Equipment BIL rating and flashovers

[MEG1122]

Hello….. we have 34.5kV distribution lines where we have experienced multiple flashovers on the pole mounted reclosers in past few years. During all failures, we either had damaged interrupters or bottom part exploded. About 70% of the failures happened during Dust storm and raining weather conditions and rest of them during normal weather conditions. Upon further investigation, we found the surge arrester grounding was vandalized on most of the locations and footing resistance to be too high. We further found out that the BIL of the installed reclosers is 150kV, where the rest of the equipment on the system had 200kV BIL.

I understand that improving the structure footing resistance and ensuring proper arrester grounding is must. But can we continue with the 150kV BIL rated reclosers? My understanding is lower BIL rating will make these reclosers the weak link in whole system and make them vulnerable to flashovers. We ran the simulation and found out to limit voltage surge in relation to footing resistance we will need 12ohm footing resistance for 200kV rated recloser and 7ohm resistance for 150kV BIL.

Any thoughts on this will be much appreciated.

 

[RRaghunath]

The Surge arrester sizing is done based on the BIL of the equipment that the surge arrester is meant to protect.
IEC 60071-1 lists 145kV as well as 170kV.
So, if the flashovers are happening, it is time to recheck the Surge arrester sizing calculation.
If sizing of surge arresters is correct, whether equipment BIL is 150kV or 200kV, surge arrester is expected to fail but not the recloser.

 

[Kiribanda]

In my opinion 150kV BIL is acceptable for 34.5kV nominal (38kV rated maximum voltage for NA installations).
Your post says "During all failures, we either had damaged interrupters or bottom part exploded".
By bottom part did you mean the recloser or the ground lead disconnector (GLD) of surge arresters?
Because since this is 34.5kV distribution line, unless the SA does not have adequate energy capability,
it will explode. Also what is the grounding method of this distribution system?
Could you kindly upload a photo of the name plate of the subject surge arresters and also the re-closers?

 

[MEG1122]

Recloser:38kV, 150kV BIL recloser.
Arrester:30kV, 24.4 MCOV
Substation transformer is solidly grounded.

 

[RRaghunath]

I agree the problem lies mainly in high resistance in earthing of the Surge arresters ("ensuring proper arrester grounding is must.").
Surge arrester will become ineffective in protecting the main equipment if the earth resistance is high. High earth resistance translates in to high residual voltage across the surge arrester (which also means high voltage across the recloser) that can cause damage to the recloser.

 

[Kiribanda]

Recloser:38kV, 150kV BIL recloser.
Arrester:30kV, 24.4 MCOV
Above given ratings (recloser BIL & MCOV of arresters) are acceptable because your 34.5kV transformer is solidly grounded.
Your area may be having heavy lightning activities meaning you need Distribution Heavy Duty Class Arresters (DH) similar to old IEC Line Discharge Class 1.
Are these arresters DH type or otherwise?
Is the lightning ground flash density >4 in your area?

 

[bacon4life]

Although the lower BIL equipment is a weak link, also consider that over voltages from lightning attenuates with distance. A lighting strike near a piece of 200 kV BIL equipment seems more likely to flash over that 200 kV BIL equipment than 150 kV BIL equipment located a few spans away.