Paralleling two power VSD's from the same 24 pulse phase shifting transformer

[samerakhaled1]

Hello,
I was wondering if anybody can help me to understand whay I can't parallel my two 24 pulse 470KVA VSD's on the same output of the same 1000 KVA 24 phase shift transformer. I understand that there might be a circulating current between the two VSD's due to this connection, if this is really the reason I am still unclear how this circulating current will present in this type of connection!!!!

 

[Drivesrock]

Hello Samerakhaled1.

Who said you can't parallel the two VSDs on the transformer? I donot expect circulating currents if connected correctly.

But there could be a reason in that the K factor of the transformer is not sufficient for the total harmonics in each secondary? The 24 pulse scheme will reduce the harmonics seen on the supply on the primary side but each secondary winding will see 6 pulse harmonics - ie high 5th and 7th etc. etc..

Make sure that the secondary voltages are within 1 or 2% of each other otherwise you will not get correct current sharing.

Was the transformer specified to supply VSDs?

 

[samerakhaled1]

I am not sure if it is specified as VSD transformer.
That is a very good point.Can you please highlight what will be the design different on the transformer if specified as VSD supply transformer.

 

[jraef]

A transformer designed for a high K Factor will have an oversized neutral and more thermal dissipation capacity in order to handle the harmonics caused by the non-linear current draw of something like the VFD rectifiers. Being that the entire purpose of having a 24 pulse transformer is to create a set of phase shifts to help prevent the effects of the harmonics from getting into the line, I cant imagine anyone designing and building one that would not have a high K Factor.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington