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Re: [TCML] MOVs for pole transformer

On 11/27/19 8:46 AM, Steve White wrote:

I have about twenty 1800 volt MOVs that I bought several years ago. I am considering replacing the safety gap of my pole transformer powered RSGTC with an arrester made from these MOVs. The RMS output of my 10 KVA pole transformer is 14.4 KV. What peak voltage value should I use for the MOV string?



I'd stick with the safety gap.
MOVs die a bit with each overvoltage. They start leaking more and more current, until they melt or catch fire from the dissipation.
A classic design error is to put the voltage on something like a 120V power "surge protector" a bit too low (like 180V or 200V), so they keep taking hits. Then they get hot and burn up.
Your RMS voltage is 14.4, so the peak voltage is 20.4kV. A 30kV string might be ok. But, consider that a pole transformer is designed for transients (the BIL might be 90kV or higher) - so the question would be "what is the MOV protecting?" 

https://www.powerpartners-usa.com/wp-content/uploads/2016/01/single-phase_product_spec_sheet.pdf?x57229
Safety gaps are essential on NSTs - they are designed to be low cost, and at the ragged edge of failure, insulation wise.
On a pole transformer, I think a safety gap is more of a diagnostic tool - if your safety gap is firing, something in the system is out of tune. 


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