Converter

A DC to DC converter assembly comprising:

• a first voltage source converter comprising:
• a first converter limb extending between first and second DC terminals and having first and second limb portions separated by a first AC terminal, the first and second DC terminals being connectable to a DC electrical network and the first AC terminal being connectable to an AC electrical network, each limb portion of the first voltage source converter including a primary switching element, the primary switching element of each limb portion being switchable to switch the corresponding limb portion into and out of circuit between the corresponding DC terminal and the first AC terminal to control the configuration of an AC voltage at the first AC terminal;
• a first auxiliary limb including a first auxiliary converter, the first auxiliary converter including at least one first auxiliary switching element, the first auxiliary limb being connected to the first AC terminal, the primary switching element of each limb portion of the first voltage source converter being switchable to switch the first auxiliary limb into and out of circuit with the corresponding limb portion and thereby switch the first auxiliary limb into and out of circuit with the corresponding DC terminal;
• and
• a first control unit to selectively control the switching of the or each first auxiliary switching element of the first auxiliary converter to control the first auxiliary limb as a current source in a primary operating mode to inject a circulation current that flows in one direction in one of the first and second limb portions to minimize a limb portion current flowing in the opposite direction in that limb portion;
• wherein the primary switching elements of the first voltage source converter are configured to switch the one of the first and second limb portions out of circuit between the corresponding DC terminal and the first AC terminal and switch the other of the first and second limb portions into circuit between the corresponding DC terminal and the first AC terminal, following the minimization of the limb portion current by the circulation current, when a minimal or zero current flows through the corresponding primary switching element of the first voltage source converter during the commutation of current from the one of the first and second limb portions to the other of the first and second limb portions;
• a second voltage source converter comprising:
• a second converter limb extending between third and fourth DC terminals and having third and fourth limb portions separated by a second AC terminal, the third and fourth DC terminals being connectable to the DC electrical network and the second AC terminal being connectable to the AC electrical network, each limb portion of the second voltage source converter including a primary switching element, the primary switching element of each limb portion being switchable to switch the corresponding limb portion into and out of circuit between the corresponding DC terminal and the second AC terminal to control the configuration of an AC voltage at the second AC terminal;
• a second auxiliary limb including a second auxiliary converter, the second auxiliary converter including at least one second auxiliary switching element, the second auxiliary limb being connected to the second AC terminal, the primary switching element of each limb portion of the second voltage source converter being switchable to switch the second auxiliary limb into and out of circuit with the corresponding limb portion and thereby switch the second auxiliary limb into and out of circuit with the corresponding DC terminal;
• and
• a second control unit to selectively control the switching of the or each second auxiliary switching element of the second auxiliary converter to control the second auxiliary limb as a current source in a primary operating mode to inject a circulation current that flows in one direction in one of the third and fourth limb portions to minimize a limb portion current flowing in the opposite direction in that limb portion;
• wherein the primary switching elements of the second voltage source converter are configured to switch the one of the third and fourth limb portions out of circuit between the corresponding DC terminal and the second AC terminal and switch the other of the third and fourth limb portions into circuit between the corresponding DC terminal and the second AC terminal, following the minimization of the limb portion current by the circulation current, when a minimal or zero current flows through the corresponding primary switching element of the second voltage source converter during the commutation of current from the one of the third and fourth limb portions to the other of the third and fourth limb portions;
• wherein the first AC terminal of the first voltage source converter is connected to the second AC terminal of the second voltage source converter via a respective one of a plurality of transmission links.