A Harmonics Elimination Method Using a Three-Winding Transformer for HVDC Transmission Systems

A high-voltage dc (HVdc) transmission system including wind turbine generating systems has been proposed. In the system, the thyristor converter and inverter are used in the same way as in traditional HVdc transmission systems. Hence, the system is suitable for large-scale wind farms and it has high reliability. However, since the traditional HVdc transmission systems generate harmonic currents, a large number of ac filters with capacitors are required. In this paper, a novel harmonics elimination method using a three-winding transformer for the HVdc transmission systems without capacitors is proposed. The theory of the proposed method is discussed and the simulation model of the system is derived first. The method is confirmed through theoretical and experimental investigations to be effective when a static load is connected to the output terminal of the system. It is shown that the proposed method is useful to eliminate harmonic distortions generated at the inverter. Finally, current waveforms and voltage waveforms of the system when a receiving-end grid is connected to the output terminal of the system are discussed.