Volker Hinrichsen
Germany, Reg.-No. 382
Group 33
Pref. Subj. No. 1, Qu. 5.4
Voltage and temperature distribution along high-voltage metal-oxide arresters
As further explained in [1], 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
1,5 m ... 2 m. In case of an ideally linear voltage distribution the maximum non-uniformity factor umax/umean is
umax/umean = 1
with
umax ... voltage across the most stressed MO resistor
umean = utotal/n
with
utotal ..... 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), umax/umean easily reaches values of two and more. On the
other hand, the MO resistors show an increasingly resistive behaviour if u/umean reaches or exceeds values of
u/umean » 1,2. 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 umax/umean) 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. umax/umean = 1,19 compared to umax/umean = 1,21) 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 [1].
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
60099-4. 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.
1 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 1 of 3
Volker Hinrichsen
Germany, Reg.-No. 382
Group 33
Pref. Subj. No. 1, 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 1,20 ... 1,22, operating temperatures up to 15 K
above ambient temperature and a temperature spread of 10 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.
1
P/P0
0,8
0,6
150°C
130°C
115°C
0,4
0,2
0
0
200
400
600
800
1000
1200
t [h]
Fig. 1: Accelerated aging test with U = Uc at different temperatures
1
P/Po
0,8
U = 1.15 Uc
0,6
U = 1.20 Uc
U = 1.25 Uc
0,4
U = Uc
0,2
0
0
200
400
600
800
1000
1200
t [h]
Fig. 2: Accelerated aging test with J = 115 °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 1,20 ... 1,22 have successfully been in service for nearly
twenty years.
Page 2 of 3
Volker Hinrichsen
Germany, Reg.-No. 382
Group 33
Pref. Subj. No. 1, Qu. 5.4
References
[1]
Hinrichsen, Göhler, Lipken, Breilmann
Economical overvoltage protection by metal-oxide surge arresters integrated in 420-kV centre-break
disconnectors - Substation integration, design and test experience
CIGRÉ Conference Paris 2000, paper 33-104
[2]
Hinrichsen
Monitoring of High Voltage Metal Oxide Surge Arresters
VI Jornadas Internacionales de Aislamiento Eléctrico
Bilbao, 22./23.10.1997, Paper 6.4
Page 3 of 3
Voltage and Temperature Distribution of MO Arresters
General
1800
1600
1400
H [mm]
1200
1000
800
H= 1200 mm
H= 1465 mm
600
H= 1805 mm
400
200
0
0,7
0,8
0,9
1
1,1
1,2
1,3
1,4
1,5
1,6
U/Umittel
Þ Grading rings necessary for
arrester heights > 1,5 m ... 2 m
Surge Arresters and Limiters
EV HBA2 Hin 08.00
CIGRÉ 2000/333/1
Voltage and Temperature Distribution of MO Arresters
"Capacitive" vs. "Resistive" Grading
1
1
capacitive
resistive
capacitive
h / hmax
h / hmax
resistive
0
1
u / umean
0
1
u / umean
• Same values of umax/umean despite different amounts of resistive grading
• Making partly use of resistive grading leads to more compact grading systems
Þ Due to the effect of resistive grading not only the non-uniformity
factor of voltage but also the temperature distribution is important
Surge Arresters and Limiters
EV HBA2 Hin 08.00
CIGRÉ 2000/333/2
Voltage and Temperature Distribution of MO Arresters
Measurement of Temperature Distribution
3500
3500
3 2
5
1
4
3000
3000
Position of the full ring
Position of the full ring
2500
2500
Position of the half ring
Height / mm
Height / mm
Position of the half ring
2000
Position of the intermediate flange
1500
2000
Position of the intermediate flange
1500
1000
1000
500
500
4 25 3
1
0
0,4
0,6
0,8
0
1
1,2
1,4
1,6
1,8
U/Umean
Fig. 7: Calculated voltage distribution along the
arrester (Ur = 336 kV) at applied maximum
line-to-earth voltage of the system (Um = 420 kV)
Surge Arresters and Limiters
2
0
2
4
6
8
10
12
14
dT / K
Fig. 9: Measured temperature distribution along the
arrester (Ur = 336 kV) at applied maximum line-toearth voltage of the system (Um = 420 kV)
EV HBA2 Hin 08.00
CIGRÉ 2000/333/3
Voltage and Temperature Distribution of MO Arresters
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 umax/umean
Þ Absolute values of electric field stress not decisive
Surge Arresters and Limiters
EV HBA2 Hin 08.00
CIGRÉ 2000/333/4
Voltage and Temperature Distribution of MO Arresters
Criteria for Effective Grading - Aging of MO resistors
Accelerated aging test::
• Part of operating duty test acc. to IEC 60099-4
• Quality assurance for running MO production
Standard test conditions:
• J = 115 ºC, U = 1,05 ·Uc, t = 1000 h (6 weeks)
Acce le ra te d Ag ing Te s t a cc. to IEC 99-4
2
1,8
Normal Behaviour
1,6
Aging Resistor
1,4
P/Po
1,2
1
0,8
0,6
0,4
0,2
0
0
5
10
15
20
25
30
35
40
sqrt (t) [sqrt (h)]
Surge Arresters and Limiters
EV HBA2 Hin 08.00
CIGRÉ 2000/333/5
Voltage and Temperature Distribution of MO Arresters
Criteria for Effective Grading - Aging of MO resistors
Accelerated aging test under non-standard conditions:
1
Influence of elevated temperature
P/P0
0,8
0,6
150°C
130°C
115°C
0,4
0,2
0
0
200
400
600
800
1000
1200
t [h]
1
Influence of increased voltage
P/Po
0,8
U = 1.15 Uc
0,6
U = 1.20 Uc
U = 1.25 Uc
0,4
U = Uc
0,2
0
0
200
400
600
800
1000
1200
t [h]
Surge Arresters and Limiters
EV HBA2 Hin 08.00
CIGRÉ 2000/333/6
Voltage and Temperature Distribution of MO Arresters
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.
Þ Aging performance not affected by
higher voltages and temperatures
Surge Arresters and Limiters
EV HBA2 Hin 08.00
CIGRÉ 2000/333/7
Voltage and Temperature Distribution of MO Arresters
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.
Surge Arresters and Limiters
EV HBA2 Hin 08.00
CIGRÉ 2000/333/8
Voltage and Temperature Distribution of MO Arresters
Conclusions
Nearly 20 years of positive service experience with:
• umax/umean in the range of 1,20 ... 1,22
• operating overtemperature up to 15 K
• temperature spread within the arrester up to 10 K
(All values measured under worst case conditions in the laboratory)
Surge Arresters and Limiters
EV HBA2 Hin 08.00
CIGRÉ 2000/333/9