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VFD tripping failure

We are developing a 1.8MW medium voltage drive. Inverter o/p voltage is 3.3kv and stepped up to 6.6kv using a transformer. Now the issue is, we are able to operate VFD directly with motor. When we connect transformer, We are getting vcesat error from the gate drive. By reducing v/f ratio, we are able to energize the transformer now.
But when we connect transformer along with motor, it is tripping though we have gone for lowest frequency and lowest v/f ratio.

If the motor is inserted after the xfrmr is stabilized with inverter, it appears like a DOL staring of motor. Agreed, the voltage and frea are lower, but inrush currents may be shooting up. I would try to connect xfrmr and motor from beginning and repeat the tests. I hope the xfrmr and motor have the same v/f profile.

If the VFD is starting a zero hz and accelerating the frequency, the transformer core will saturate. The first few hz will make the transformer input impedance look VERY low and the VFD output current much too high. See if the VFD has a minimum starting frequency choice like 6 hz or so. Also, is the input winding of the transformer set up as a wye, or delta? If the input is delta, there is a good chance of an overvoltage trip of the inverter, depending on dead time of PWM output.

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