How can I correct harmonics with K-rated harmonic mitigating (HMT) or drive isolation transformers?

How can transformers either reduce my harmonics or deal with the harmonics by living with the harmonics or maybe canceling some of the harmonics?

Each type of transformer is going to have some inherent impedance. For example, using a transformer with 5.75 impedance is just like adding a line reactor. With an isolation transformer, we inherently reduce the harmonic currents, just like a choke, a DC choke and an AC line reactor would. The harmonics are reduced just by having that transformer. The key is to match up the size of the transformer to the load. Otherwise, we would have a lot less impedance than we actually think we do. 

K-rated transformers are intended to "live" with harmonics. For example, with 100% computer power supply load or switch mode power supplies, the transformer would have to be derated considerably, otherwise it would overheat. Standard transformers overheat signficantly compared to K-rated transformers. If the kVA load on that transformer is not reduced, the transformer will likely need to be replaced in three to five years. 

A K-rated transformer is similar in physical size to a 112.5 kVA transformer, but with a 75 kVA label on it. In fact, it's not altogether different. We actually oversize them and build them a little beefier to handle the harmonic currents and the heating that's associated with that. 

The K-rated transformer has some additional benefits. For example, in the case of a delta-wye transformer, we can circulate the 3rd harmonic currents in the delta just like any delta-wye transformer—whether a K-rated or standard—and those 3rd harmonics will be eliminated from the primary system. 

Harmonic currents come from the load and the 3rd harmonics accumulate on the neutral. The harmonics come back and they circulate in the delta winding on the primary. Therefore, we don't have those 3rd harmonics on the 480 V side.

An HMT that is a delta zigzag works a little differently. In this case, the 3rd harmonics coming from the load will actually get eliminated—some from phase A and phase C will cancel each other out on the secondary winding. The benefit is that the harmonics don't circulate on the primary winding, resulting in less losses and a reduction of current going through the transformer.

Phase shifting is another really good way to use transformers to reduce harmonics. For example, for a system approach, two 6-pulse drives can be used to create a 12-pulse system. Four transformers could also be used to create a 24-pulse system. The benefit of adding transformers to the system is to do phase shifting and cancellation. 

There are benefits of using transformers in regard to harmonics. In conclusion, there are three good ways to use transformers to deal with, live with and negate harmonics on the power system. 

We can use them like line reactors to reduce the actual amount of current coming out of the load, or choke the currents coming out of the load to restrict the current that is flowing back. 

Transformers can also be used to negate some of the 3rd harmonics. The transformers prohibit the 3rd, 9th, 15th—the odd multiples called triplens—from coming back on the system through the delta windings on the primaries, or on the secondaries in the case of delta zigzag windings.

Lastly, we can use transformers in combinations to do phase-shifting to cancel the harmonics between multiple sources of load, such as VFDs  and computer power supplies.