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Crack on Surge Arresters
2

Crack on Surge Arresters

Crack on Surge Arresters

(OP)
Almost all the surge arresters in all the feeders of MV switchgear (6kV) got cracks as shown in the picture.

What are the reasons for it apart from dirt, moisture and inadequate capacity?. The said surge arresters were in operation for just more than a year. We have planned to replace it with another surge arrester with the same dimension with increased MCOV? will that help? In general, when the surge arrester starts conducting due to over voltage, why dont the ground fault protection activate?

RE: Crack on Surge Arresters

The MOV may only conduct for a small fraction of a cycle. That can be too fast for the protection.

RE: Crack on Surge Arresters

It could be inadequate discharge duty rating also.
Better to subject a sample for failure analysis to know whether the failure is voltage related or current related, before going for deciding on new specs.

Rompicherla Raghunath

RE: Crack on Surge Arresters

Looks like they did their job. Time to replace :)

Maybe a lightning strike in the station?

RE: Crack on Surge Arresters

Ask the manufacture if they have seen it before and can give advice.
Questions to consider.
Is it in an area that would be likely hit by lightning?
Are they rated line to neutral and could be exposed to line to line voltage?
Are they rated high enough for the voltage (does the switch gear operate at higher than expected voltages)

RE: Crack on Surge Arresters

what is the material? is it a polymer or ceramic..?

RE: Crack on Surge Arresters

Over-tightening?

RE: Crack on Surge Arresters

What is the system earthing?

The surge arrestor selection should consider the system neutral point earthing. If there is a selection issues, then re-verify the rating vis-à-vis system neutral earthing.

RE: Crack on Surge Arresters

i think over tightening the lightenting arrestor. Maybe get a torque wrench and look at the specs are for this arrestor?

if it was lightening would not you have some burns or ozone residue?

RE: Crack on Surge Arresters

(OP)
The system voltage is 6kV, ungrounded and the arrester MCOV is 7.25kV and we have planned to replace it with 8.4kV and the manufacturer offers to replace it with the same dimension. I have a concern with similar dimensions or will there be difference in metal oxide block structure from the earlier ones?

RE: Crack on Surge Arresters

(OP)
There is a transformer in the station 35kV/6kV. If there is a lightning strike on 35kV lines, How will it affect the 6kV side? like normal step down ratio? We have these arresters installed in the 6kV Panel which is around 30 meters from the secondary of the transformer.

RE: Crack on Surge Arresters

The lightning arrestor appears to be an indoor type. i.e. mounted inside the switchgear cubicle.

If the cable is use from the transformer to switchboard, mostly the lightning surge gets attenuated. By the same token, the lightning struck at transformer primary, would not generally transfer to the secondary incomer, due to the attenuation, in the cable and transformer.

In an ungrounded system, an earth fault could result in such surge due to the arcing ground phenomena

Here you should also differentiate between the lighting and the surge. Though majority of the people use the term surge arrestor and lightening arrestor interchangeably, technically they are different. Normally in the cable connected switchgears you install the surge arrestor.

So review your application and the characteristics. Is it applied as lightning protection or as a surge protection? Then select the over voltage protective device accordingly.

RE: Crack on Surge Arresters

To add to Krisys, the lightning on primary of the transformer would be stopped by the lightning arresters connected to primary terminals of transformer.

Rompicherla Raghunath

RE: Crack on Surge Arresters

(OP)
Thank you Krisys for your explanation. These surge arresters are mounted inside the MV switchgear; All the feeders have surge arresters equipped with, Does the Switching transient voltages damages these arresters as it is switched ON/OFF by VCB's

RE: Crack on Surge Arresters

Over voltage due to VCB switching vis-à-vis the surge arrestor rating for the VCB feeder shall be based on the VCB manufacturer's recommendation.

RE: Crack on Surge Arresters

Without complete details difficult to find out RCA. Looks like failure from over voltages due to wrong selection of LA voltage class. LAs are used to mitigate surges from VCB. Where VCB is located, after or before VCB ? Please refer to any of the following Application guides that cover well the Selection and application of MV surge arresters. IEC 60099-5 ed3.0 -2018, IEEE C62.22-2009 .Also you can refer to a good ABB guide on the same subject from their site-Selection and application of MV arresters -http://inside.abb.com/cawp/gad00539/36bb5322dd98f5... - Also see www.arresterworks.com

RE: Crack on Surge Arresters

(OP)
Hi PRC,

The surge arrester is mounted before the VCB in Incoming VCB and after the VCB in outgoing feeders

RE: Crack on Surge Arresters

Please give the rating of the 6 kV VCB and also the transformer connection and earthing details (earth fault factor) on 6kv system.

RE: Crack on Surge Arresters

(OP)
The VCB rating is 7.2kV, 1250A, 25kA/3s and the transformer is of Y-Y ungrounded at both the primary and secondary. What is "earth fault factor"?

RE: Crack on Surge Arresters

Earth fault factor is the extent that line to ground voltage will rise in healthy phases with a line to ground fault in one phase. That is the voltage seen by LA. In your case it is 1.732 and with 1.1 factor, 7.2 seems OK. But many years back I had seen a case(before the metal oxide arrester days) where 11kV tertiary of an auto-transformer required 18kV to stop LA failures.

RE: Crack on Surge Arresters

"11kV ...required 18kV to stop LA failures" - I guess this solution to stop LA failures would have initiated cable / equipment failures in 11kV system.
I don't see it appropriate to install LA with MCOV of >12kV considering that the system (i.e. all the equipment installed in 11kV system) is designed with a max system voltage of 12kV.
It is the earth fault detection and removal process that needs to be improved and instead increasing the MCOV of LA could be counter-productive (by shifting the failures elsewhere).
I haven't worked in an ungrounded system but I read why the concept of ungrounded system has gone out of popularity and is replaced by high resistance or medium resistance grounding concepts. It is the difficulty in identifying the faulty feeder and consequent delay in detecting and removal of earth faults that give rise to 'arcing grounds' which can lead to overvoltages much higher than the equipment rating.
I thought the above will be helpful to OP as he is also planning to increase the MCOV of the LAs.

Rompicherla Raghunath

RE: Crack on Surge Arresters

In my view it is better to review the system grounding and provide at least high resistance grounding if not low resistance grounding. Now a days in the medium voltage you may not find many installations with ungrounded neutral.

In the low voltage the unrounded systems are still used, for the specific (critical???) applications. But in MV and HV I don't envisage any justification to have an ungrounded system.

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