Field suppression of turbo-alternators

The paper is concerned with the field suppression of synchronous machines by means of a discharge resistor connected across the field winding and with the suitability of such a method for alternators of 100MW and above. Expressions are given for the field current when a 3-phase fault is applied to the armature terminals, and also for the currents and voltages during field suppression. The effect of the discharge resistance value upon the field over-voltages, the field ciucuit-breaker arc duration and the stator fault damage is discussed. Many field suppression tests have been made on turbo-alternators in conjunction with short-circuit tests on the alternator. Results of some of these are presented and are compared with values of theoretical expressions derived as illustrated in the Appendix. The duty on the main and discharge contacts of the field circuit-breaker and methods of proving their suitability for the duties are considered. The use of a synthetic circuit for proving the interrupting capacity of the main contacts is unsuitable if the arcing time is long. Other forms of field suppression are available. These may give improved field suppression but either require more complex circuits or are unlikely to be suitable for very large alternators. It is concluded that the value of the discharge resistance should be determined by the permissible field over-voltate and circuit-breaker arc duration.