A Novel Method Based on Self-Power Supply Control for Balancing Capacitor Static Voltage in MMC

Abstract

The modular multilevel converter (MMC) is one of the most attractive converter topologies for high-voltage DC transmission (HVDC) systems, but it needs at least 10 minutes during the system uncontrolled precharge stage to verify the stability and reliability of sub-modules (SMs), making the capacitor static voltage balancing a key issue. This paper proposes a novel method based on self-power supply control for balancing capacitor static voltage. Because of the influence of self-power supplies on capacitor voltages, the method can keep the capacitor static voltage balanced by controlling the input characteristic of SMs self-power supplies. The control signals of self-power supplies have a fixed frequency and duty ratio, and they can be determined based on capacitor voltage sorting results and self-power supply output support capacitor.

Existing System

Curve Fitting Method.

Proposed System

In this paper, a novel method based on self-power supply control for balancing capacitor static voltage is proposed. The proposed method can control the capacitor static voltage balancing indirectly by regulating the input characteristic of the self-power supplies. Based on the SM capacitor voltage sorting results and self-power supply output support capacitor, the capacitor static voltage balancing control signals with fixed frequency and duty ratio can be generated. The input impedance and input current of self-power supply with lowest SM capacitor voltage can be regulated by using the control signal, and the capacitor voltage balancing can be controlled indirectly. Compared with previous works, the proposed method has less computation, and it does not rely on IGBTs and additional complex circuits except for the self-power supplies. This can save on the software and hardware costs. The method can also improve the equalizing resistances and reduce the active power losses of SMs.

Architecture

Voltage balancing based on self-power supply load switching circuit

Voltage balancing based on self-power supply mutual providing energy circuit

Voltage balancing based on self-power supply input characteristic regulation