Abstract: A protection system for a thyristor valve composed of a plurality of thyristors connected in series. The protection system includes a forward voltage detection circuit for generating a forward voltage signal, a first reverse voltage detection circuit for detecting that a reverse voltage in excess of a first reverse voltage level is applied to the thyristor to generate a first reverse voltage signal, and a second reverse voltage detection circuit for detecting that a reverse voltage in excess of a second reverse voltage level larger than the first reverse voltage level is applied to the thyristor to generate a second reverse voltage signal.

Abstract: A device for controlling a thyristor converter including series connected thyristors. The device includes a plurality of forward voltage detection circuits, each of which detects forward voltage of one of the thyristors and generates a forward voltage signal based on the detected forward voltage, respectively. The device includes a first circuit for generating a conduction command signal which commands the thyristor converter to conduct, and a second circuit for generating a plurality of light gate signals and a gate monitoring signal based on a plurality of modified forward voltage signals and the conduction command signal. Each of the light gate signals is applied to one of the thyristors to turn ON thereof, respectively. The device includes a first delay circuit connected to receive the gate monitoring signal for delaying the gate monitoring signal by a first predetermined time to produce a first delayed signal.

Abstract: A synchronous condenser is provided on an AC bus line which is connected to an AC power system via a circuit breaker. The AC bus line and a DC bus line are connected via a power converter system. In such a system, if three conditions: the breaker is opened; the synchronous condenser is under operation; and the power converter operates as an inverter, are met, the power converter is driven into a zero power factor operation, i.e., an operation to cancel a capacitive reactive power on the AC bus line including AC filters in order to prevent the synchronous condenser self-excitation.