A start strategy for synchronized connection of MMCs to an AC system

Two series problems plague prevailing modular multilevel converter (MMC) start strategies: (1) during uncontrolled charging, the capacitor voltage of an MMC disconnected from the AC source is half of the capacitor voltage of an MMC connected to the AC source, and (2) the MMC system cannot synchronously connect to the AC grid. To solve these problems, a smooth-start scheme for synchronized connection of MMCs to an AC system is proposed. The proposed smooth-start process is divided into three stages: a voltage-imposed stage, a connection stage, and a regulation stage. During the voltage-imposed stage, MMCs disconnected from the AC source are charged with the proposed M-delta controller. In the connection stage, the M-delta controller facilitates synchronized connection to the AC network to diminish the impulse current, and a buffer function is used to smoothly switch the M-delta controller to a decoupled d-q controller. During the regulation stage, the reference values of the d-q controller are adjusted to rating values. Simulation results indicate that the proposed start strategy is feasible and effective. (C) 2015 Elsevier Ltd. All rights reserved.