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My Christmas present to myself, Help choose!

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GrantX

Fri Nov 23 2012, 07:55AM

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

Well, turns out after buying all the Christmas gifts for the family I still have some of my budget left (amazingly). Anyway, coupled with an extra juicy paycheck this month I've decided that I need a new HV transformer, because one can never possess enough iron and oil.

Anyway, I've been chatting to a guy on ebay, who has a monster PT up for sale, unfortunately we can't figure out the exact configuration and capacity of this unit:
Ebay Listing

From what I can see, it's got 3 windings, 16kV and 2x 120V but I can't see a VA capacity anywhere, although at 72kg I guess it's a couple of kVA.

What do you think, is it worth it, or too overpriced for what it is? Honestly, this is the cheapest PT / polepig I've seen so far, so I'm very tempted.

dex

Fri Nov 23 2012, 09:27AM

Registered Member #2566 Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147

XravenorX wrote ...

Anyway, I've been chatting to a guy on ebay, who has a monster PT up for sale, unfortunately we can't figure out the exact configuration and capacity of this unit:
Ebay Listing

From what I can see, it's got 3 windings, 16kV and 2x 120V but I can't see a VA capacity anywhere, although at 72kg I guess it's a couple of kVA.

There should be given rated VA or rated current on the plate.Strange, I can't find it on the photos too..

Dr. Slack

Fri Nov 23 2012, 10:45AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

You can see the position on the plate that's got VA written next to it, but it's resolutely blank.

I guess this means that the original spec, and the size, are all down to isolation voltage, and the VA wasn't considered important. As long as it was sufficient to drive the downstream instruments, that was enough.

The high voltage winding will be a certain minimum thickness, just for mechanical integrity, so the HV VA will be substantial, I would imagine several kVA with that weight. However, they might have taken the opportunity to short-change the low voltage windings and use thinner wire. You could ask the seller to measure the DC resistance of all the windings. Obviously the internal geometry will affect the detail, but you'd be able to tell whether the LV windings are in the same ball-park of VA or not.

Shrad

Fri Nov 23 2012, 10:57AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

it must have been used with 16kVAC busbars in high power, high inertial inductive drives, like you find in big plants with rotating furnaces or mega blowers (like hundred-kilowatt range motors)

if so, it is certainly overrated and should withstand secondary currents of half an amp

basically it's an overrated power distribution measurement transformer

Steve Conner

Fri Nov 23 2012, 02:24PM

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

It is a measurement transformer, but how can you tell that it's overrated?

Answer: The "1.2Z" means that it's rated for 1.2% error with a burden of 200VA. Very roughly speaking, that implies that it has 1.2% (or is it 2.4%?) impedance on a 200VA basis, so you could shove something like 2kVA through it easy.

Also to be pedantic, the rated voltage is 16kV/sqrt(3) which is 9.2kV, and the rated secondary voltage is not 120V but 69.3V. (these VTs are intended to be used in a set of three, star connected)

Dr. Dark Current

Fri Nov 23 2012, 09:05PM

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

...but can often be overvoltaged to the phase-to-phase voltage, as they usually seem to be designed with pretty low flux density to be accurate (and there is a lot of voltage headroom before they saturate hard). This also means that with sqrt(3) times higher voltage, you get 3 times higher power output for (approximately) the same efficiency.

GrantX

Sat Nov 24 2012, 05:13AM

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

I think I'll ask for the DC resistance of the windings and go from there. I am a little concerned about it, as I would have to place both 63V windings in series and hope that it will perform well at 230V. I don't like the idea of overvolting a HV transformer so dramatically.

I found a small dry type PT which will end up costing about the same after shipping. 400VA, 22kV : 110V, which will also be problematic, since I will need to find a large enough step down transformer to power it. Heres the link: Ebay listing.

I've also found a pole pig, but it's ridiculously overpriced, and shipping would probably be insane: Ebay listing.

It's annoying that I already own the beefiest HV transformer my work currently produces.

GrantX

Wed Dec 05 2012, 03:22AM

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

Well, I did a bit more hunting and found a really nice brand new PT. One of those common red epoxy units, 14.4kV 120:1 1500VA. It was a bit cheaper than the other alternatives, and has a substantially higher thermal rating to boot. Purchased it today, awaiting the final shipping price.

Hopefully I'll have more luck receiving this than the lost capacitor :p

radiotech

Wed Dec 05 2012, 07:37AM

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

I would pay close heed to the PCB content. After the exporter has your money,
what do you have to do import it to Australia? It looks like an old unit and the oil
may have be cleaned a time or two.

GrantX

Sat Dec 08 2012, 05:13AM

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

Finished payment and have received a list of data for the transformer (not the original datasheet, something the seller put together I think). In addition to its thermal rating of 1500VA @ ambient temp it also has a labelled impedance of "0.3Z". As Steve Conner was saying, does that imply 0.3% system impedance at 1500VA? If so, then it seems it's an even heftier transformer than what I originally thought!

It's also labelled with a BIL of 110kV, and it's ground connection (for the HV coil, I believe) reads "insulated for 10kV test, 60 seconds". From this information it would appear that it's very well insulated. With a primary rating of 14.4kV, how much higher do you think I could take it without risking damage? I've seen similar PTs on youtube wired directly into 240V for almost 30kV output, but I wouldn't think that would be very easy on the transformer!

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