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.