DC Magnetization Remedy

The primary reason causing this loss is the harmonic current introduced by the inverter. I am not sure the proper K-rated transformer is used. The transformer is supplying the nonlinear load of inverter and large harmonic current flows through it which is causing heat loss in the transformer. Such a transformer (which supplies 100% non-linear load) should have rated a K-factor of at least 20.

- MS

My initial response is to capacitively couple your main to transformer connection. This will remove any DC component your main voltage source is providing into your transformer. But this will likely be a very large, expensive capacitor.

You should not have a significant DC component to your AC feed though. I suspect that somewhere else downstream of the main power transformer (not the one connected to your inverter) there is a significantly unbalanced load affecting one of the three phase's current draw. This could be anything from one phase winding of a three phase motor being shorted or open, to a cracked three phase transformer core that opens under a heat load. Have your electrician measure and record each of the three phase currents at every three phase distribution point at your facility with a clamp on ammeter. This will help you to identify where the unbalanced loading is happening.

Thanx for your valuable answers. Dear samad, when we ordered the transformers for our VFDs we asked the vendor that we required the transformers that can handle harmonics and we have about 6 transformers specially designed for the purpose of feeding power to the 56 VFDs connected in 2 large DC Bus Systems usually called the Sectional Drives for Paper Machine. These transformers are 2 MVA of rating and the total load of drives is about 8 MW for the whole paper machine including the winder part. K-factor was not discussed at that time it was expected that all the protection calculations required for the transformers will be done by vendor. We only wanted the safe operation.

Best Regards,

I hope the vendor considered the K-rated transformer for dealing with the harmonics. Sometimes, the standard transformers are de-rated for the harmonics considerations. De-rating may minimize the copper losses for the harmonics but unable to deal with the core losses caused by the eddy current. Harmonic currents are the multiples of fundamental frequency and hence produce more heat in the core due to the high frequency. The well-designed K-Factor transformers minimize the winding eddy current losses through the use of parallel conductors and other winding techniques. I would suggest you to contact the vendor and find out whether the transformers are designed for K-rating for the harmonics current.

By the by, what makes you feel that the transformer losses are due to dc magnetization? I never heard of such issue.

- MS

Actually I thought about it due to the Thyristor controlled large rectifiers of DC Bus Systems with large reactors in series. Now since Thyristors are uni-directional and the inductive load of the motors as well as the large reactors in series with the DC Bus Systems are making the load as highly inductive and the voltage buildup across this heavy inductive load in each cycle is against the supply load resulting in reverse current to the supply. Now Thyristors will block this current will DC Current is fed back to transformer secondary resulting in the DC Magnetization or DC Saturation of core. This will increase the core losses of transformer and noise due to harmonics.

It is easy to see if the transformer's heating is from DC current or harmonics.

Have a competent electrician clamp the secondary winding cable one at a time while in operation. If the clamp sticks and he cannot open it under power, you have DC. If he can measure the three phases, you have harmonics.

Of course you can have both!

DC current is usually caused by a diode failure or its corresponding fuse if applicable. Use a diode tester with power removed.

Harmonics need a 3-5% line chokes to be almost acceptable. Other methods are possible but are much more expensive and risky.

DO NOT ADD A SIMPLE CAPACITOR BANK BETWEEN THE TRANSFORMER SECONDARY AND THE DRIVE.