An example of a three-phase power distribution network is illustrated in Figure 1 below.
In the UK, voltages of 132 kV, 110 kV, 66 kV, 33 kV and 11 kV are typically used to provide primary distribution, with a 380–415 V three-phase and neutral low voltage supply to smaller consumers, such as residential or smaller commercial premises, where 220–240 V single-phase to neutral is taken off the three-phase supply.
3-Phase Power Distribution Network
Distribution voltages in continental Europe are typically 110 kV, 69 kV and 20 kV, but practice varies from country to country. In the USA, voltages of 138 kV, 115 kV, 69 kV, 34.5 kV, 13.2 kV and 4.16 kV are employed.
The transformer stepping down from the primary distribution to the low voltage supply may be pole-mounted or in a substation, and it is close to the consumers in order to limit the length of the low voltage connection and the power losses in the low voltage circuit.
In a national power system, many thousands of transformers and their associated circuit breakers or fuses / protective devices are required for distribution to low voltage circuits, in contrast to high-voltage transmission and primary distribution systems, where the number of substations is in the hundreds.
The progressive introduction of small-scale distributed generation (DG) is now a major issue for distribution networks.
It will be noted from Figure 1 that the primary and low voltage distribution systems are connected in a radial configuration. Circuit loops between adjacent substations are avoided because these can lead to circulating currents, which may increase the power losses and create difficulty in protection schemes.
However, tie circuits between adjacent lines and cables are available to reconfigure the network when a portion of the low voltage circuit is out of service for maintenance or because of failure.
These tie circuits are controlled by a normally open switch which can be closed manually within a few minutes, although an increasing trend is for automation of this operation by Supervisory Control and Data Acquisition (SCADA) systems.
Urban and suburban areas
In urban and suburban areas, much of the primary and low voltage distribution system is underground, with readily accessible substations sited in cellars or on small secure plots. Industrial sites may also have a number of substations incorporated into buildings or secure areas.
These may be controlled by the works engineer or operated and maintained by an electricity distribution company.
In rural areas and in more dispersed suburban areas, many three-phase overhead lines operating at 10-15 kV or 27-33 kV are supported for many miles on poles which may be of wood, concrete or steel lattice. The 380–415 V three-phase supply is taken from these lines through a small pole-mounted fused input/output transformer.
If the maximum load to be taken is below about 50 kW, the supplies for homes or farmsteads may be derived from a single-phase 10–15 kV supply.
Typically, a rural primary distribution system supplies up to 50 step-down transformers spread over a wide region. The lines in such a system are vulnerable to damage by tree branches, snow and ice accumulation and lightning strikes and it therefore has lower reliability than underground systems in urban areas.
Considerable ingenuity has been applied to protection of this type of system with the use of auto-reclosing supply circuit breakers and automatic reconnection switches.
Repairs to the low voltage system are still dependent, however, on consumers notifying a loss of supply.
Earthing of distribution systems
The proper earthing of distribution systems is of prime importance in order that excessive voltages do not appear on connections to individual consumers.
It is the practice in UK and some other countries to connect to earth the neutral conductor of the four-wire system and the star point of the low voltage winding on the step-down transformer, not only at the transformer secondary output, but also at every load point with a local meter and protective fuse.
This is known as the Protective Multiple Earth (PME) system, which is designed to ensure that all metallic covers and equipment fed from the supply are bonded so that dangerously high voltages do not hazard lives.
Reference: Newnes Electrical Power Engineer’s Handbook – D.F. Warne (Purchase hard copy from Amazon)