Transformer Impedance for the VSD

The transformer is designed to supply two three phase bridge rectifiers with the three phase applied to one rectifier phase shifted relative to the other.

The value of the impedance is usually not critical except for affecting the voltage regulation.

The important point, is the impedance of the transformer as it is seen by each bridge rectifier.

Both impedances must be equal or there will be an imbalance in the current between the two rectifiers and the harmonic cancellation will be reduced. You could also cause the failure of one of the bridge rectifiers if the curent imbalance is too bad.

Best regards,

Mark Empson

http://www.power-harmonics.co.nz

I think, You asked for the leakage reactance, causing the eddy currents & consequently, voltage distortions. This is understood to be to control the harmoics i.e. practially, 5th harmonics causing neusance firing of the thyristors.

If similar problem encounters with the ordered transformer, you may put filter

solve the problem.

@ harry potter

It's a 12 pulse rectifier, that means the 5th harmonic is quite low.

What I understand is: the lower the impedance of the source, the 'rigid' the source will be, as there will be only small distortion at the voltage of the source.

But in the catalog of siemens drive, it mentions about Minimum leakage reactance for the transformer. It means it would be better to have higher impedance.

Could somebody please help me with this issue?

Thanks

Higher leakage impedance smooth out the current and reduce the ITHD at the expense of VTHD. If nothing else is feed from the transformer secondaries, the VTHD is not an issue (usually) for the drive.

With high leakage inductance, the rectifiers current will be less "spiky" than when using a hard supply.

You could add supplementary line chokes between the transformer and the drive to get the desired inductance.