4 Common Arrangements of Distribution Substations

The general technique of supplying large quantity of power to factories or other complexes is by means of high-voltage (HV) supply, typically at 11 kV, occasionally at 6.6 kV. The installation consists of a main substation at the point of entry, with HV cables to supply subsidiary substations located near load centres. Where necessary, it may be possible for the consumer to obtain two supplies from the power authority, especially in cases where loss of supply could give rise to a particular hazard for example in a hospital. The two supplies may be provided from separate sources.

The distribution system typically consists of either radial feeders or a ring main using underground cables to supply the subsidiary substations. Four main distribution arrangements include:

Radial Feeder Arrangement

In a radial feeder arrangement, each feeder is controlled by its own automatic circuit breaker. A cable or transformer fault will result in loss of supply only to the one substation, but supply cannot be restored until the repair is done. 

Ring Main Arrangement

The ring main arrangement allows for any individual transformer substation to be isolated for inspection or test without interrupting the supply to other substations. Any fault occurring on the system will result in a total shut down, but the fault can be isolated and the supply restored to all substations except one, unless the fault is in the cable between substations C and D, in which case all substations can be re-energized.

Distribution substation with outgoing cables controlled by switch and fuses

The distribution substations have their outgoing cables controlled by switches and the transformer by a fuse switch. Thus, every transformer can be separately switched, and should a fault occur and one of the fuses blow, the fuse switch is arranged to open all three phases automatically, hence disconnecting the transformer from the system.

The scheme would typically be operated with the ring open, but any cable fault will result in part of the system being shut down until the fault is located, when power can be restored to all the substations.

High-voltage (HV) installation with automatic circuit breakers

By employing automatic circuit breakers throughout as illustrated in Figure 1.3 above, a greater degree of security of supply is achieved. The system can be operated as a closed ring and a greater security can even be achieved by duplicating the transformers as illustrated in the figure above.

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