Transformer problems?

[bluesman70]

I was at a customers site and I was troubleshooting on the VFDs ( Variable Frequency Drives) AC of course.It was tripping out on high DC link voltage...anyway that not the issue.I measured the following on the motor.

L1-ground = 440 V AC
L2- ground = 440 V AC
L3- ground = 55 V AC

I went to the power distribution and it was the same there.
The first thought I had but not sure: They must have a open Delta Transformer where one leg is tied to ground..the L3 leg I believe...is that possible?.Or please tell me whats going on.If you can provide me a PDF with any form of drawing that would be great.Thanks so much!

 

[ScottyUK]

Sounds like a corner grounded system with a poor ground connection. Ain't something we see over here in Europe so hopefully one of the US contributors will pipe up soon.

FWIW some VFDs do not like the corner grounded system and trying to use on on such a supply leads to problems and or damage to the drive. VFDs are usually designed to work on a grounded star (wye) supply. The input filters are the cause of the problem if I recall correctly: I think there's some discussion on here somewhere but the search facility has hidden it.


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If we learn from our mistakes I'm getting a great education!

 

[bluesman70]

Hi Scotty
Thnaks for your reply.Yes you are right.I told the customer to not run the VFDs until they had solved this grounding problem due to damage.What do you mean by corner grounding system?.I was thinking about putting on input chokes but since there were 20 VFDs that would be expensive and not really solve the problem since the root cause to the problem is grounding.

 

[dpc]

Corner grounding - a delta system where one corner of the delta is grounded. But the voltage measurements don't seem consistent with this.

It would be interesting to know what the phase-phase voltage are.

 

[bluesman70]

Hi DPC
Well as far as I remember they were normal.
L1-L2= 485V
L1-L3= 486V
L2-L3 = 485V

Ya doesnt it seem like the L3 leg is grounded since it is 55 V ?.Thanks

 

[dpc]

As Scotty said, 55 V indicates some potential difference between the "grounded" corner and earth potential. It should be basically zero.

I'd check the grounding connections at the transformer(s) and at the service panel.

Corner-grounded deltas are a PITA. They are uncommon, so some "expert" may have removed the bonding jumper from the corner to ground, leaving the neutral to float.

Or you have what is supposed to be an ungrounded 480 V system that has one leg grounded somewhere, or nearly so.

If this is a utility service, they can tell what it is supposed to be - then troubleshooting can begin.

 

[opmgr1]

How is the transformer connected? GND star-delta, delta-delta, etc.
Is this a single 3-phase unit or 3-single phase units bank together?
If these are single-phase units bank together are there any of the units with secondary breakers (CSPs)?
Does this 3-phase source supply any single phase loads?
I have a theory as to what may be happening but need these issues clarified first.

 

[bluesman70]

Hi Opmgr1
Iam sorry I dont have this information.I just dont know,I was the technician that was troubleshooting on the VFDs and I know our equipment has to have a wye ( star) connection on the secondary side with the neutral connected to ground for the VFDs to operate safe..I guess if you measure from ground to L1,L2,L3 there should be 277V right ?. 480 / 1,73

thanks

 

[VTer]

There would be 480/277Y if it is a Wye connected transformer. In case of 480V corner grounded delta, with let’s assume L3 grounded at the transformer, the voltages should be:
L1-Ground = 480V
L2-Ground = 480V
L3-Ground = 0V
Also between any of the phases the voltage should be 480V as well. In this case you will not have 277V at all on your system. If you can, please verify transformer nameplate supplying the VFD. The nameplate should indicate connection type. My advice is to start troubleshooting from both sides of “your” system (transformer to VFD and VFD to transformer)if possible.


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[davidbeach]

It could still be a wye connected transformer, just not grounded. It is an ungrounded system, probably delta, but may be wye. Maybe it is supposed to be a corner grounded delta, but that isn't working well.

A properly maintained ungrounded system should show very close to the same voltage to ground from each phase, nominally 277V on a 480V system, but that won't make it suitable for use with VFDs that were designed for use only on a grounded wye system.

