ADJUSTABLE-SPEED DRIVE RETROFIT FOR ORMOND BEACH FD FANS

The boiler on Southern california Edison's 750 MW Ormond Beach Unit 2 was extensively modified to reduce minimum load from 250 NW to 36 MW. With the unit operating at minimum load frequently at night, it became economically attractive to install adjustable-speed drives (ASDs) on each of the 6250 hp Forced Draft Fan Motors. The ASD technology that was used is a fourth generation current-source, GTO-PWM inverter that is used in a configuration fostered by a series of five ASD field tests conducted for EPRI by Bechtel in the period from 1984 to 1989. This ASD concept has features that ensure the high degree of reliability that is needed in these high capacity utility power plant applications. These features are as follows: 12 pulse inverter to minimize harmonics to the utility bus. UPS for ASD control power to eliminate outside influences on the operation of the ASD. Input transformer and ground at the motor side of the inverter to prevent high line-to-neutral voltage on the motor. Selective harmonic elimination inverter control to preclude electrical resonance between output filter and motor. Liquid cooling of thyristors to prevent temperature-related failures. The Ormond Beach ASD system is the world's largest and most sophisticated of its kind. These 7200hp ASDs have operated for 3 years with no backup system, attesting to the reliability that can be achieved with this approach. This paper describes the application concept that has evolved from the EPRI tests, the GTO-PWM ASD technology, field test results of power savings, measured harmonics at utility bus and at the motor, effect of THD reduction by using an input harmonic filter, and operating experience to date. Fan inlet vanes have been removed, and no ASD bypass was provided.