In order to protect converter equipment which includes GTO switching-off thyristors and having d-c inputs and a protective inductance connected in series therewith against short circuit currents, a short circuit path for d-c current flowing from a d-c supply source on the one hand, and on the other hand, an energy storage device, is inserted into the d-c circuit, closed via an auxiliary thyristor, of the converter equipment in such a manner that through the change of the state of storage of the energy storage device, a countervoltage is impressed on the d-c terminals, which leads to the forcible commutation of the switching-off thyristors. The energy storage device advantageously comprises an auxiliary winding of a limiting choke serving as a protective inductance. The fault current, which increases steeply in the event of a short circuit via the protective inductance and the auxiliary thyristor, causes, due to the magnetic coupling, the generation of a counter voltage in the supplemental winding required for forcible commutation of the switching-off thyristors. For monitoring the current and for generating the auxiliary thyristor firing energy, a monitor likewise fed inductively by means of the protective inductance may be provided.

The power conversion system of this invention comprises line-commutated power converting means in which a line-commutated power conversion circuit that performs line-commutated commutation and a coupling diode are coupled to form a DC circuit and for converting DC power into AC power or AC power into DC power. Further, the system comprises self-commutated power converting means, in which a self-commutated power conversion circuit is coupled to the coupling diode in order to form a DC circuit and for reducing reactive power, or the reactive power and harmonics generated by the line-commutated converting means. Accordingly, their respective strengths can be made use of and their mutual weaknesses can complement each other.

A method is provided for determining a shorted thyristor cell in a bridge that supplies a load from a source, the bridge including a plurality of the thyristor cells. The method includes the step of sequentially gating each of the cells to a conducting state, so that only one cell is gated at one time; providing at least one current transformer in the bridge; generating a current flow that passes through the bridge including the one cell that is gated; observing current in the at least one current transformer to determine a short in one of the cells the that is not gated; and determining a shorted cell based on the step of observing current in the at least one current transformer.

A transient protection circuit for an inverter has a rectifier capable of supplying direct current from an alternating current power source, where protection is provided against a voltage transient in the AC power source. A DC bus is fed by the rectifier. A bus capacitor is connected across the DC bus. Solid state switches are connected to the DC bus for supplying alternating current to a load. The DC bus voltage is sensed. In response to a transient rise in the DC bus voltage, a resistor is connected across the bus capacitor to provide a discharge path, the solid state switches are disconnected from the DC bus, and after the transient dies away from the DC bus, the solid state switches are reconnected to the bus and the discharge resistor is disconnected from across the bus capacitor.