging performance of the MO resistors, as long as they show decreasing power losses with time in the standard accelerated aging procedure. It is generally questionable if another behaviour should still be tolerated, as MO resistors with decreasing power losses represent the state of art today. EV HBA2 Hin 8. CIGRÉ 2/333/8

12 Conclusions Nearly 2 years of positive service experience with: u max /u mean in the range of,2...,22 operating overtemperature up to 5 K temperature spread within the arrester up to K (All values measured under worst case conditions in the laboratory) EV HBA2 Hin 8. CIGRÉ 2/333/9

Damping resistor design consideration

http : //www.cigre.org CIGRÉ B4-054 CIGRÉ Winnipeg 2017 Colloquium Study Committees A3, B4 & D1 Winnipeg, Canada September 30 October 6, 201717 Damping resistor design consideration Elassad Yann MS Resistances