Because oil is still a principal source of standby energy, diesel engine driven generating sets are used on a large scale as a back-up to the public electricity supply. Such sets comprise a diesel engine, coupled to a generator with appropriate control gear for operation, instrumentation and protection.

Usually the sets are electrically started from 12 V or 24 V batteries and arranged to run up automatically on mains failure, take over the load and shut down again when the mains supply returns. In some cases the load is separated into non-essential and essential groups and only the latter is then supplied from the standby set during mains outage.

Diesel generating sets are rated according to their electrical output expressed in kW at a load power factor usually assumed to be 0.8 lagging. The smallest units, up to say 10kW, are often arranged for l.v. single-phase output but the greater majority, between say 10 and 1000kW, have a three-phase output, some of the larger sizes generating at 3.3–11kV.

The most popular size for general industrial standby use lies in the 150 to 500 kW range and where greater standby capacity is required, two or more sets may be operated in parallel. Multiple set schemes frequently involve less capital outlay than a single larger unit and also provide greater flexibility, easier maintenance schedules and better system reliability.

To minimise initial cost, diesel sets are generally run at relatively high speeds and four-pole generators are very common running at 1500 rev/min for 50 Hz output or 1800rev/min for 60Hz.Very small sets may use two-pole generators (3000/3600 rev/min) while larger sets may sometimes use six-pole generators (1000/1200 rev/min).

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