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Modeling and control of zero-sequence current in multiple grid connected converter

Because of energy shortage and environment pollution, the renewable energy, especially wind energy has become more and more considerable all over the world. Direct drive wind energy conversion systems based on multipole permanent magnet synchronous generator (PMSG) have some advantages such as no gearbox, high power density, high precision and easy to control. In our research project, a direct drive wind energy conversion system is developed. In order to reduce the current of each IGBT and reduce the harmonic current to the grid, a multiple ac-dc-ac converter which is suitable for high power wind energy conversion systems is used. A zero-sequence circulating current will exist in the parallel converters if the zero-sequence current is not controlled. The zero-sequence current can result in circulating current in two parallel converters. So zero-sequence current is harmful for power semiconductor. Modeling and control methods of the zero-sequence current are introduced in this paper, as well as the control scheme of the grid side converter. Simulation and experimental results verify the analysis and the conclusion.