Generator, wye-wye and a delta-delta

[looper89]

I have been asked to trouble shoot a 750 kW genset. The genset is feeding two transformers, one is a 500kVA 480V-2470V grounded wye-wye and the other is a 150kVA 480V-4160V delta-delta transformer. Everything works fine when just the delta-delta transformer is connected but once the wye-wye is energized the genset begins to oscillate and voltage fluctuates about 50V. There is a neutral running to the genset.

I have not been to the site and do not know if there are any neutral currents. The loads are primarily three phase induction motors.

Any ideas would be appreciated.

 

[waross]

The first thing to do is tweak the stability control on the AVR.
The other first thing to do is check whether the ground isolation switch is open and if the neutral is connected to ground instead of to neutral.
respectfully

 

[rbulsara]

I would eliminate the neutral between the Y-Y xfmr and the gen and see what happens. Do not ground 480V side of the transformer N. Make sure the 4160V side N is grounded.

Is this a new installation or everything was fine untill now?

 

[looper89]

The wye-wye transformer is new and the delta-delta has been in service for some time.

The 500kVA transformer is 480V-12470V. I noticed that I mislabeled it.

Thanks for the comments.

 

[waross]

Hi Rafiq
I understand that this is a wye/wye transformer. The secondary voltages and phase angles will be all over the map if there is any unbalance in the load. Very unstable. The neutral must be connected to stabilize the secondary voltages and phase angles.
Respectfully

 

[rbulsara]

Bill:

Not knowing all the facts difficult to comment. I do agree that normally neutrals of a Y-Y xfmrs are internally connected to each other and only grounded at the secondary. However primary side neutral is never connected, it may be grounded. Remember the source netural is grounded at the generator too. This should be adequate to stabilise the voltages. Apparently the primary neutral connection is not helping in this case so it may be worth while to see it helps disconnceting it. It is still a 3 wire input (primary) regarless of the type of loads on the secondary side.

 

[looper89]

The installation was investigated and it was found that the neutral wasn't tied to ground like I was told. Once it was attached everything began to operate as expected.

Thanks for your time.

 

[waross]

Hi looper89;
Thanks for the feed back.
The transformer neutral should be tied to the generator neutral, and then either the generator neutral or the transformer neutral should be grounded. If the wye:wye transformer neutral isn't tied to the generator neutral the three secondary voltages will be in inverse proportion to the secondary loads. The phase to neutral voltage on the lightest loaded phase may approach phase to phase voltage. The phase angles will be all over the map.
A wye:delta transformer is a special case and the wye primary neutral is left floating. A wye:wye transformer should always have the primary neutral connected to the source neutral.
Grounding the transformer neutral will work, however it may lead to complications in the event of a ground fault. Connecting to ground instead of the neutral will also interfere with ground fault protection on units so equipped.
I'm glad you have got your machine working well. Please forgive my nit-picking.
Respectfully