High Leg on Delta-Delta Transformer causing problems

[jjdad]

I was called in to review a problem with a delta-delta transformer that is connected to 240V 3-phase in and running 480V 3-phase out. This transformer is only feeding one machine that requires 480V 3-phase.

The Buss feeding the transformer apparently has a grounded delta, the voltage between each leg is 240V, phase to phase. Voltage to neutral is 120V at legs A & C, but it is 240V between leg B and neutral.

The transfomer only has the three phase wires feeding both in and out of the transformer. No neutral is connected to the load, or the transformer.

The problem is the machine will trip the main breaker to the packaging area. This breaker is two breakers upstream from the breaker feeding the transformer. The transformer breaker doesn't trip. When the breaker trips, the middle leg of the breaker sparks excessivly.

While I understand the issues with a grounded delta, what could be causing the tripping? Is it possible, something at the machine is grounded improperly, or simply an overload?

What is interesting, is the machine runs most of the time, it just trips off occasionally.

Thanks for the help!

 

[rbulsara]

jjdad:

Try answering rock1's questions.

 

[jjdad]

Answering previous questions and today's results.

Here is the one line diagram

200A Main 240V three phase delta system with "high leg"
This breaker feeds
|
Distribution panel 1 with multiple breakers
100A breaker (A) in this panels feeds
|
Distribution panel 2 with multiple breakers
100A breaker (B) in this panel feeds
|
Distribution panel 3 with multiple breakers
100A breaker (C) in this panel feeds
|
30kVA Delta-Delta Tranformer 240 to 480 volt step up

The breaker that always trips is 100A breaker (A) in distribution panel 1.

We shut everything off in all these panels, except the circuit feeding the transformer. Turned the tranformer breaker on and it stayed on. Nothing trip. At least during this test.

The Breaker (C) was on and the system was working properly.

Amperage readings on the 240V side of the transformer
A - 11.1
B - 10.3
C - 3.8

Amperage readings on the 480V side of the transformer

A - 4.1
B - 4.1
C - 0

Voltage measurements on 480 Volts side

AB - 491
BC - 492
AC - 496

Voltage measurements on 480 volt side to ground
Remember this transformer is not grounded nor does it have a neutral connected anywhere in this circuit either. This measurement was taken at the incoming power disconnect on the 480v machine. The ground wire runs all the way back to the distribution panel 3. The ground wire does not go through the transformer.

A-G 278
B-G 245
C-G 327

We shut off breaker (c) waited a minute then turned it back on. It tripped. There was nothing else on in distribution panels 1, 2 or 3. We reset the breakers then tried again. It tripped. But sometimes it will hold.

Breaker (A) trips about 80% of the time when you turn Breaker (C) on to the transformer. Breaker (B) NEVER trips.

The machine is the only device connected to the step transformer. The voltage and amperage readings are what the systems sees when the breaker is turned on and the 480 volt circuit is seeing with the machine powered up, but not running. There are three motors, which are all controled by drives. The only 480 direct connecte motor is a 2HP vacuum pump motor, but it is not on until started by the control system. Normally it is off. There are three small drives on this machine, all controlling 1HP motors. But the drives are not "on" during power up.

This has been one weird problem, and I am wondering if there is more than one problem involved here.

Again, thanks to all, so far.

 

[rbulsara]

You still need to post the amperes with all motors running.

From what I read in your last post, that the breaker tripping while 'energizing' the transfomer is possibly due to high inrush current.

Does the breaker trip, when the tranformer is already energized and motors are running?
or does it trip only when switching the transformer on?

 

[jjdad]

The breaker only trips when energizing the transformer. It doesn't trip when the machine is running.

I just spoke with the machine manufacturer and they say there is a control tranformer on this machine that has a high inrush current and the circuit breaker that it tripping needs to have a higher delay setting.

We shall see.

 

[rbulsara]

I can assure you its not the control tranformer but the 30kVA transforer causing the high inrush.

But it is the inrush that is causing the breaker trip. Untill you upgrade the breaker (and possibly the wiring), leave the transformer engergized all the time.

 

[busbar]

First, rbulsara has done an outstanding job of pro bone remote-control troubleshooting and advice. Not sure I’d have the patience to type out all the piecemeal questions and information he has given. I hope it is very sincerely recognized by the OP and duly appreciated.

It’s time to consider reviewing the overcurrent-devoice provisions of NEC Article 450. If this reference is not clearly understood, pay for some help. Enough manhours of handholding…

 

[rbulsara]

Thanks, busbar. It means a lot coming from you.

But you are right that this forum is no substitute for hiring right professionals.

 

[jbartos]

Suggestion to jjdad (Electrical) May 26, 2004 marked ///\\
What has me baffled, is the breaker that always trips is two breakers upstream from the transformer. The breaker feeding the transformer and the breaker feeding that power panel never trip. All of the three of these breakers are rated at 100A. Apparently no coordination has been applied here.
///In some cases, the upstream breaker trips, if a loop in the downstream installation materializes and some irregularity in the loop exists, e.g. load is connected to the upstream CB as well as downstream CB via voltage displacement angle of the transformer, "wild leg" of the upstream transformer is connected to 120V load, etc. Therefore, the power distribution circuit tracing and load examination are needed as well as a correct single line diagram of the as-built power distribution.\\\