Zero-Sequence Current Harmonics in Face-to-Face Interconnected-Converter System

Nowadays, more and more converters are interconnected face-to-face with each other to achieve higher power and equivalent switching frequency. This interconnected converter system appears in advanced testing benches for power electronics, paralleled-connected converters, or open-winding motor drive systems. Typically, the interconnected converters are powered by the same dc voltage source, thus inevitably creating a zero-sequence current path in the ac terminals. Consequently, undesired current harmonics, which do not exist in typical 3-phase 3-wire converter systems, are introduced, with increased power loss and extra stresses. These current harmonics have complicated frequency spectrum under different mission profiles, which have significant impacts on sampling results. However, these impacts of the current harmonics are seldom discussed. In this article, the time-domain models of the high-order zero-sequence current harmonics in an interconnected converter system are established under various operating conditions. Impacts of the high-order zero-sequence current harmonics, including current waveform distortions, sampling distortions, and control errors, are comprehensively identified and analyzed, which can help to predicate the current waveforms and eliminate the impacts of the current harmonics from more aspects. Simulations and experiments are provided to validate the analysis.