Presence of LV back-up generators

(see Fig. D18)

LV backup-up generator is the association of an alternator mechanically powered by a thermal engine.

No electrical power can be delivered until the generator has reached its rated speed. This type of device is therefore not suitable for an uninterrupted power supply. Depending, if the generator is sized to supply power to all or only part of the installation, there is either total or partial redundancy.

A back-up generator runs generally disconnected from the network. A source changeover and an interlocking system is therefore required (see Fig. D18).

The generator back-up time depends on the quantity of available fuel.

Fig. D18 – Connection of a back-up generator

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

• Sensitivity of loads to power interruption (see Voltage Interruption Sensitivity for definition),
• Availability of the public distribution network (see Service reliability for the definition),
• Other constraints (e.g.: generators compulsory in hospitals or high buildings)

In addition 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 (only short interruption 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).

Determining the generator apparent power, depends on:

• installation power demand of loads to be supplied,
• transient constraints that can occur by motors inrush current for example.