drive isolation transformer

[don_resqcapt19]

Many years ago when we installed VFDs, we often used a drive isolation transformer. For the last 15 years or so, we have not being doing that.
I am looking at installing a 600 hp, 460 volt drive that will be supplied from a 2500kVA transformer in an industrial facility.. Most of the other load is across the line start motors with a few smaller VFDs and a couple of lighting transformers.
Is there any need for a drive isolation transformer or a line side reactor for this installation?

 

[Jraef]

If your source kVA is more than 10x the drive kVA, you absolutely want at least a line reactor.

DITs in the “olden days” were necessary to decouple the line notching caused by SCR voltage control front ends on the old 6 step inverters. The advent of diode front ends on voltage source PWM drives did away with that need.

A DIT is only necessary now if you have a DC drive, or to deal with a delta source. PWM VFDs are not designed for delta sources, the components are generally selected for Wye, where the voltage reference to ground is lower. The DIT will always be a Wye secondary. But if you do use a DIT, you don’t need the line reactor, it will serve that function too.

 

[JoeStillman]

Some drives have an internal zig-zag transformer to make it twelve-pulse. They're much better for harmonics. I wouldn't put a DIT upstream of that.

Jraef, did you mean less than 10x drive kVA? A stiffer source would be more immune to noise. His 600HP on a 2500 kVA is going to be way more than 1/10 in load / source. I would definitely specify reactors for that.