GATE DRIVE

A hard drive with sharp rise time is necessary to turn-on the device quickly with large initial mdi/dt. A single short pulse is adequate to turn-on the device,provided the current in the device does not go below the holding current during the conduction period(about 120) of the device. Sometimes, long pulses (or a pulse train lasting the required conduction period) are applied to avoid the blocking of the device due to discontinuous conduction.

             The locus of possible gate trigger points is bounded by two lines A and B. There are also lower and upper limits on direct gate voltage and current. The HVDC valve are functions of the junction temperature. The thyristor in a modern HVDC valve are fired from the optical signals sent from the circuits at the ground potential. These signals in turn are generated from the converter controller. The source of firing energy is provided at each module by a power supply unit which is charged from the forward voltage across the device when it is not conducting. The diagram of typical module with one thyristor per module. The gate pulses are generated in the gating and logic units in response to optical signals,transmitted via fibre-opti light guides separately to each module. The gating and thyristor status are also monitored at the ground level the signals sent via another set of light guides. The breakover diode (BOD) is used to sense overvoltage across the device and protect it by generating pulse. BOD firing is also used as a back-up if the gating and logic unit fails.

AC Voltage grading is provided by resistor RD and capacitor CD in series. This circuit also damps the oscillations produced by step changes in the voltage, which occur during commutation of current from one valve to the next. It also limits the rate of rise of voltage on a late firing module.

   

        The resistor RDC provides for DC voltage grading and this circuit is also used for voltage measurement. The saturable reactor is used to protect the device against di/dt. CH and RH are used as grading circuits for high frequency voltages.