Hi.
I've wye-wye 750/415V,60KVA epoxy filled dry type power distribution transformers.The power supply to this txf is given through the output of a converter whose phase to phase voltages are balanced,but its phase to earth voltage is having high frequency voltage spikes of peak 2.5KV.I've observed that in some txfs this phase to earth voltage in primary is coming on secondary side also.So.I meggered that txf b/w its primary and secondary terminals and its value is coming around 0.24Mohm.In other txfs this value is coming around 800Mohm.Kindly help me to understand this behaviour?
Thanks
Change grounding through inductor, instead of direct grounding......
As ground provides path for the high frequency signal, but is not the power signal hence will be loaded when load is applied and will vanished, or connect ground through inductor.
If I have understood your problem correctly, the Megger value between pri and sec of some transformers are showing a much lower value compared to others and these are also passing the high frequency spikes through while others are not.
The problem appears to be a leakage path between pri and sec and I suspect the terminal board. Please remove the winding terminals from this terminal board, leaving them free in air and then do Megger test again to confirm.
By the way, I presume you are providing a 4-wire supply to the input from the converter, otherwise a 3-wire input Wye-Wye transformer without a Delta tertiary will tend to distort the output waveform by not being able to provide a path for triplen harmonics of magnetizing current. But this problem will happen for all transformers & not selectively.
The input to the txf is 3phase 3 wire and the output is 3phase 4 wire with ungrounded neutral.The distortion due to triplen harmonics should be seen in the phase to neutral voltages.I've observed the phase to neutral waveforms -they seem balanced and the distortion is very negligible.Here the phase to earth & neutral to earth noise is coming on secondary.