Query on Surge Arrestor Failure - Reg. 1

[Shiv K Kumar]

I have been working in the electrical construction, operation & maintenance field for about 35 years now. I have worked on all voltages from 240V to 420kV. All along my experience, I have never heard of a surge arrestor blast. For me, surge arrestor is such a device, which would act as an insulator until the voltage across it does not increase beyond its rated voltage. And, when the voltage across the surge arrestor goes beyond its rated voltage, the surge arrestor conducts the surge to earth/ground. But, as soon as the surge dies down, the surge arrestor reclaims its insulating properties and becomes an insulator again, ready for the next surge. Isn’t it true?



But, of late, I have been hearing & experiencing quite a lot of surge arrestor blasts, particularly, indoor polymeric surge arrestors, installed in Medium Voltage VCB Switchgear Panels. There is nothing wrong with the selection (viz.) the MCOV, the rated voltage, the nominal discharge current, the rated line discharge class, the rated pressure relief class, etc. The selection is based on manufacturer’s guidelines and on site conditions.



This blast phenomenon is more prevalent in:

i) 33kV Class Switchgear (though there are isolated incidences of SA failures/blasts at 12kV & 24kV)

ii) More so in Solar Photovoltaic Power Plants (At the same voltage level, there are no such failures reported from any other power plants’ or process industries’ installations)

iii) More when the cable length is more, say, of the order of a few kilometres

iv) Initially, there were recurring PT failures, but, later, when the PTs were strengthened with a higher Overvoltage Factor or when the PTs were removed, the Surge Arrestors started failing/blasting.

v) It is unique in VCB Switchgear Panels (At least, I am not aware of such failures with other types of switchgear).



Now, my questions are:

a) What is the reason for such surge arrestor blasts?

b) Why is it predominant only in 33kV Voltage Level?

c) Why is it unique in Solar Power Projects?

d) What has cable length got to do with it?

e) Could the PT failures also be for the same reason?

f) Is this phenomenon unique to switching with VCBs?

g) Doe it have anything to do with the contact material, contact profile, contact design in the vacuum interrupter?

h) How to avoid such failures/blasts?



I request your expert opinion on this.

 

[dpc]

Surge arresters can certainly fail and they generally fail in exciting ways. Arresters are intended to conduct for impulses - very high voltage but very short duration. Sustained overvoltage beyond the MCOV of the arrester can certainly cause explosion of the arrester. I have seen a few after the fact. As arrester engineer once told us - arresters are a simple product - no moving parts. If they have moving parts, you'd better not be in the way.

As to specific causes - many arresters are selected assuming a solidly-grounded system, using L-N voltage. If the system ever becomes ungrounded, arrester failure can result. PT failures brings to mind possibility of ferro-resonance - this can create high voltages as well. Extremely long, lightly loaded 33 kV cables can create high voltages due to the charging current.

Check arrester ratings. If local solid grounding cannot be assured, arrester MCOV should be at least 10% above maximum expected Line-to-Line voltage. Configure your relays to capture any high voltage events.