METHODS AND DEVICES FOR CONTROLLING ACTIVE POWER FLOW IN A THREE- PHASE MODULAR MULTILEVEL CONVERTER

摘要

The invention relates to methods and devices for controlling unbalanced active power flow in a three phase mo dular multilevel converter 20. The converter 20 comprises a first and second converter 4 5 both comprising three phase legs arranged in a wye connection. The first and second converters 4 5 are interconnected in a double wye connection and their neutral paths are independently floating. The method 200 comprises: detecting an active power flow in the phase legs; determining a zero sequen ce voltage the determination providing magnitude and phase of the zero sequence voltage; re computing the magnitude of the zero sequence voltage while keeping the phase of the zero sequence voltage fixed the magnitude being re computed with the requirement that the resulting voltage over the phase legs is smaller than or equal to a maximum allowed leg voltage the re computed magnitude and the phase giving a re computed zero sequence voltage; imposing the re computed zero sequence voltage on the neut ral point of the first and second converters thereby reducing the active power flow determining remaining active power based on the re computed 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 that will counteract remaining active power; and imposing the DC current on the phase legs.