IS i 3043 - 1987 23.2.3 Multiple Low Voltage Generator Earthing ( Synchronous Machines ) 23.2.3.1 Generator. operating in isolation from other supplies — When low voltage generating sets are operated in parallel, the energy source earth ing method is influenced by the magnitude of the circulating currents, particularly third harmonic, which can arise when generators are connected as four-wire machines. If the magnitude of the circulating current due to the nature of the load or the design of the generators is excessive when the neutrals are connected, then a neutral earth ing trnsformer or star-point earthing switches are required.
c) Generator star point switching — When this arrangement is adopted, it is necessary before the first generator is started for its star-point/neutral earthing switch to be closed. When subsequent sets are started, their star-point earthing switches remain open. This avoids the circulating* current problem, but it is essential that electrical and mechanical interlocks on the starpoint/earth switches ensure the integrity of the energy source neutral earth connection at all times and under all possible operating conditions.
23.2.3.2 Standby generators (without mains paralleling facility ) — The alternative neutral Hence, Lhree alternative neutral earthing earthing arrangements for standby generators are arrangements are possible for parallel operation as set out in 23.2.3.1 for generators operated in isolation from an electricity board supply. The as follows: earthing arrangements are shown in the following a) All generator neutrals connected — With this drawings: arrangement, the neutral busbar in the a) All generator neutrals connected (see main low voltage switchgear is connected Fig. 25 ); through a bolted link to an earthing con b) Neutral earthing transformer (seeFig. 26 ); ductor and independent earth electrode. and b) Neutral earthing transformer — By providing c) Alternator star-point switching ( Fig. 27). a neutral earthing transformer solidly con For standby generators with no mains para nected to the busbars, the system neutral can remain earthed at all times whilst any lleling facility, the changeover arrangement has number of generators can be connected to to prevent inadvertent connection of the genera tor outputs and electricity board supply. the busbars as three-wire machines. PRIME MOVER
R Y B N GENERATOR
CHANGEOVER FACILITY AUTOMATIC OR MANUAL
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PRIME MOVER
EXTRANEOUS J. CONDUCTIVE *-PARTS INDEPENDENT EARTH ELECTRODE NOTE 1 — Cable sheath of provided/shown •
Nora 2 — PNE link of provided/shown
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LfEJ NOTB 3 — Changeover switch could be 3-pole with linked neutral. FIG. 25
Low
VOLTAGE STANDBY GENERATORS WITH NEUTRALS GONNECIED
58
INCOMING ELEC BOARD LV SUPPLY