Trapezoidal Wave Operation of T-Type DC/DC Converter for High-Power Applications

All-DC collection and transmission is a promising solution for integration of large-scale renewable energy in desert and deep-sea areas. High-power DC/DC converter is the key equipment in this all-DC system. However, current T-type DC/DC converter (T-DC) based on modular multilevel technology faces high current stress and limited power capacity. In this paper, trapezoidal wave operation of T-DC is proposed to reduce device current stress and thus increase power capacity. Waveform constraints are clarified, and both symmetrical trapezoidal wave (STW) and asymmetrical trapezoidal wave (ATW) operation are proposed. Compared with the traditional sinusoidal wave operation, the proposed STW and ATW operation both offer higher power capacity, reduced capacitor energy storage, and improved efficiency in a wide voltage ratio range. The advantages of ATW become more pronounced at higher voltage ratios. In addition, to mitigate significant fundamental-frequency current ripple on the DC side caused by unbalanced T-DC losses between phases, an arm energy balance method based on internal AC and DC voltage regulation is proposed, eliminating the ripple without the need for a DC filter. The detailed analysis, control schemes, simulation results, and scale-down prototype are provided to validate the effectiveness of proposed trapezoidal wave operation method.