Overvoltage protection in series thyristor string for static var compensator

A gate pulse generator, comprising:

• a thyristor switch being formed of a first block of series-connected thyristors and a second block of series-connected thyristors, said second block being anti-parallel connected to said first block;
• first detector means for detecting a first voltage applied across said thyristor switch to generate a first voltage signal when the first block thyristors is forwardly biased;
• first firing means for firing said first block thyristors in accordance with a first conduction period signal and said first voltage signal;
• first expanding means for expanding by a first predetermined period an active period of said first conduction period signal from time when said first voltage is substantially zero;
• second detector means for detecting a second voltage applied across said thyristor switch to generate a second voltage signal when the second block thyristors is forwardly biased;
• second firing means for firing said second block thyristors in accordance with a second conduction period signal and said second voltage signal;
• and
• second expanding means for expanding by a second predetermined period an active period of said second conduction period signal from time when said second voltage is substantially zero;
• means for generating a first phase signal when said first block thyristors are too be turned on, a second phase signal when said second block thyristors are to be turned on, and an ON signal when said thyristor switched capacitor is to be activated,
• wherein said first firing means includes first generator means for generating said first conduction period signal when both said first phase signal and said ON signal are generated.

A gate pulse generator for a static var compensator comprising a thyristor switch obtained by series-connecting a plurality of antiparallel-connected thyristors, a capacitor ON/OFF-controlled by said thyristor switch, and a reactor series-connected to said capacitor, comprising:

• flip-flop means, being set by a first phase signal having a predetermined phase difference from a voltage applied to said static var compensator, for providing an output signal, said flip-flop means being reset by an output from a conduction detector of the thyristors operated during a predetermined period starting from the generation of a second phase signal which is generated in a phase opposite to the first phase signal;
• voltage detecting means for detecting a voltage applied to said thyristor switch to provide a voltage signal;
• and
• means for detecting a logical AND of the output signal from said flip-flop means and the voltage signal from said voltage detecting means, and for applying a gate pulse having a predetermined width to the thyristors constituting said thyristor switch when the detected logical AND is set at "1".