The next‐generation medium‐voltage power conversion systems

This paper describes the next‐generation mediumvoltage power conversion systems based on transformerless cascade PWM converters and bidirectional isolated dc‐dc converters. A 350‐V, 10‐kW and 20‐kHz dc‐dc converter is designed, constructed and tested as a core circuit for medium‐voltage power conversion systems. It consists of two single‐phase full‐bridge converters using eight IGBTs and a 20‐kHz transformer with a nano‐crystalline soft‐magnetic material core and litz wires. The transformer plays an essential role in achieving galvanic isolation between the two full‐bridge converters. The overall efficiency from the dc‐input to dc‐output terminals is accurately measured to be as high as 97%. Loss analysis clarifies that the overall efficiency may reach 99% or higher when SiC‐based power devices are used. In addition, this paper presents the 6.6‐kV transformerless STATCOM(STATic synchronous COMpensator) intended for achieving reactive‐power control in industrial and distribution power systems. It is characterized by direct connection to the 6.6‐kV grids, thus bringing significant reductions in cost, weight and size to the 6.6‐kV STATCOM. Key words: bidirectional isolated dc‐dc convertermultilevel convertersreactive powerSTATCOM Notes H. Akagi is with the Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, S3–17, 2–12–1, O‐okayama, Meguro, Tokyo, 152–8552, Japan. (Tel/Fax: +81–3–5734–3549; Email: akagi@ee.titech.ac.jp)