In or relating to chain-link converters

A method of identifying a faulty sub-module of a chain-link converter, the chain-link converter including a plurality of sub-modules, each sub-module including at least one switching element and an energy storage device which together are selectively operable to provide a voltage source, the method comprising the steps of:

• measuring a switching characteristic of each sub-module of the chain-link converter;
• establishing a switching characteristic signature of the chain-link converter;
• and
• identifying the or each sub-module with a switching characteristic that deviates from the switching characteristic signature as being a faulty sub-module;
• wherein the step of measuring a switching characteristic of each sub-module of the chain-link converter includes measuring a switching frequency of each sub-module of the chain-link converter;
• the step of establishing a switching characteristic signature of the chain-link converter includes establishing a switching frequency signature of the chain-link converter from the measured switching frequencies;
• and
• the step of identifying the or each sub-module with a switching characteristic that deviates from the switching characteristic signature as being a faulty sub-module includes identifying the or each sub-module with a switching frequency that deviates from the switching frequency signature.