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 modular 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-sequence 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 neutral 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.