Fault tolerant AC-DC chain-link converter

A converter comprising:

• a first terminal and a second terminal, the first terminal configured for connection to a first network, the second terminal configured for connection to a second network;
• at least one switching module arranged to interconnect the first terminal and the second terminal, the or each switching module including at least one module switching element and at least one energy storage device, the or each module switching element and the or each energy storage device in the or each switching module arranged to be combinable to selectively provide a voltage source, the or each switching module switchable to control a transfer of power between the first and second networks;
• the or each switching module including a discharge circuit, the or each discharge circuit including a discharge switching element and a discharge resistor, the or each discharge switching element switchable to switch the corresponding discharge resistor into and out of the corresponding switching module;
• and
• a controller configured to selectively control the switching of the or each discharge switching element in:
• a normal mode to switch the corresponding discharge resistor into the corresponding switching module at a first switch-in voltage level of the or each corresponding energy storage device during a normal operating state of the converter so that a normal operating current flowing in the corresponding switching module is divided between the corresponding discharge resistor and the or each corresponding energy storage device, wherein the or each first switch-in voltage level is lower than a voltage level corresponding to a maximum voltage capacity of the or each corresponding energy storage device;
• and
• a fault mode to switch the corresponding discharge resistor into the corresponding switching module at a second switch-in voltage level of the or each corresponding energy storage device during a fault operating state of the converter so that a fault current flowing in the corresponding switching module is divided between the corresponding discharge resistor and the or each corresponding energy storage device, wherein the or each second switch-in voltage level is lower than the corresponding first switch-in voltage level.