Harmonic interaction between 1500 kVA supply transformer and VFD load at an industrial plant

The application of variable-frequency drives (VFDs) in industrial and commercial facilities has seen a tremendous increase. However, when multiple VFDs are fed from one dedicated power transformer, care should be taken not to drive the transformer into saturation. At one industrial location, nine VFDs rated at 200 HP each, and fed from a single 1500 kVA air-cooled transformer, showed saturation phenomenon, which had the potential of causing serious damage to the transformer. This paper discusses the input AC current waveform asymmetry caused by the interaction of the transformer and the nine 200 HP VFD load fed from it. The term waveform asymmetry is used when the positive half of the input AC current cycle is not equal to the negative half of the input AC current cycle. This results in the existence of DC components along with even harmonics in the input at current waveform. The various tests and the eventual solution to the waveform asymmetry caused by the VFDs is presented. The value of the inductance in the DC smoothing inductor (DC-link choke), which is included as part of the VFD, was found to contribute significantly to the waveform asymmetry. Experimental results showing the performance of a reconfigured DC-link inductor to reduce its interaction with the power transformer are presented.