A Hybrid Isolated Bidirectional DC/DC Solid-State Transformer for DC Distribution Network

The isolated bidirectional DC/DC solid-state transformer (DCSST) plays an important role in connection of renewable energy sources, voltage conversion, bidirectional power transmission and electrical isolation for DC distribution network. To connect the renewable energy, energy storage and DC loads with DC distribution network, a hybrid DCSST scheme is proposed in this paper. Compared with the existing DCSST schemes, the proposed hybrid DCSST is composed of modified full-bridges and modular multilevel converter (MMC) topology, achieving flexible connection and large voltage boost ratio for renewable energy system, realizing fault handling capability, avoiding the application of DC circuit breakers and reducing the volume and construction cost. Besides, the high-frequency-link (HFL) voltages in proposed scheme are hybrid waveform and adjusted adaptively to match HFL transformer voltage ratio, reducing the current stress, reactive power and power loss of DCSST under the fluctuation of power flow from renewable energy sources and distributed loads. The topology, operation principle, fault handling process and hybrid wave HFL voltages adjustment strategy of hybrid DCSST scheme are investigated. The experimental results verify the correctness and effectiveness of proposed scheme, promoting the adaptability and practicality for DCSST and DC distribution network.