METHOD FOR CHARGING MODULAR MULTILEVEL CONVERTER

摘要

A method for charging a modular multilevel converter includes: firstly, electrifying DC side of a converter; after voltages of submodules are stabilized, deblocking the converter, turning on all the submodules, then reducing the number of turned on submodules in phase unit; when over-current occurs on a bridge arm, temporarily increasing the number of turned on submodules to suppress the over-current; after the voltages of the sub-modules are stabilized, continuously reducing the number of the turned on submodules until the number of the turned on submodules in the phase unit is finally equal to the number of working submodules of the bridge arm, so as to smoothly transit to a normal operation state. The DC side is charged, such that the voltages of the submodules reach a working voltage before the converter normally operates, and an impacting current is avoided in the charging process by using a proper control strategy.

主权项

1. A method for charging a modular multilevel converter, the converter comprising at least one phase unit, each phase unit consisting of an upper bridge arm and a lower bridge arm, each bridge arm consisting of several submodules with identical structures and an electric reactor in series, the submodule consisting of an IGBT half bridge and a capacitor in parallel and comprising three working states: an on state, an off state, and a blocking state, the charging method comprising the following steps:
(1) electrifying DC side of the convener; (2) deblocking the converter and turning on all the submodules after voltages of the submodules stabilize; (3) reducing, the number of turned-on submodules in the phase unit; (4) determining whether over-current occurs in a bridge arm; if yes, going to Step (5); otherwise, going to Step ((6); (5) increasing the number of turned-on submodules in the phase unit, and returning to Step (4); (6) determining whether the voltages of the submodules are stable; if yes, going to Step (7); otherwise, returning to Step (4); (7) determining whether the number of turned-on submodules in the phase unit is greater than the number of working submodules in one bridge arm; if yes, returning to Step (3); otherwise, going to Step (8); and (8) keeping the number of turned-on submodules in the phase unit constant; wherein an end-of-charge condition is that charging processes of all the phase units in the converter reach Step (8).