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Potential Transformers in series

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LAN_403

kilovolt

Fri Oct 24 2014, 11:06AM

Registered Member #2018 Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74

Dear highvoltage friends out there

recently I got the following two resin potted potential transformers for free from a power company:

6364

They have a turn ratio of 16'000V:100V each, but actually they are designed for 24kV permanent operation. At first I wanted to connect the high voltage windings in series for getting up to 48kV, but then I thought this might be a bad idea, because the potential against earth doubles as well, and there is always a capacitive coupling between earth and the high voltage winding, even though these are isolated transformers with two high voltage bushings. What do you think: Is it a risk to connect them at the high voltage side in series? Shall I better parallel them for double current output?

Thanks for your advice.

Best regards
kilovolt

hen918

Fri Oct 24 2014, 01:14PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

You are correct, there could be a risk of insulation failure, I would parallel them and use them along way away from anything remotely flammable, conductive or human!

Patrick

Fri Oct 24 2014, 03:25PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

if in series, the high side one is likely to see secondary-to-primary insulation failure. im suspicious, they say 24kv contiouous use. but id worry with two beautiful transformers, you might kill one or both.

Thomas W

Fri Oct 24 2014, 03:50PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

I would suggest asking this guy: high1voltage1rules. Hes one of my friends and has a fair few PTs and good experiance with them...

...

Fri Oct 24 2014, 05:14PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I am unsure how those are wound, but you can almost always get away with putting 2 transformers in series, so long as you ground the midpoint instead of one end. Then each transformer only sees its normal rated voltage, but the voltage measured end to end will be double.

Patrick

Fri Oct 24 2014, 09:11PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

As long as it's mid-point grounded. Then it would be like a NST, but he said 48kv.

Sigurthr

Sat Oct 25 2014, 01:38AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Midpoint ground them, too beautiful to risk destruction on.

kilovolt

Sat Oct 25 2014, 03:39PM

Registered Member #2018 Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74

Hello All

Many thanks for your advices. I think, I will connect them only in parallel.
I'm not sure, if midpointgrounded is okay, because in normal operation, such a transformer is driven with floating highvoltage winding and if I'm right, this means half of the voltage on every terminal against earth (capacitive coupled). If one highvoltage terminal of every transformer is grounded, the other terminals have the full potential of one transformer against earth.

Thanks for your help.
Best regards
kilovolt

radiotech

Wed Oct 29 2014, 09:23AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

PTs are intended to operate at a standard burden on the LV to keep core flux at a level to
ensure accuracy.

Standard practice does see secondaries in series, however they are connected to
a constant voltage source limited to the nameplate voltage, with a BIL rating
perchance the line is hit by lightning.

Somewhere, there is a table, like the one below, to better describe how the
unit is wound, inductance wise.

PT secondary inductance is thousands of Henrys.

kilovolt

Wed Oct 29 2014, 10:15AM

Registered Member #2018 Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74

Hello radiotech

Thank you for your interesting answer.

Standard practice does see secondaries in series, however they are connected to
a constant voltage source limited to the nameplate voltage

Do you mean the configuration in a threephase grid on the high voltage side, right? I think this is another case, because, as you state as well, they are connected to a constant voltage which doesn't exceed the rated value of the transformer against earth potential. In my case, if I connect them in series, the voltage against earth is not defined.

Unfortunately I'm not able to interpret your table.

Best regards
kilovolt

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