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Standby Generators - Definition, Uses and Disadvantages

A diesel or gas turbine generator set can be installed in a building to provide electricity when the public supply fails. This is a complete form of protection against all possible interruptions of the main supply.

The generator can be large enough to supply all the needs of the building and its output can be connected to the ordinary mains immediately after the supply authority’s meters and it then provides standby facilities for the entire building.

A diesel or gas turbine generator set can be installed in a building to provide electricity when the public supply fails. This is a complete form of protection against all possible interruptions of the main supply.

The generator can be large enough to supply all the needs of the building and its output can be connected to the ordinary mains immediately after the supply authority’s meters and it then provides standby facilities for the entire building.

It is cheaper, and may be adequate for the risk to be guarded against, to have a smaller generator serving only the more important outlets. In this case, the distribution must be arranged so that these outlets can be switched from main to emergency supply at one point and so that there is no unintentional path from the emergency generator to outlets not meant to be served by it. In effect the building is divided at the main intake into two distribution systems and only one of them is connected to the emergency changeover switch. It is also possible to install a completely separate system of wiring from the emergency generator to outlets quite distinct from the normal ones.

Standby Generators

This may be the simplest thing to do in a small building or when the emergency supply is required to serve only one or two outlets. It has the disadvantage that individual pieces of equipment have to be disconnected from one outlet and reconnected to another. Whilst this may not be acceptable in a hospital it may be quite in order in a large residence or hostel to have one or two emergency power points into which vacuum cleaners and other domestic equipment can be plugged when the main power supply is interrupted.

It takes 8 to 10s for a diesel generator to come to full speed. With the system just described this period is needed to bring the emergency supply into action after the mains have failed and, therefore, during this period there is no supply to the load. In some applications an interruption even of this short duration is not acceptable, and a more complex arrangement is necessary. In one system the diesel engine is coupled to a clutch the other side of which is connected to a squirrel cage induction motor. The induction motor drives an alternator through a flywheel, and the alternator supplies the load. Under normal conditions the induction motor is connected to the mains and the set operates as a motor alternator supplied from the mains.

When the mains fail the motor is disconnected from the mains and the diesel engine is started. As soon as it reaches its running speed, the clutch operates and the alternator is driven through the shaft of the motor by the diesel engine. During the time it takes for the engine to come up to speed the alternator is kept going by the flywheel. The automatic controls required for this arrangement are similar to those already described. Clearly this scheme is much more expensive and involves some permanent losses in the motor alternator set. It is used only for comparatively small power outputs for special purposes, such as telecommunications and power for aircraft landing systems.

Standby generators are normally supplied as complete units on a stand.  The diesel engine is a normal engine with a governor, and it would be outside the scope of this book to enter on a description of diesel engines. The alternator is directly coupled to the engine and has an automatic voltage regulator. The commonest type of alternator used is the screen protected, provided with brushless excitation. It is directly coupled to the engine and in smaller sizes may be overhung. In larger sizes it is supported at both ends from the set base plate. A separate exciter is mounted within the casing on the main shaft. In most modern sets the automatic voltage regulator is one of the static types. Finally, there is a control panel with voltmeters, ammeters, battery charger, incoming and outgoing terminals and the relays and circuits for the automatic start and stop control. A fuel tank is needed for the diesel engine, but this is normally supplied as a separate item and fixed independently of the generator set, with a short fuel pipe between them.

Diesel engines are noisy and it is prudent to arrange some form of sound-attenuating enclosure. The enclosure must have openings for fresh air to the engine and for the engine exhaust, and these openings will be found to limit the degree of silencing that can be achieved. Several manufacturers supply diesel generator sets complete in an enclosure which provides silencing and is also weatherproof, so that the set can be installed outdoors. Similar generating sets can, of course, be used to supply power to a building under normal conditions. In the UK it is not economic for consumers to generate their own power, but there are still parts of the world where it is a reasonable proposition.

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