The two thyristors connected in an anti-parallel connection, located between connections C and D, can briefly (approx. 5 s) conduct the pulse current. The overvoltage triggers a break-over diode (BOD) on the trigger circuit which in turn triggers the blocking thyristor and conducts the firing current past the blocking thyristor through a diode connected in an anti-parallel configuration to its gate/cathode. Independent of the polarity of the overvoltage, the break-over diode is always operated in the same direction using a bridge rectifier and the firing current is limited using series resistors. The thyristor fires within just a few microseconds and the voltage decreases quickly down to the forward voltage (1 to 1.5 V). The load current increases the temperature of the thyristor within just a few seconds and the thyristor and the stack construction (in the case of units 7VV3003-5...) absorb the thermal energy. As a consequence, the load cycle can only be repeated after a cooling time has elapsed (see Technical specifications).

The following also applies to units 7VV3003-5...:

The fast de-excitation option (G11) is connected to the firing circuit in such a way that the thyristors can be triggered at any time by controlling at least one of the three fast relays – that are independent of one another. This assumes that there is sufficient voltage. Generally, this is approximately 5 % of the trigger voltage. Each of the three relays can be controlled with 24 V DC, 110 V to 125 V DC or 220 V to 240 V DC.

The voltage detection for the de-excitation/discharge resistor is connected to the external de-excitation/discharge resistor. When the voltage detection responds, the supplying converter must be blocked or the current controlled down to zero. The voltage detection module requires an external 24 V DC power supply with min. 100 mA.

The following generally applies:

The de-excitation/discharge resistor is an external device and is not included in the scope of delivery of the SICROWBAR DC. Its resistance must be so low that even at the highest load current, the voltage is still under the destruction limit of the supplying converter and/or the winding to be protected. The lowest possible resistance is defined by the supply voltage and the maximum load current of the converter (dimensioning the fuses). The required de-excitation time must also be taken into account when dimensioning the value of the resistance.