Method for balancing a chain-link converter in delta configuration

A control apparatus for controlling a chain-link power converter, said converter comprising three phase legs, each of which phase legs comprising a plurality of series-connected converter cells, each of which cells comprising a DC capacitor, the phase legs being connected in a delta configuration, the control apparatus comprising:

• processor circuitry;
• and a storage unit storing instructions that, when executed by the processor circuitry, cause the apparatus to:
• detect an unsymmetrical voltage condition at a terminal of the converter;
• determine a ratio between a zero sequence and a negative sequence component of a compound current to be injected into the converter, wherein an amount of the negative sequence component is not zero, based on the detected unsymmetrical voltage condition;
• calculate the compound current comprising the zero sequence component and the negative sequence component in accordance with the determined ratio;
• and inject the compound current into the converter to control the converter in view of the detected unsymmetrical voltage condition, wherein the control apparatus is integrated in the chain-link power converter.

A non-transitory computer-readable storage medium having stored thereon a computer program for controlling a chain-link converter, said converter comprising three phase legs, each of which phase legs comprising a plurality of series-connected converter cells, each of which cells comprising a DC capacitor, the phase legs being connected in a delta configuration, the computer program comprising computer program code which is able to, when run on processor circuitry of a control apparatus for the converter, cause the control apparatus to:

• detect an unsymmetrical voltage condition at a terminal of the converter;
• determine a ratio between a zero sequence and a negative sequence component of a compound current to be injected into the converter, wherein an amount of the negative sequence component is not zero, based on the detected unsymmetrical voltage condition;
• calculate the compound current comprising the zero sequence component and the negative sequence component in accordance with the determined ratio;
• and
• inject the compound current into the converter to control the converter in view of the detected unsymmetrical voltage condition.