# Reality check



## wildleg (Apr 12, 2009)

seems ok to me. you need to make sure there's no factory bond to case, though, and bond case to ground, right ?


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## ampman (Apr 2, 2009)

wow a 750 kva thats big what is bil


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## RIVETER (Sep 26, 2009)

Did you have to do any drying out of the windings since it was so old?


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## nitro71 (Sep 17, 2009)

wildleg said:


> seems ok to me. you need to make sure there's no factory bond to case, though, and bond case to ground, right ?


Wildleg is on to something here. How do you create a ground path on the secondary side of this transformer. Seems like this is creating a ungrounded(might be off on my terminology) system.

Can you take one of the H leads and create a ground off of it?


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## JohnR (Apr 12, 2010)

Is there any way to center tap one of the phases? Tap 5 off the B leg should do it. so long as you don't need modification.

I was thinking of trying this so I could put my Genset at the back of my shed with a couple of 15KVA's but never did. I never got the second tran

Make sure you put a 100W lightbulb under the Tranny for a few hours to dry out the windings before cranking it up.


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## JohnR (Apr 12, 2010)

ampman said:


> wow a 750 kva thats big what is bil


as it says on a FP site, 


*Basic Impulse Insulation Levels (BIL)*​ 



Outdoor electrical distribution systems are subject to lightning surges. Even if the lightning strikes the line some distance from the transformer, voltage surges can travel down the line and into the transformer. High voltage switches and circuit breakers can also create similar voltage surges when they are opened and closed. Both types of surges have steep wave fronts and can be very damaging to electrical equipment . To minimize the effects of these surges, the electrical system is protected by lighting arresters but they do not completely eliminate the surge from reaching the transformer. The basic impulse level (BIL) of the transformer measures its ability to withstand these surges. All 600 volt and below transformers are related 10 KV BIL. The 2400 and 4160 volt transformers are rated 25 KV BIL.


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## Mike in Canada (Jun 27, 2010)

The transformer has been sitting inside a dry factory for months, so I wasn't planning on taking any special pains in drying it out. The case is bonded, certainly. I'll open it up again today and look it over before I put power to it. The transformer is being reinstalled after being moved from another state maybe 7 months ago, and is part of the same system it was running before, so presumably it was working just dandy in the past doing the same job. It all 'looks' good, but big transformers just worry be a bit. They just seem so... fragile.

Mike


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## drsparky (Nov 13, 2008)

I don't suppose they put the shipping bolts back in before they moved it.


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## Zog (Apr 15, 2009)

Mike in Canada said:


> The transformer has been sitting inside a dry factory for months, so I wasn't planning on taking any special pains in drying it out. The case is bonded, certainly. I'll open it up again today and look it over before I put power to it. The transformer is being reinstalled after being moved from another state maybe 7 months ago, and is part of the same system it was running before, so presumably it was working just dandy in the past doing the same job. It all 'looks' good, but big transformers just worry be a bit. They just seem so... fragile.
> 
> Mike


You arre testing it before you energize right? TTR, IR, DAR, PI, and winding resistance at the very least.


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## RIVETER (Sep 26, 2009)

Zog said:


> You arre testing it before you energize right? TTR, IR, DAR, PI, and winding resistance at the very least.


I agree with you but what does all those other letters mean?


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## Zog (Apr 15, 2009)

RIVETER said:


> I agree with you but what does all those other letters mean?


Transformer turns ratio
Insulation Resistance
polarization Index
Dielectric Abrosbtion Ratio


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## Navyguy (Mar 15, 2010)

This is a standard Delta / Wye transformer, nothing special except if I am reading the tag and your post correctly you are using it backward.

Am I correct or have I misread something?

Cheers


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## micromind (Aug 11, 2007)

This connection is the same as using a 480∆ - 208/120Y being fed with 208. 

I've done this several times. The largest one I've done was a 150KVA, but it'd be the same with a larger one. 

The connection is simple, just connect the 208 to X1, X2, and X3, leave X0 unconnected, and remove the ground jumper from X0 if there is one. 

The high voltage side will need to be grounded in some fashion. You can simply ground any one of the H terminals, I usually ground H2. This gives you a grounded B system. 

