Maintenance on station class surge arrestors.

[Electic]

Has anyone heard replacement of 115kV arrestors (MOV type) is required on a periodical basis?

 

[itsmoked]

I cannot say on that particular case. But it does make sense to me as all MOVs only have a finite cycle life. They are slowly altered every time they shunt.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ELEP]

We do following maintenance,
1. Washing/cleaning periodically(9 month to 1 year) to avoid dust accumulation
2. Earth resistance measurement(2years)
3. Track of Earth leakage current & surge counter readings(monthly).
There might be earth leakage current indicator close to earthing point of Surge arrester, check if it has crossed the safe limit. Contact vendor for max leakage earth current.

 

[xabproject]

Electic,

SA are replaced based on following criteria in most networks:
1. When the Continuous / Steady State Leakage current as measured in the leakage current meter is 4/6/8/10 times the value at commissioning instant
2. More than twice the number of discharges in any specific period, as per the adopted maintenence period / practice, in comparison with another identical unit's activity within or in a nearby similar installation
3. Pre and Post monsoon readings of the counter exceeding expected values based on isokeraunic level of the installation.

Periodic replacement without regard to any one or all the above could be expensive.

Regards

 

[Electic]

Thank you Gentlemen. I appreciate these good answers that are not easy to find "out there".

 

[magoo2]

Unless you're in a coastal area or one that is subject to a lot of contamination, I don't think as a general practice most utilities monitor leakage current on 115 kV arresters. We don't and I know we're not alone in this regard.

 

[Electic]

Project is in a coastal area, however; I have been led to believe that current leakage across MOV substrate can be occur as a function of aging and usage (number of hits). Eventually the MOV will fail, usually as a short circuit. I was not aware of a Preventative Maintenance procedure to prevent this.

This current leakage would occur internally, and the units are supposed to be environmentally sealed (though there have been some problems with sealing) so I suspect that internal current leakage (failure to electrically seal) would not only be a coastal problem.

Thank You.

 

[dpc]

One nice thing about MOV arresters - when it fails, you will know it.

I heard an engineer who designed surge arresters say that it was a very simple product - no moving parts. And if it ever has any moving parts, you'd better get out of the way!

 

[itsmoked]

Except it's my understanding that they actually are a giant mass of moving parts. A bazillion little particle matrices that physically change each trigger. Hence the cycle life.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[electricpete]

Monotiring leakage current is simple IF provisions are provided when the arrester is installed (for example insulate lightning arrestor from transformer tank and run the ground connection in an insulated lead down to ground level for clamp-on check.

Do some arresters come with operation counters? I hadn't heard of that.

Doble testing is a good test for lightning arresters. Of course you've got to pay attention to the resutls. We had a lightning arrester fail. Went back and look at the trends... sure enough power factor had been trending up for several readings in a row.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

actually, I don't remember if it was power factor or watts loss that was trenidng up.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[majesus]

Monotiring leakage current is simple IF provisions are provided when the arrester is installed (for example insulate lightning arrestor from transformer tank and run the ground connection in an insulated lead down to ground level for clamp-on check.

Very interesting thread.
I am designing the gnd system for a 67kV Substation where there is two lightning arrestors (one just before the high side of the step-down XFMR and the other on the main incoming post).

http://i48.photobucket.com/albums/f223/majesus/gnd.jpg

I am sizing the earth connection for the MOVs and I was curious on how this is done. So I did a search. Interestly I read that the earth connection is suppose to be insulated and isolated from the XFMR tank. Why?

If I size a #4/0 AWG stranded bare copper run from the GND point of the MOVs down the side of the XFMR's tank to a gnd rod, will this be a problem?

 

[Electic]

A couple of comments:

We did have operation counters installed on a CT around the ground lead from the arrestors. These were valuable in determining whether the failed arrestor had extra duty.

The ground lead from the arrestor might be insulated for a couple of reasons:
it could help with testing the arrestor (though that could also be disconnected at the top during testing)

In the event of an arrestor discharge, large current flow will dissipate thru this conductor and it would be best to direct that to the ground grid and not thru the transformer tank. The failed arrestor in original question had burn marks across the small isolators at the foot of the arrestor (special devices for mounting) and I would imagine this would be worse if a bare 4/0 were leaning against the tank.

These are simply observations that might be confirmed if you speak to a transformer manufacturer.

 

[jghrist]

Monotiring leakage current is simple IF provisions are provided when the arrester is installed (for example insulate lightning arrestor from transformer tank and run the ground connection in an insulated lead down to ground level for clamp-on check.

If you insulate the ground lead, then the voltage across the transformer insulation during a lightning surge will include the L·di/dt drop in the ground lead. This practice may make testing easier, but it reduces the protective margin on the transformer. Perhaps a spark gap between the arrester base and the tank could be used to limit the voltage.

 

[majesus]

YUP... Makes sense with me... I wonder what insulation rating to use ... the Voltage potential shouldn't be too high as it is shunting current to GND and the Voltage the cable sees would be the Vdrop due to the cable's impedance.... Say a 4/0 AWG, non-shielded 5kV insulation rating shall be more than adequate.

 

[bacon4life]

We do Doble tests on them every couple of years when the associated equipment is out of service for testing. We don't have leakage current monitors or operation counters.