Control circuit
One circuit includes first and second primary terminals for connection to first and second power transmission lines and a current transmission path extending between the primary terminals and having current transmission path portions separated by a third primary terminal. A first current transmission path portion includes at least one primary switching element connected in series between the first and third primary terminals, the second current transmission path portion includes an energy conversion block connected between the second and third primary terminals, and the energy conversion block includes at least one primary energy conversion element for removing energy from the power transmission lines. The control circuit further includes a converter limb connected across the second and third primary terminals that includes an auxiliary converter. The control circuit further includes a control unit which controls the auxiliary converter to selectively provide a voltage source.
1. A control circuit comprising:
first and second primary terminals for respective connection to first and second power transmission lines;
a current transmission path extending between the first and second primary terminals and having first and second current transmission path portions separated by a third primary terminal, the first current transmission path portion including at least one primary switching element connected in series between the first and third primary terminals, the second current transmission path portion including an energy conversion block connected between the second and third primary terminals, the energy conversion block including at least one primary energy conversion element for removing energy from the power transmission lines;
a converter limb connected across the second and third primary terminals, the converter limb including a chain-link converter, wherein the chain-link converter includes at least one energy storage device for storing and releasing energy;
a control unit which controls the chain-link converter to selectively provide a voltage source to minimize a voltage across the or each primary switching element prior to switching of the or each primary switching element; and
further including a charging circuit having first and second auxiliary terminals, the first auxiliary terminal being connectable to the first power transmission line, the second auxiliary terminal being connected to the chain-link converter, the charging circuit including at least one auxiliary switching element, the or each auxiliary switching element being switchable to cause a current to flow between the first and second power transmission lines via the chain-link converter to charge the or each energy storage device.
2. The control circuit according to claim 1 wherein the first current transmission path portion including a plurality of primary switching elements connected in series between the first and third primary terminals.
3. The control circuit according to claim 1 wherein the chain-link converter includes a plurality of series-connected modules, each module including at least one secondary switching element and at least one energy storage device, the or each secondary switching element and the or each energy storage device in each module combining to selectively provide a voltage source.
4. The control circuit according to claim 1 wherein the converter limb further includes an inductive element connected in series with the chain-link converter.
5. The control circuit according to claim 1 wherein the control unit controls the chain-link converter to provide a variable voltage source in a ramping mode to ramp up or down a voltage across the second current transmission path portion.
6. The control circuit according to claim 1 wherein the control unit controls the chain-link converter to provide a zero voltage source in a bypass mode to permit current to flow in the converter limb while the first current transmission path portion is switched out of circuit.
7. The control circuit according to claim 1 wherein the charging circuit includes a resistive element with a higher resistance than the energy conversion block.
8. The control circuit according to claim 1 wherein the second auxiliary terminal is connected to a junction between the inductive element and the chain-link converter such that a branch is formed between the third primary terminal and the second auxiliary terminal, the branch including the inductive element.
9. The control circuit according to claim 1 wherein the chain-link converter further includes at least one secondary switching element being switchable to selectively control the chain-link converter to provide a voltage source or block the chain-link converter from providing a voltage source.
10. The control circuit according to claim 9 wherein the control unit controls the switching of at least one secondary switching element of the chain-link converter in a blocking mode to block the chain-link converter from providing a voltage source while the or each primary switching element is switched to switch the first current transmission path portion into circuit.
11. The control circuit according to claim 1 wherein the control unit controls the chain-link converter to selectively charge or discharge at least one energy storage device when current flows in the converter limb.
12. The control circuit according to claim 11 wherein the chain-link converter includes at least one secondary switching element, and the control unit controls the switching of the or each secondary switching element of the chain-link converter to selectively charge or discharge at least one energy storage device when current flows in the converter limb.