In the system using a generator and an internal- combustion engine (Fig. 9.8), the normal flow of power is from the plant auxiliary a-c system. If the commercial power fails, the internal-combustion engine is started. As soon as the proper voltage and frequency are established at the generator terminals, the transfer switch connects the load to the generator. This system is widely used in nuclear
power plants as the emergency auxiliary-power source. The main disadvantage of this system is that the load is interrupted for the time interval required to start the engine and transfer the load to the generator; typically this is 10 to 15 sec.

A-C

INPUT

The system shown in Fig. 9.8 can be modified so that the generator will float across the line by being driven continuously by the engine. Whenever the normal a-c source failed, the generator would deliver power im­mediately without interruption. With this scheme the engine starting period is eliminated. However, the engine runs constantly, and transients are present on the line when switching. Power directional relays and synchronizing equipment are required for proper operation of the transfer switch. Since the engine is running continuously, increased maintenance and operating costs are involved. This is a distinct disadvantage.

9- 5.5 Synchronous Motor-Generator—Flywheel- Clutch—Internal-Combustion-Engine Systems

(a) Nonisolated System. In the nonisolated system shown in Fig. 9.9, the critical load is normally fed directly from the plant auxiliary a-c power system in parallel with a synchronous machine operating as a motor driving a flywheel. Whenever normal plant auxiliary power is inter­rupted or a frequency or voltage anomaly in excess of preset tolerances is experienced, the synchronous motor

and critical bus are disconnected from the system. The synchronous motor instantaneously converts to generator operation to supply the critical bus with interim power, with the stored energy of the flywheel being transferred to
drive the generator. The engine, when furnished, would simultaneously be started and then connected to the load when it is up to speed. The engine is recommended only for those systems requiring operating time, after plant auxiliary-power failure, in excess of the stored-energy capability of the flywheel.

Under fault conditions this type of system is subject to a power dip during the time required to isolate the critical a-c load from the plant auxiliary source In addition, since the critical load is normally fed from the plant auxiliary source, it is subjected to any transients occurring on that system. At best this system (with the engine) is justified only where the plant auxiliary source is very unreliable.

(b) Isolated System. The isolated system shown in Fig 9.10 offers a significant improvement over the nonisolated system (Fig. 9.9) in that complete isolation from the plant auxiliary source is obtained. The system consists of a synchronous motor with a stored-energy flywheel unit, which is fed from the plant auxiliary source and drives a synchronous generator that feeds the critical bus A standby engine is used whenever the duration of the outage exceeds the capability of the inertial unit. During normal operation voltage — and frequency-sensing devices monitor the incoming power line for variations beyond acceptable limits.

Should an unacceptably large voltage or frequency excursion occur, the synchronous motor is disconnected from the plant auxiliary-power source, and the stored

energy in the flywheel is used to drive the synchronous generator. The standby engine, if furnished, is simul­taneously started and brought up to synchronous speed in about 10 to 60 sec, depending on the stored-energy content of the flywheel, at which time the engine is connected to the generator shaft by the magnetic clutch. The frequency stability under engine operation is maintained by a highly sensitive load — and frequency-sensing governor that closely controls the speed of the engine. Should the voltage and frequency of the plant auxiliary power return to acceptable values and remain for a preset time period, the synchronous motor is synchronized to the source, the clutch is deener­gized, and the engine is returned to standby condition.

On loss of plant auxiliary power, the system begins to draw energy from the flywheel and causes it to lose speed. Since the frequency is directly related to the speed of the flywheel-driven motor—generator unit, the frequency is soon reduced to a value below an acceptable limit to the critical bus.

The overall frequency regulation of this system is equal to that of the plant auxiliary-power system.