Presence of LV back-up generators

(Fig. D16)
Here we only consider LV back-up generators.
The electrical power supply supplied by a back-up generator is produced by an alternator, driven by a thermal engine.
No power can be produced until the generator has reached its rated speed. This type of device is therefore not suitable for an uninterrupted power supply.
According to the generator’s capacity to supply power to all or only part of the installation, there is either total or partial redundancy.
A back-up generator functions generally disconnected from the network. A source switching system is therefore necessary.
The generator can function permanently or intermittently. Its back-up time depends on the quantity of available fuel.

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Fig. D16: Connection of a back-up generator

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The main characteristics to consider for implementing LV back-up generator:

• Sensitivity of loads to power interruption,
• Availability of the public distribution network,
• Other constraints (e.g.: generators compulsory in hospitals or high-vise buildings)

The presence of generators can be decided to reduce the energy bill or due to the opportunity for co-generation. These two aspects are not taken into account in this guide.
The presence of a back-up generator is essential if the loads cannot be shed for an indefinite duration (long interruption only acceptable) or if the utility network availability is low.
Determining the number of back-up generator units is in line with the same criteria as determining the number of transformers, as well as taking account of economic and availability considerations (redundancy, start-up reliability, maintenance facility).