No passive or active device designed to mitigate harmonics converts harmonics ‘back to useful Electric energy.’ In fact, all of the passive and active harmonic mitigation devices consume some small amount of real power (useful energy) in order to operate.
Unsatuarable zigzag reactors are also known as zigzag transformers. There are two main uses for these devices.
1) Neutral current mitigation: The device attaches to the neutral and the three mains conductors. It is designed to conduct the third harmonic (the largest and most harmful harmonic on the neutral) to the mains conductors equally per phase. This process reduces the neutral harmonic away from the loads beginning at the point of connection to the neutral and increases the harmonic current on the mains. But since it is divided equally on the mains, it has a much less detrimental effect on the mains.
2) Multipulse rectifiers: Autotransformers can be designed with zigzag secondary connections to create phase shifting that results in reduced harmonic current on the mains side of the transformer. The equipment are three phase loads with two or more rectifiers. In some cases, two or more independent nonlinear loads may be connected in this fashion to create a quasi-multi-pulse system.
To reitierate the important point, no harmonic mitigation device, passive or active, converts harmonics back to useful electrical energy. All harmonic mitigation equipment consume some level of real power while functioning.
The purppose of harmonic mitigation equipment is to limit the effects of harmonics on the mains side of their connection within the electrical system to permit fuller utilization (increase energy efficiency) of the electrical system. This also stops the detrimental heating and voltage distortion that causes other equipmnet to fault or fail.