High-Frequency Operation of a DC/AC/DC System for HVDC Applications

This paper presents a dc/ac/dc system using a transformer coupling two modular multilevel VSCs. The system consists of two front-to-front connected VSCs coupled through a transformer. The ac link is internal to the dc/ac/dc system and therefore the operating frequency can be selected freely and used to tradeoff the volume of the passive components against the power losses in the semiconductor devices. In such a system, the capacitors occupy a large fraction of the volume of the cells but a significant reduction in volume can be achieved by raising the ac frequency. Using high frequency can also bring benefits to other passive components such as the transformer but also results in higher switching losses due to the higher number of waveform steps per second. The outcome of the study shows that a frequency of 350 Hz provides a significant improvement in volume but also a penalty in losses compared to 50 Hz. This leads to a trade off between volume and losses which has been explored in this study and verified by MATLAB/Simulink results with a transistor level model.