High power grid system with thyristor controls

A method of operating a line commutated converter for a high-voltage, direct current converter, wherein the line commutated converter 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:

• at least one upper thyristor valve, and at least one lower thyristor valve, connected in series;
• an associated branch extending from between the at least one upper thyristor and the at least one lower thyristor;
• and
• at least one thyristor-based capacitor module for each phase, the method comprising a 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 a 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 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 an instant of commutation start;
• (ii) bypassing the main capacitor from the main circuit loop at an 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 instant of commutation start;
• 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 which comprises a converter transformer, wherein a capacitor module is connected to the line commutated converter 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 a voltage polarity of a first of the auxiliary capacitors;
• (v) inserting the main capacitor into a circuit with a negative polarity of the main capacitor connected to the valve side;
• (vi) reversing the voltage polarity of a second subsidiary capacitor;
• (vii) inserting the main capacitor into the circuit, with a negative side of the main capacitor connecting to a valve side;
• (viii) reversing a voltage polarity of a third subsidiary capacitor;
• (ix) inserting the main capacitor into a circuit with a positive side of the main capacitor connected to a valve side;
• (x) reversing a voltage polarity of a fourth subsidiary capacitor.