 

[rbulsara]

bluesman70:

Did you measure the L-G voltages with load (motor) on? or off?

Also how far away from the source (grounding point) did you measure them?

With load on and away from the source, you will measure the source voltage minus the voltage drop in the condutor due to the load current.

If the VD is indeed 55V, your readings seems somewhat consistent. 440+55=495V which could be the voltage at the source.

Measure the voltages with load off.

 

[sibiu]

I've had this same problem twice with a delta 480 ungrounded system. After isolating the problem the voltage came back to normal. My question is how to protect each individual machine because when this occurs it affects the whole factory and it take quiet some time to pin point where the short is.

 

[davidbeach]

Isolation transformer for each drive when on anything other than a solidly grounded wye system or drives suitable for use on ungrounded systems; those are your two options.

 

[bluesman70]

Hi everybody!
Thanks so much for your answers.They were all helpful.I found out that the whole building is ungrounded...this building was built in the late 50's and it never got grounded.Now I am understanding why the drives were faulting on high DC link voltage,because the voltage across the capacitors is way too high.The normal DC link voltage should be about 650V DC.There are 2 capacitors in series with a ground between them and with no ground this voltage will be up to over 900V DC with no grounded system..so this explains the high DC link voltage fault.Yes they are thinking now of getting isolation transformers.I dont know of drives that will work without problems without a ground system.

 

[waross]

Have you considered grounding the system? There may be a lot of resistance from some old timers to any change. If so, expect to hear a lot of arguments that do not have a valid technical basis. You may counter most of those arguments by reminding them of the issues and cost of not grounding the system. That would be high DC bus voltages, VFD damage and isolation transformers. It is often cheaper, quicker and more dependable to fix problems at the source (ground the ungrounded system) than to treat the symptoms (install isolation transformers).

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rockman7892]

I had similar problems with a drive tripping on a DC overvoltage link when the drive was not even running a load. This drive was on a HRG system. I figured the drive was tripping due to the flucuations on the incoming voltage due to some sort of transient upstream (cap switching etc...) All voltages on this 480V drive checked out fine.

After reading this thread I became more curious about this but cannot seem to understand how being ungrounded is causing the voltage across these capacitors to be high? I'm guessing that its just due to normal capacitive coupling to ground which is high and causing the drive to therefore have a high DC voltage.

 

[MagneticFlux]

davidbeach,

Im curious about your suggestion. Could you explain your thinking? I have seen in installation manuals where some VFDs will not function on delta systems. Why is that?

bluesman,

The 55 volts on the grounded leg, could it not be the result of a High Resistance Ground system? Have you checked to see if such a device is in the system? Not that it is important at this point, I believe davidbeach is onto the solution.

 

[davidbeach]

The isolation transformer will give you a grounded wye system for the VFD rather that it being connected to the ungrounded delta system.

The power electronics of most VFDs are designed with the assumption of a grounded wye system as the supply source, allowing for the use of lower voltage ratings on components than would be necessary for an ungrounded system.

 

[MagneticFlux]

davidbeach,

I should have been a more specific on the question, I understood the reasoning for the transformer, I was not certain as to why some VFDs will only work on a WYE system.

Thank you... that was worth a star.

I asked the motor/drive manufacturers tech dept. that very question and got a long winded "not completely sure" kinda answer that left me more puzzled...

 

[waross]

MagneticFlux;
The problem is not whether the system is delta or wye, it is the voltages that may be experienced and the voltage rating of the components.
A machine designed for use on 480 Volt, grounded wye must have components suitable for 277 Volts to ground.
A machine designed for a 480 Volt ungrounded delta system or a 480 Volt HRG wye system must have components suitable for 480 volts to ground.
A ground on one phase of a floating delta system or an HRG wye system will drive the voltage to ground of the other two phases to 480 Volts (line voltage).

Bill
--------------------
"Why not the best?"
Jimmy Carter