How to test surge arresters

It requires special test equipment that reads in the teraohm range

See http://www.suparule.com/docs/MI2077TeraOhm5%20kV.pdf for example

The 64 kvs arrestors are the most sensitive and will be the first to activate. If these are in series. They will usually be the only ones to activate.

Thank you for the link. That testor is next on my wisk list.

I have been using a very similar MEGGER that goes to 5kV and measures the Teraohm range like the one you mentioned. Is this really the BEST test method for these arresters?

It is the only way that I know.

Dear Sir,

please give me some idea that how can i test the Surge arrester with the help of Megger,

You didn't mention whether or not the "stacked" 318 kV arresters are equipped with external grading rings but they certainly should be. Also unknown is the age, class or type of surge arrester (presumably station class).

The bottom 64 kV unit may be more susceptible to failure due to non-uniform voltage grading from line to ground. Ideally, it should be uniform from top to bottom of the stack but that may not always be the case and a disproportionate share of the line to ground voltage will appear across the most capacitively decoupled bottom unit. External contamination can also upset both the internal and external grading systems if the arresters contain internal gaps.

The most effective means of evaluating the internal condition of the arresters is to measure the internal grading current. However, it is always best to have a benchmark with which to compare. Best practice is to measure and record the arrester grading current at installation when new then routinely monitor it from time to time throughout its service life, under the same test conditions (clean, dry, same environment, etc.).

One effective means of measuring and monitoring arrester grading current is to ground the arrester through a milliammeter in the ground lead. That is sometimes done with a discharge counter (to count the number of arrester operations) containing an integral milliammeter. Either requires that the arrester be mounted on an insulating sub-base to isolate it from the grounded structure (transformer tank, in this case) such that the arrester is grounded only through its ground lead. It's important to recognize that the ground lead milliammeter will measure both the internal grading current and external leakage currents.

There are some commercial surge arrester testers on the market but you might want to take a look at www.doble.com. Doble Engineering has been involved with surge arrester testing and monitoring for many years and may be able to help you. They also have performance records and data on various surge arresters.

Good luck.

Yes they are station class GE Tranquell arresters. And they are equiped with corona rings. I am at a very large plant so we have numerous sets of this particular arrester station and all their ages vary form brand new to about 6 years old.

DOBLE testing looks like the method of choice for arrester testing. It would definitely be more comprehensive than a MEGGER test. The fact that it uses a 60Hz source at 10kV makes it superior to a 5kV DC source when testing arresters made to perform at 64kVrms and above.

My only question is what did you mean by "the most capacitively decoupled bottom unit"? And how exactly would this affect voltage drops?

Thanks for the great input thus far.

The line end corona rings that you mention perform two functions. They do shield the line end casting and connections but, more importantly, they also provide external capacitive voltage grading (grading rings). I mentioned previously that the ideal surge arrester will have uniform voltage distribution both internally and externally such that the voltage drop per unit length (per MOV block in this case) will be identical from top to bottom under normal operating conditions. That is seldom actually achieved in practice so there is usually some variation. Optimal grading of long stacked individual units is usually more difficult to achieve due to the effects of capacitance coupling through the metal end castings between units.

Although the conditions that resulted in failure were not disclosed, it was noted that the failures have all been bottom units. That suggests the duty on the blocks in the lower unit is more severe under the conditions that caused failure.

An electrostatic field plot would show the equipotential field and the resulting voltage distribution over the arrester length. The bottom unit would normally be most decoupled simply because it is the furthest from the energized line and therefore the least influenced.

I would suggest contacting the Ohio Brass Company (www.hubbellpowersystems.com) for assistance. You could also call Mike Comber at the Ohio Brass Company in Wadsworth, OH USA at (330)-335-2361 extension 1329. I'm sure he can help you.

I have never heard of the capacitive voltage grading that you speak of. Where exactly is this capacitance and how does it affect the operation of the arrester stack? Usually line capacitance doesn't become a factor until you have many miles of power line.

Trust me. The surge arrester housing is a capacitor and the internal metal-oxide-varistor (MOV) blocks are almost pure capacitors at their MCOV (Maximum Continuous Operating Voltage) rating. If one were to energize the arrester at its MCOV rating and monitor grading current through the arrester one would find that current leads the applied MCOV AC voltage by almost exactly 90 degrees. If that voltage is then increased until the non-linear MOV blocks begin to conduct, the current will shift and at full conduction the current through the arrester is entirely resistive. In fact the arrester conduction starting voltage is defined in industry standards as the AC RMS voltage at which the resistive component of current = 1.0 milliamp crest.

In addition the the inherent capacitance of the arrester, stray capacitance due to surrounding energized components is also coupled to the arrester.

Very informative post. I didn't realize that was how an MOV worked. I assumed they were purely resistive at their MCOV. Thanks for your help.