Abstract: Protecting a gate fully controlled thyristor that is controlled by a drive circuit including a turn-on circuit and a turn-off circuit against overvoltage, is accomplished by monitoring the anode voltage of said gate fully controlled thyristor relative to its cathode voltage. Upon overshoot of the voltage of a predetermined limit, a control signal is generated. This control signal causes the negative voltage source that is connected at the gate of the gate fully controlled thyristor to turn off. The gate fully controlled thyristor is then turned on by means of the turn-on circuit.

Abstract: A method and apparatus for commutating a load-independent current between the valve arms of a static converter device. Each valve arm has a valve device with blocking capability in the reverse direction, and a valve damping circuit. The valve device of a current-assuming valve arm is actuated at the time of commutation, and the valve device of the current-delivering valve arm reaches a zero current level upon commutation. In order to prevent a dangerous overvoltage via the inductance of the valve damping circuit, actuation of the valve device in the current-assuming valve arm is delayed until the valve device in the current-delivering valve arm substantially attains a zero current level after being turned off. During this delay, the current can commutate to the valve damping circuit of the current-delivering valve arm and generate a positive voltage there, thus expediently avoiding a reverse current and its dangerous consequences.

Abstract: Three voltage transformers have primary windings whose first ends are connected, respectively, to three a-c leads of a static converter and whose second ends are connected together to form an artificial neutral point. A high value resistor is connected between the neutral point and ground and a current-measuring means measures the current flowing in the resistor and generates an alarm signal in the event such current indicates a ground in the converter. Also disclosed is an arrangement including two high voltage converters connected to each other through smoothing chokes to form a frequency changer or converter in which the two converters have substantially identical circuit arrangements for detecting grounds with the circuit arrangements for detecting grounds having their outputs connected to each other.