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GrantX

Thu Jun 13 2013, 03:36AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

So I got my hands on a brand new 14.4kV 1500VA PT recently. It had a 120:1 ratio and an insulation class of 15kV (BIL 110kV), so I decided to power it at 115V input from an isolation transformer in order to avoid any damage.

However, after about 30 mins of accumulated runtime the secondary failed short for no apparent reason. At the time I had the PT ballasted for a maximum of 7A @ 115V, and was testing a 1nF string of capacitors. I placed the capacitor string in series with the PT's output, and used a grounded chicken stick to check that the caps could survive the full 14kV across it. The poorly made capacitor string survived fine. The PT did not. As soon as I drew some faint sparks from the series capacitors I heard the PT flash over internally with a horrible muffled grinding sound.

I confirmed that the capacitors weren't anywhere near resonant value, and I double-checked that only 115V ever made it to the PT. I guess it had to be a manufacturing fault of some kind and I just got really unlucky? I've never heard of a PT failing so easily before, and I've witnessed people throwing 240V into 120V PTs on Youtube without them failing instantly. My PT was brand new from Instrument Transformers Inc. part of GE, does anybody know if their transformers are low-quality all round, or did I just get a dodgy one?

I was going say "oh well, that's a thousand-dollar lesson" but I haven't really learned anything in particular...

EDIT: Is there perhaps a way to free the core and windings from the epoxy? I know that PTs have very large cores and instead of simply throwing away the 60kg inert lump, it would be nice to at least salvage something from the wreckage. Rewinding the transformer for a low-kilovolt output would be nice, sort of like a 'super-MOT'.

Perhaps I could build a chemical bath out of a large plastic tub and some liquid solvent? However, I wouldn't have a clue what to use to dissolve the epoxy, I've never been a chemistry whiz.

Wolfram

Sat Jun 15 2013, 07:22PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

I'm sorry for your loss.

How do you know it wasn't resonant with the leakage inductance of the transformer? From the description of what happened, it sounds like that can have been the problem.

GrantX

Sun Jun 16 2013, 04:53AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Wolfram wrote ...

I'm sorry for your loss.

How do you know it wasn't resonant with the leakage inductance of the transformer? From the description of what happened, it sounds like that can have been the problem.

True, I can't be 100% sure that there wasn't a resonant condition. The capacitor was 1.1nF (6x 6.8nF 4kV caps in series) and the PT was inductively limited back to 7 Amps (115x7 = 805VA). Using the same ballast inductors I managed to get a resonant condition at 45Vac input with a 30nF rolled polystyrene capacitor (IIRC).

Really not sure what went wrong. The exact model is: PTG5-1-110-1442F, by Instrument Transformers Inc. I was told by a youtuber that "all PTs are only rated for 100V in", but the datasheet confirms 120V/14.4kV so I don't think I was overvolting it.

At any rate, I'm not going to try importing another expensive transformer, instead I've bought a large amorphous metglass core with a 150mm x 150mm winding window, just in case I can't get to the PT's core. Any suggestions on how to free the PT from its epoxy prison?

Ash Small

Sun Jun 16 2013, 10:54AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

GrantX wrote ...

Any suggestions on how to free the PT from its epoxy prison?

While there are solvents that will soften it, I'd suggest carefully applying heat. Most epoxies aren't rated for much over 100 degrees C, so it should be possible without damaging the core. You'll need good ventilation, though.

Antonio

Sun Jun 16 2013, 09:55PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Maybe boiling it in water until the epoxy softens, and then =carefully= cutting the epoxy away with a chisel. (I have a nice scar caused by doing exactly this...)

Steve Conner

Mon Jun 17 2013, 11:42AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If you bought it brand new, surely you got a warranty? I don't see how you could have exceeded the 110kV BIL in this experiment.

GrantX

Mon Jun 17 2013, 01:41PM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Steve Conner wrote ...

If you bought it brand new, surely you got a warranty? I don't see how you could have exceeded the 110kV BIL in this experiment.

"Brand new" from the factory, according to the Ebay seller, but they seemed reputable enough. Who knows.

I've had the transformer for about two months, in which I clocked about 30 minutes of run time with an arc lamp ballast and variac, never exceeding 120V. I'd tested a few caps at lower voltages without experiencing anything strange, and used resistive loads up to 120V input for several minutes on end without any sign of corona or unusual current draw.

I don't mind the idea of heating the transformer or boiling it in water, but I don't know if I currently have means necessary. I could use a hot air gun and chip away at it, but it will probably be tedious/messy. Shame I don't have a big enough oven!

HV Enthusiast

Mon Jun 17 2013, 02:59PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

I would contact the seller and inform him that the transformer is failed and they should be able to give you the manufacturer information.

If this is a GE potential transformer that is brand new, I find it very suspect that it would fail like this. These things are virtually indestructible. They are designed EXTREMELY ROBUST for utility power applications, so they have to be designed to take a HUGE beating.

If it failed that early, its either a manufacturing defect, or perhaps these were a bad lot that the seller got through surplus.

Dr. Dark Current

Mon Jun 17 2013, 07:43PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Hi
Sorry to hear that. Me and my friend have abused PTs really well, using large resonant capacitor banks, running the core well beyond saturation (an inductor in series with the LV winding allows for higher peak voltages than when running from a fixed voltage supply), the 22/sqrt(3) kV (12.7 kV AC nominal) transformers were outputting beyond 80 kV repeated peaks without any failure. So I guess your transformer had a manufacturing fault, for this reason I would contact the seller.

-Jan

johnf

Mon Jun 17 2013, 07:58PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Grantx
You now have shorted turns on your secondary. Even it you are not at resonance the peak voltage developed on the secondary output would have been well past thethe transformers ratings.
this gets worse with low resistance windings as the Q goes up along with the corresponding voltage developed across the caps. I have used this to resonate superconducting magnets to get the required voltage for extreme AC fields in the 3-4tesla @ 200hz. In this case I was using 10kV caps from Plastic Capacitors Inc an I was using an audio amp with an impedance step down transformer to drive the series resonant circuit of caps ans superconducting magnet windings. With a couple of hundred watts of audio power the caps were developing 5-6kV across them

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