My transformer doesn't have a BIL rating on its nameplate but I assume that it is similar to other 10 KVA pole transformers. I am currently using a horn gap. It has rarely fired. I've been operating this coil for 3 years now with no maintenance required. I've had these MOVs sitting around so I thought that I would put them to use. It sounds like I should just stick with my safety gap. MOVs have always bothered me somewhat anyway because they are sacrificial in nature. "If it ain't broke, don't fix it".
Steve White
Cedar Rapids, Iowa
----- Original Message -----
From: "jimlux" <jimlux@xxxxxxxxxxxxx>
To: tesla@xxxxxxxxxx
Sent: Wednesday, November 27, 2019 10:58:09 AM
Subject: 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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