A harmonics elimination method using a novel three-winding transformer for HVDC transmission systems

A high-voltage direct current (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, the traditional HVDC transmission systems cause a large number of harmonics in the sending-end grid current and the receiving-end grid current. In this paper, a novel harmonics elimination method using a three-winding transformer with negative leakage inductance for the HVDC transmission systems is proposed. The theory of the proposed method is discussed and the simulation model of the proposed system is developed first. The proposed method is confirmed to be effective when a resistance 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. Also, to obtain an accurate simulated result, the simulation model is modified to take in the effects of the excitation circuit of the transformer and its magnetic hysteresis loop. Finally, current waveforms and voltage waveforms of the transformer when receiving-end grid is connected to the output terminal of the system are discussed.