Elimination of commutation failure by hybrid HVDC system

A line commutated converter, LCC, for a high-voltage, direct current, HVDC, power converter, the LCC comprising at least one bridge circuit for connection to at least one terminal of a DC system, each bridge circuit comprising a plurality of arms, each associated with a respective phase of an AC system, each arm comprising:

• an upper and lower thyristor connected in series;
• an associated branch extending from between the upper and lower thyristors;
• at least one capacitor module for each phase, the or each capacitor module operable to insert a capacitor into the respective arm of the bridge circuit;
• wherein the capacitor modules are operable to insert a first capacitor into the bridge circuit during a first commutation period, wherein commutation is provided from a first initially conducting thyristor in a first initially conducting arm to a first subsequently conducting thyristor in a first subsequently conducting arm;
• wherein the capacitor modules are operable to insert the first capacitor into the first initially conducting arm to increase an effective commutation voltage, thereby reducing a current flowing through the first initially conducting thyristor and charging the capacitor;
• and
• wherein the capacitor modules are operable to insert the first capacitor into the bridge circuit during a second commutation period, wherein commutation is provided from a second initially conducting thyristor in a second initially conducting arm to a second subsequently conducting thyristor in a second subsequently conducting arm.

A method of operating a line commutated converter, LCC, for a high-voltage, direct current, HVDC, converter, wherein the LCC comprises at least one bridge circuit for connection to at least one terminal of a DC system, each bridge circuit comprising a plurality of arms, each associated with a respective phase of an AC system, each arm comprising:

• an upper and lower thyristor connected in series;
• an associated branch extending from between the upper and lower thyristors;
• and
• at least one capacitor module for each phase, the method comprising:
• inserting, by the or each capacitor module, a capacitor into the respective arm of the bridge circuit in either polarity.

A method of operating a line commutated converter, LCC, for a high-voltage, direct current, HVDC, converter, wherein the LCC comprises at least one bridge circuit for connection to at least one terminal of a DC system, each bridge circuit comprising a plurality of arms, each associated with a respective phase of an AC system, each arm comprising:

• an upper and lower thyristor connected in series;
• an associated branch extending from between the upper and lower thyristors;
• and
• at least one capacitor module for each phase, the method comprising:
• inserting, by the or each capacitor module, a capacitor into the respective arm of the bridge circuit;
• wherein the capacitor modules insert a first capacitor into the bridge circuit during a first commutation period, wherein commutation is provided from a first initially conducting thyristor in a first initially conducting arm to a first subsequently conducting thyristor in a first subsequently conducting arm;
• wherein the capacitor modules insert the first capacitor into the first initially conducting arm to increase an effective commutation voltage, thereby reducing a current flowing through the first initially conducting thyristor and charging the first capacitor;
• and
• wherein the capacitor modules insert the first capacitor into the bridge circuit during a second commutation period, wherein commutation is provided from a second initially conducting thyristor in a second initially conducting arm to a second subsequently conducting thyristor in a second subsequently conducting arm.

A line commutated converter, LCC, for a high-voltage, direct current, HVDC, power converter, the LCC configured as a rectifier for converting from AC to DC, and as an inverter for converting from DC to AC and comprising at least one bridge circuit for connection to at least one terminal of a DC system, each bridge circuit comprising a plurality of arms, each associated with a respective phase of an AC system, each arm comprising:

• an upper and lower thyristor connected in series;
• an associated branch extending from between the upper and lower thyristors;
• and
• at least one capacitor module for each phase, the or each capacitor module operable to insert a capacitor into the respective arm of the bridge circuit,
• wherein the capacitor modules are operable to insert a capacitor into the bridge circuit during a commutation period, wherein commutation is provided from an initially conducting thyristor in an initially conducting arm to a subsequently conducting thyristor in a subsequently conducting arm, to increase an effective commutation voltage, and
• wherein during a fault of the AC system, the effective commutation voltage is controlled to provide a controllable reactive power support to the AC system, whereby the rectifier and inverter, respectively, supply a positive reactive power to the AC system, with a negative firing angle control for the rectifier and a negative extinction angle control for the inverter.