Another way is to use a ground monitor. It's a device that has a wye-connected grounding transformer, some electronics, and a few relay outputs. You can use the relay outputs for an alarm, or to shunt-trip the feeder breaker. 

Reverse feeding of transformers like this is very common at power plants. Almost all transformers over about 1KVA can be reverse fed without problems. 

One slight issue that I can see is the transformer is designed for 440 volts, and if the source is actually 480, you might end up with more than 600 on the high side, even if the taps are all the way at the ends of the windings. (1&2).


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## Mike in Canada (Jun 27, 2010)

micromind said:


> The connection is simple, just connect the 208 to X1, X2, and X3, leave X0 unconnected, and remove the ground jumper from X0 if there is one.


So far so good...



> The high voltage side will need to be grounded in some fashion. You can simply ground any one of the H terminals, I usually ground H2. This gives you a grounded B system.


This seems odd. If one 'corner' of the delta output is grounded then how will I be able to use it for three phase output? This transformer is running a 350A 600V system with many, many, motors. 



> One slight issue that I can see is the transformer is designed for 440 volts, and if the source is actually 480, you might end up with more than 600 on the high side, even if the taps are all the way at the ends of the windings. (1&2).


 This is a very serious issue, indeed, and one that I've been stewing about for two months. We had the power company come in the turn down the taps on the substation because it had been running over 490V, and now at least it's down to 480, which is at least within hailing distance of 440. I'm only really worried about the control transformers. 

The real kicker, here, is that due to supply problems on the part of the customer (who is supplying the parts) the materials to hook up to the switchgear only arrived this week, and because they had to 'prepare' for it being down the install happened on Friday. I get on a plane on Monday and fly away from here. I have no opportunity for more materials, or even much testing. I'll be working tomorrow (Sunday) and all night until I get on the plane on Monday. I'm way, way behind for various reasons (partly because I'm lousy at estimating how long these things should take) so I won't have all day to play with the transformer. I think, honestly, the only testing it's going to get is an ohmmeter from H to X and that's about it.

This particular job has been a toughie... I was called away in the middle of it for three weeks, and despite the many phone calls to 'the lads' wherein they claimed everything was delightful, little got done and much had to be fixed when I got back, and that killed my scheduling dead. I've got a wife pregant with twins at home that I haven't seen in far, far too long, and now I'm starting to rant. I should stop now. Anyway, thanks for your help. I'm not understanding why the tranformer needs to be corner-grounded, though.

Mike


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## micromind (Aug 11, 2007)

I don't know about the Canadian code, but the USA code requires every system (with a few exceptions) over 50 volts to be grounded. 

If you ground one of the phases of a transformer, you still have 3 phase power, and can still use a 3 phase panel and 3 pole breakers. There's no neutral though, as it is a delta connection. 

The voltages of your system (if phase B is grounded) will be A-B=600V, B-C=600V, C-A=600V; and A-GND=600V, B-GND=0V, C-GND=600V. 

This system (though 240 or 480 V around here) used to be somewhat common in industrial buildings, but is more rare these days. 

It takes a bit of getting used to, especially if it uses a single phase panel (completely legal, provided the breakers aren't slash rated, like 120/240, but are straight rated, like 240), but in my opinion, it's safer to have a system grounded than to let it float.


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## brian john (Mar 11, 2007)

Grounding one corner of the secondary at 600 VAC will not affect the ability to feed 3 phase loads, the voltage is still 600 VAC nominal phase to phase but isA to ground 600 VAC, B to ground 600 VAC, C to ground "0" VAC (assuming you ground the "C"phase.


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## mattsilkwood (Sep 21, 2008)

brian john said:


> Grounding one corner of the secondary at 600 VAC will not affect the ability to feed 3 phase loads, the voltage is still 600 VAC nominal phase to phase but isA to ground 600 VAC, B to ground 600 VAC, C to ground "0" VAC (assuming you ground the "C"phase.


 And it will trip the breaker under a ground fault condition.


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## Mike in Canada (Jun 27, 2010)

I asked the customer how they felt about a corner-grounded system, and they said, paraphrased, 'hell, no!' so that isn't happening. I left it ungrounded (with the case bonded, of course) and that should do it, yes?

