High Power Grid System with Thyristor Controls

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 thyristor valve or valves, and lower thyristor valve or valves, connected in series;
• an associated branch extending from between the upper and lower thyristors;
• and
• at least one thyristor-based capacitor module for each phase, the method comprising the step of;
• inserting, by firing one or more thyristors within the at least one thyristor-based capacitor modules, a main capacitor into the respective arm of the bridge circuit;
• and
• further comprising the step of firing one or more first subsidiary thyristors within the at least one thyristor-based capacitor modules in order to allow current to flow through one or more L-C oscillation circuits thereby generating one or more L-C, oscillations, each L-C oscillation causing both a bypass thyristor and a first subsidiary thyristor to switch off.

A method of switching of a thyristor based half-bridge capacitor module for use in commutation of a line commutated converter, LCC, wherein the capacitor module comprises a thyristor, a main capacitor, a plurality of auxiliary capacitors and a plurality of diodes, the method comprising:

• (i) inserting the main capacitor into a main circuit loop at the instant of commutation start;
• (ii) bypassing the main capacitor from the main circuit loop at the end of commutation;
• (iii) reversing polarity of a first auxiliary capacitor;
• (iv) firing the thyristor to form a current path;
• (v) turning off the thyristor and inserting the main capacitor into the main circuit loop with a short lead time just before the start of commutation;
• and
• (vi) reversing polarity of a second auxiliary capacitor.

A method of switching a thyristor based full-bridge capacitor module for use in commutation of a line commutated converter, LCC, which comprises a converter transformer, wherein the capacitor module is connected to the LCC between a valve side and a secondary side of the converter transformer and comprises a main capacitor, at least four auxiliary capacitors and a plurality of diodes, the method comprising:

• (i) initial charging of the main and auxiliary capacitors;
• (ii) inserting the main capacitor into a circuit to provide an additional commutation voltage;
• (iii) bypassing the main capacitor;
• (iv) reversing the voltage polarity of a first of the auxiliary capacitors;
• (v) inserting the main capacitor into the circuit with its negative polarity connected to the valve side;
• (vi) reversing the voltage polarity of a second subsidiary capacitor;
• (vii) inserting the main capacitor C into the circuit, with its negative side connecting to the valve side;
• (viii) reversing the voltage polarity of a third subsidiary capacitor;
• (ix) inserting the main capacitor into the circuit with its positive side connected to the valve side;
• (x) reversing the voltage polarity of a fourth subsidiary capacitor