Performance and modelling of 70kVdc power supply with solid-state crowbar

Abstract

The experimental activities of tokamak research involve development of high power RF and microwave sources for fusion related heating and current drive applications. High power RF and microwave tubes like Klystron, Gyrotron and Tetrode are in general operated with high voltage DC power supplies. These HVDC power supplies of the order of 70kVdc, must be equipped with necessary arc fault protection in addition to general over current and over voltage protection.The arc fault protection must act within few microseconds to prevent permanent damage to the RF tube, window, etc. When an arc fault is detected, output voltage of the DC power supply is short circuited using a crowbar device (generally Ignitron, Thyratron, thyristor, rail-gap, etc.) that operates in few microseconds. This diverts the fault current from the load to crowbar device, thereby protecting the load. This is necessary as conventional protection in the power supply input takes ∼100ms to switch-off. The crowbar device must be able to take the fault current till the circuit breaker placed at power supply input is switched off.The arc fault protection is tested for its effectiveness by “wire-burn” test. Full power short circuit of ∼1.5MW DC power supply puts enormous stress on the power supply, utility and the crowbar, therefore frequent wire-burn testing is to be avoided.This report presents simulation of wire-burn test using PSIM software. Optimization of the component values without conducting actual wire-burn test could be achieved.