isolation transformers vs line reactors
With VFD's I have seen both isolation transformers and line reactors used to help isolate harmonics from the main power system. Is there a particular reason why one is used over another or what are the pro's and con's for both when used as an isolation means with a VFD application?
Drive Isolation Transformers were mostly associated with DC drives and older Current Source or 6 step AC inverters, both of which have pretty much gone by the wayside now. Those drive technologies created a lot of common mode noise and the shielded isolation transformer attenuated it so that it didn't get back onto the supply. As a general rule, modern PWM drives don't have that problem to begin with, so the isolation is no longer necessary. Isolation transformers do virtually nothing for harmonics, it tends to pass right through them.
That said, when you REALLY want to do something about harmonics in PWM inverters, one of the ways is to use 12 pulse or better yet, 18 pulse front ends. Those drives then do once again need transformers to create the Delta-Y phase shifts going into the 2 (12 pulse) or 3 (18 pulse) sets of rectifiers on the VFD's front end. The isolation issue is just part of the design and the harmonics are reduced by virtue of the phase shift that comes from using them, but again, it isn't the isolation itself that mitigates the harmonics.
You typically see reactors as a low-cost method of mitigating harmonics on basic 6 pulse inverters. They help, but by themselves they are rarely the entire solution. Usually they need to be combined with tuned caps as part of an LC filter.
If you are specifying new equipment then yeah, 12 or 18 pulse drives are good, but if you have existing 6 pulse drives then a mix of half line reactors and half delta-wye isolation transformers in front of 6 pulse drives will take care of almost all harmonic problems and is the cheapest option. The reason for using both is that at the common bus of these drives, the harmonic voltage distortion of the reactor fed drives cancels out with the harmonic voltage distortion of the isolation transformer to give a much less distorted waveform. If your common bus is close to the drives this is most effective, but if the common bus is at the main service, then tuned filters at each drive would work better.