An analysis of the major disturbances that have occurred over the past few years has demonstrated the importance of maintaining, if at all possible, continuous operation of system generating units. Generally, hydroelectric units are less vulnerable than thermal units to frequency excursions and, in the event of trip out, can be re-Synchronized and fully loaded in a fraction of the time required by thermal units. On the other hand, large thermal units that are tripped from either low or high frequency and at the same time lose their auxiliaries present a much more difficult operating problem in restoring generating capacity to the system. Even though system help or standby units may be available to restart auxiliaries, much valuable time is consumed in restarting the unit and restoring it to full load. Under the worst conditions of a major system collapse and when no outside supply is available, at the time of separation from the system each thermal unit should be able to hold on to its own station service transformer supply. However, in the event that this station service supply is lost, provision should be made to supply, either by an in-house steam or gas turbine generator or diesel engine generator, auxiliary lube oil and hydrogen seal oil pumps, turbine turning gear, emergency lighting, battery chargers, and supplies for communication and public address systems in order to prevent serious damage to the unit for shutdowns over 30 to 45 minutes' duration. Some vital auxiliaries, such as auxiliary lube oil pumps, condensate extraction pumps, and boiler feed pumps can be made especially reliable by providing a steam turbine drive. Copyright © 1968 by The Institute of Electrical and Electronics Engineers, Inc.
