Shunt thyristor rectifiers for the generrex excitation system

Abstract

A shunt thyristor rectifier circuit has been applied to the new GENERREX excitation system for large steam turbine-generators. This shunt thyristor rectifier system allows less than one-eighth of the number of silicon controlled rectifiers (SCR's) to control the generator field current as in a full-SCR bridge, because the current is carried by diodes instead of SCR's. In the shunt thyristor circuit the SCR's serve only as shunt "trimmers". Current carrying capacity and demonstrated reliability of diodes are enough higher than that of SCR's that rectifier reliability, based on substantial field data from both diode and SCR equipments, should be several times as great as for a full SCR bridge. Augmenting this inherent advantage from the circuitry, a new water cooling system provides 0.015°C per watt total external thermal resistance per cell. This low external thermal resistance permits effective utilization of new 53 mm diodes which can carry up to 2050 amperes per path continuous current with this cooling system. These large cells minimize the number of cells required and thus increase reliability. Redundant series cells, compartmented parallel bridges with separate disconnects, a redundant bridge, and other maintenance-oriented features decrease the probability that a component failure could cause loss of excitation. All parts of this rectifier were designed exclusively for generator excitation. No compromise to make the parts serve other markets was required. The resulting design is unique in several respects, and well suited for its single intended purpose.