Methods and Devices for Controlling Active Energy Flow in a Three-Phase Modular Multilevel Converter

摘要

Abstract Methods and Devices for Controlling Active Energy Flow in a Three-Phase Modular Multilevel Converter. The present invention relates to methods and devices for controlling an unbalanced active energy flow in a three phase modular multi-level converter (20). converter 20 comprises first and second converters 4,5, both comprising three phase steps arranged in a y-connection. the first and second converters (4, 5) are interconnected in a double y connection, and their neutral paths are in independent fluctuations. method (200) comprises: detecting an active energy flow in the phase steps; determining a zero sequence voltage, determining providing the amplitude and phase of the zero sequence voltage; the recalculation of the magnitude of the zero sequence voltage while holding the phase of the zero sequence voltage fixed, the magnitude being recalculated with the requirement that the resulting phase step voltage be less than or equal to a maximum allowable step voltage , the recalculated magnitude and the phase giving the recalculated zero sequence voltage; the imposition of the recalculated zero sequence voltage to the neutral point of the first and second converters whereby the active energy flow is reduced by determining the remaining active energy based on the recalculated magnitude of the zero sequence voltage; determining a dc current giving a product with a dc voltage of the first and second converters (4,5) which will neutralize the remaining active energy; and the imposition of the dc current on the phase steps.