Unloaded Transformers humming Wildly

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Rho_HV

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Without going into details, I have three very small control power transformers (50VA) connected to my three phase line side feeding a 1000HP VFD. Transformers are used to detect Ground Faults. Anytime there is a ground fault on DC bus (bus between rectifier and inverter) transformers start humming crazy and if not disconnected from 600VAC, they fail in matter of seconds.
My question is: what causes a an unloaded transformer to hum? low frequency content? excessive harmonics? Here is what I think the current waveform through transformers' primary looks like during DC fault:

Capture.JPG
 
Are you trying to put a DC current through a coil designed for AC? I don't really follow exactly what you are doing here.
 
kwired said:
Are you trying to put a DC current through a coil designed for AC? I don't really follow exactly what you are doing here.
Click to expand...
No.
I kind of want to keep my question simple, without going into details. Transformer are part of ground fault detection circuit monitoring a rectifier and inverter. Transformers start to humm profusely and fail in seconds when there is a fault on DC bus.
I am just wondering what would cause an unloaded transformer to hum wildly and even fail? DC or lower frequency content?
 
LMAO said:
No.
I kind of want to keep my question simple, without going into details. Transformer are part of ground fault detection circuit monitoring a rectifier and inverter. Transformers start to humm profusely and fail in seconds when there is a fault on DC bus.
I am just wondering what would cause an unloaded transformer to hum wildly and even fail? DC or lower frequency content?
Click to expand...

During the fault is there DC current applied to the transformer? I don't fully understand where or how these transformers are connected. Typical AC coils will not have inductive reaction when DC is applied and therefore the coil is nothing more than a short circuit instead of a load.
 
What is the voltage rating of the transformers, and to what voltage could they be subjected in the event of a ground fault ?

Consider for example an ungrounded or impedance grounded delta system with 480 volts between phases.
That would give 277 volts to ground under normal conditions, and if the small transformers have 277 volt primaries all will be well normally.
However if a ground fault occurs, then two of the 277 volt transformers will be subjected to 480 volts and liable to fail very quickly indeed.
Broadly similar arguments apply at other voltages, remembering that solidly grounded systems may not be as solidly grounded as expected in the event of large fault currents.
 
sounds like you have a ground reference on the ac input side, and when the dc bus shorts to ground also you have a direct short thru those little xfmrs of the 600v line to ground.

your keeping it simple is not helping anyone help you understand it; draw a wiring diagram of how they and the 1000hp 600vac vfd is wired....
 
LMAO said:
Without going into details, I have three very small control power transformers (50VA) connected to my three phase line side feeding a 1000HP VFD. Transformers are used to detect Ground Faults. Anytime there is a ground fault on DC bus (bus between rectifier and inverter) transformers start humming crazy and if not disconnected from 600VAC, they fail in matter of seconds.
My question is: what causes a an unloaded transformer to hum? low frequency content? excessive harmonics? Here is what I think the current waveform through transformers' primary looks like during DC fault:

View attachment 6713
Click to expand...
Is zero current in your diagram where the current goes level for a bit?
If that's what really happens, then you would be passing direct current through transformer primary which would saturate it.

Do you have a sketch of how the circuit is connected?
 
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