BYPASS OPERATION BY CYCLIC FIRING OF THE BRIDGE THYRISTORS.

By-passing of a thyristor bridge is used in large converter systems such as in hvdc transmission lines for fault conditions. In normal voltage-control operation, an increase in the total power factor of the whole convertor system can be achieved by relieving a few bridges from current by using external bypass valves. However, a fully rated external bypass valve is relatively expensive because it has to carry continuously the full direct current. A novel way of continuous operation of the bridge thyristors in a cyclic firing method is described. This method allows the designer to eliminate the external bypass valve and still get the bypass mode of operation.