Excitation Systems Performance witn Motor-Driven Exciters

Two criteria of satisfactory systems performance are the ability of the main generator to retain stability and the ability of the exciter set to recover after a disturbance. Normal emergency throw-over times are generally between one and two seconds. This study indicates that for these throw-over times a very substantial margin in system performance exists, with the use of a special design of motor-driven exciter set with an inertia constant H of 5 and a maximum torque of about 325 per cent rated. This exciter set does not require a shorter throw-over time than any other essential station auxiliary. The high torque characteristic of the induction motor enables satisfactory system performance to be obtained with voltage at the motor terminals as low as 70 per cent of normal both on steady state and on re-energization after throw-over. With this exciter, for a 3-phase short-circuit fault at or near the generator terminals, the maximum permissible switching time closely approaches the actual critical switching times which vary between approximately 11 and 20 cycles for the conditions studied. Practical switching times are generally below ten cycles. Additional special design considerations of the exciter set as presented in reference 1 further improve performance for the short-circuit fault operation.