Method and apparatus for controlling discharge of a thyristor-switched capacitor

A method of discharging a static VAR compensator connected to a AC electrical system including a capacitor connected in series between the AC electrical system and two anti-parallel thyristor switches which are selectively controlled by external control signals comprising the steps off

• determining when a substantially constant voltage exists across the capacitor;
• applying a periodic series of firing pulses to one of the thyristors to gradually discharge the capacitor thereby relieving stress on and minimizing ratings for the capacitor and the thyristors;
• and
• determining when the magnitude of the voltage across the thyristor is within a predetermined threshold range.

A system for gradually discharging a static VAR compensator, comprising:

• a power transmission line for providing AC power;
• a capacitor connected to the transmission line;
• switching means for controllably switching the capacitor across the transmission line;
• a control circuit for periodically activating the switching means gradually reducing the capacitor voltage;
• and
• means for detecting when the voltage across the switching means is within a predetermined range.

A system for rapidly discharging a static VAR compensator, comprising:

• a power transmission line for delivering AC power;
• a capacitor connected to the transmission line;
• switching means for controllably switching the capacitor across the transmission line;
• control means for activating the switching means during a single time period when a substantially constant voltage exists across the capacitor to rapidly reduce the capacitor voltage to zero;
• and
• means for detecting when the voltage across the switching means is within a predetermined range.