Patent ID: 8760122

Claim:
A method for supporting an AC system via a cell based voltage source converter, said converter having an AC side connected to an AC power line of the AC system and a DC side connected to a DC power line of a DC power transmission system and a group of phase legs connected in parallel between two DC poles for connection to the DC power transmission system, the phase legs further comprising cells connected to each other in cascade, where each cell comprises a string of switching elements in parallel with a cell capacitor and each of at least some cells further include a battery module group comprising at least one battery module connectable in parallel with the cell capacitor, the method comprising the steps of: controlling the switching elements of the cells for converting AC power to DC power or vice versa, obtaining data regarding a power requirement of the AC system via measurements of the AC power system, selecting battery module groups of a number of active cells for fulfilling the power requirement, the selecting of battery module groups being made in dependence on the power needed for fulfilling the power requirement and a cell switching scheme used in the converter and comprising selecting cells that are active the longest in a given time interval before those that are active a shorter time, and connecting the selected battery module groups in parallel with corresponding cell capacitors for exchanging power with the AC system.