Mike


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## Bob Badger (Apr 19, 2009)

Mike in Canada said:


> I asked the customer how they felt about a corner-grounded system, and they said, paraphrased, 'hell, no!' so that isn't happening. I left it ungrounded (with the case bonded, of course) and that should do it, yes?
> 
> Mike


Nope.

Here under the NEC you would be required to provide ground fault indicators.


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## Bob Badger (Apr 19, 2009)

micromind said:


> I don't know about the Canadian code, but the USA code requires every system (with a few exceptions) over 50 volts to be grounded.


I would not say 'few exceptions' 250.21 gives us some leeway but it also requires ground fault indicators.


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## Mike_586 (Mar 24, 2009)

Up here everything up to 150V has to be grounded, over 150V has to be either grounded or have ground fault indication.


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## Mike in Canada (Jun 27, 2010)

Just to give a bit more background, this transformer is powered by a 600A 3-phase circuit breaker in a 2000A Siemens switchgear, which is in turn powered by a 1200A 3-phase circuit breaker is a 1600A Square-D switchgear. Given the 3-phase circuit breakers is it still *mandatory* that the output of this transformer be corner-grounded? The customer considers corner-grounding to be a safety hazard. None of the other sources in this plant are corner-grounded except one, which I was told is 'very old' and will be changed to 'normal' as soon as suitable work needs to be done.

Mike


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## oldtimer (Jun 10, 2010)

Mike in Canada said:


> Just to give a bit more background, this transformer is powered by a 600A 3-phase circuit breaker in a 2000A Siemens switchgear, which is in turn powered by a 1200A 3-phase circuit breaker is a 1600A Square-D switchgear. Given the 3-phase circuit breakers is it still *mandatory* that the output of this transformer be corner-grounded? The customer considers corner-grounding to be a safety hazard. None of the other sources in this plant are corner-grounded except one, which I was told is 'very old' and will be changed to 'normal' as soon as suitable work needs to be done.
> 
> Mike


 Why not call your inspector, and ask. He/She will give you the correct answer. Then you are covered.


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## dr electron (May 3, 2009)

I have a similar situation as this poster. A customer bought a warehouse/ office which has a 120/208 3 phase service. He has some equipment that he already owns that is 480 volt 3 phase. Rather than changing the entire service I thought about using a transformer backward but, as is mentioned here, was wondering about the best way to ground what will be the 480 volt output


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## HARRY304E (Sep 15, 2010)

dr electron said:


> I have a similar situation as this poster. A customer bought a warehouse/ office which has a 120/208 3 phase service. He has some equipment that he already owns that is 480 volt 3 phase. Rather than changing the entire service I thought about using a transformer backward but, as is mentioned here, was wondering about the best way to ground what will be the 480 volt output


Where have you been it has been 2 years twenty days since the last post in this thread..:laughing:


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## Wireless (Jan 22, 2007)

HARRY304E said:


> Where have you been it has been 2 years twenty days since the last post in this thread..:laughing:


He didn't have the similar situation then!


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## brian john (Mar 11, 2007)

dr electron said:


> I have a similar situation as this poster. A customer bought a warehouse/ office which has a 120/208 3 phase service. He has some equipment that he already owns that is 480 volt 3 phase. Rather than changing the entire service I thought about using a transformer backward but, as is mentioned here, was wondering about the best way to ground what will be the 480 volt output


THe 480 output will be 480.

Assuming you ground the system the voltage to ground, say "B" phase is grounded then

"A" to ground 480
"B" to ground 0
"C" to ground 480

"A" to "B" 480
"B" to "C" 480
"C" to "A" 480

The equipment could care less.

As I have posted in the past I would get a 208 delta to 480/277 wye then you have a ground wye where you have 277 available, if necessary.

But other than confusing for some electricians the back fed transformer will work adequately.


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## HARRY304E (Sep 15, 2010)

Wireless said:


> He didn't have the similar situation then!


He could have started his own thread...:whistling2:


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## brian john (Mar 11, 2007)

HARRY304E said:


> He could have started his own thread...:whistling2:


New post to old threads that are pertinent to a ongoing topic are fine IMO.


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