High-voltage input power supply for modular multi-level converter

A modular multi-level converter (MMC) is composed of many sub-modules (SMs). Each SM needs a power supply to drive insulated-gate bipolar transistor (IGBT) and control power. The MMC system has a power of 100MW, a dc link voltage of 250kV, and a total of 125 SMs. This means that each SM has a rated input voltage of 2kV. This high input voltage should be converted to low load voltage by the power supply. Because of the high input voltage, voltage balancing techniques and the selection of an appropriate topology should be considered in the design of the power supply. The most common and popular approach in the design of the power supply is the input-series output-parallel (ISOP) method. In this paper, ISOP two-transistor flyback converters based on common-duty control are used. If all the converters are exactly equal, common-duty control guarantees the voltage balancing. However, in a real application, the DC/DC converters are not identical. Unbalancing in the transient state and no-load is severe. To reduce this voltage unbalance in the transient state and no-load, some techniques to balance the input-series voltage are introduced. Proposed techniques and topologies are analyzed and verified by experimental results.