Systems, methods, and apparatus for controlling a voltage source converter

A method of controlling a voltage source converter including a plurality of converter limbs, each of the plurality of converter limbs extending between first and second DC terminals and including first and second limb portions separated by an AC terminal, each of the first and the second limb portions including a chain-link converter having a plurality of series-connected modules, each of the plurality of series-connected modules including at least one switching element and at least one energy storage device, the at least one switching element and the at least one energy storage device of each module combining to selectively provide a voltage source (VIND) whereby the corresponding chain-link converter is operable to provide a stepped variable voltage source, the method comprising:

• (a) establishing during an operating cycle of each chain-link converter a respective utilization peak based on an actual peak number of modules providing a voltage source (VIND);
• (b) establishing a respective target utilization for each chain-link converter based on a desired number of modules providing a voltage source during an operating cycle of the corresponding said chain-link converter;
• (c) determining a respective control function for each chain-link converter based on a difference between the corresponding established utilization peak and the corresponding target utilization which alters the peak number of modules providing a voltage source during a subsequent operating cycle of the corresponding said chain-link converter so as to drive the corresponding utilization peak towards the corresponding target utilization (UT), wherein the step of determining a respective control function which alters the peak number of modules providing a voltage source during a subsequent operating cycle of the chain-link converter includes determining a respective control function which modifies an operating cycle average module voltage of the modules within the chain-link converter;
• and
• (d) combining the respective control function into a series of direct current and circulating alternating current components to manage the overall operation of the voltage source converter.

A voltage source converter comprising:

• at least one converter limb, the at least one converter limb extending between first and second DC terminals and including first and second limb portions separated by an AC terminal, at least one of the first and the second limb portions including a chain-link converter having a plurality of series-connected modules, each of the plurality of series-connected modules including at least one switching element and at least one energy storage device, the at least one switching element and the at least one energy storage device of each module combining to selectively provide a voltage source (VIND) whereby the corresponding chain-link converter is operable to provide a stepped variable voltage source;
• and
• a control unit operatively associated with at least one chain-link converter and programmed with respect to the at least one chain-link converter to:
• (a) establish during an operating cycle of the chain-link converter a utilization peak based on an actual peak number of modules providing a voltage source;
• (b) establish a target utilization based on a desired number of modules providing a voltage source during an operating cycle of the chain-link converter;
• and
• (c) determine a control function based on a difference between the established utilization peak and the target utilization which alters the peak number of modules providing a voltage source during a subsequent operating cycle of the chain-link converter so as to drive the utilization peak towards the target utilization, wherein the step of determining a control function which alters the peak number of modules providing a voltage source during a subsequent operating cycle of the chain-link converter includes determining a control function which modifies an operating cycle average module voltage of the modules within the chain-link converter.