POWER DISTRIBUTION SYSTEMS
Electricity distribution is the final stage in the delivery of electricity to end users. A distribution system’s network carries electricity from the transmission system and delivers it to consumers. Typically, the network would include medium-voltage (2kV to 34.5kV) power lines,substations and pole-mounted transformers, low-voltage (less than 1 kV) distribution wiring such as a Service Drop and sometimes meters.
First commercial distribution of electric power
Simplified diagram of AC electricity distribution from generation stations to consumers. Transmission system elements are shown in blue, distribution system elements are in green.
In the very early days of electricity distribution (for example Thomas Edison’s Pearl Street Station), direct current (DC) generators were connected to loads at the same voltage. The generation, transmission and loads had to be of the same voltage because there was no way of changing DC voltage levels, other than inefficient motor-generator sets. Low DC voltages (around 100 volts) were used since that was a practical voltage forincandescent lamps, which were the primary electrical load. Low voltage also required less insulation for safe distribution within buildings. The loss in a cable is proportional to the square of the current, and the resistance of the cable. A higher transmission voltage would reduce the copper size to transmit a given quantity of power, but no efficient method existed to change the voltage of DC power circuits. To keep losses to an economically practical level the Edison DC system needed thick cables and local generators. Early DC generating plants needed to be within about 1.5 miles (2.4 km) of the farthest customer to avoid excessively large and expensive conductors.
Introduction of alternating current
The competition between the direct current (DC) and alternating current (AC) (in the U.S. backed by Thomas Edison andGeorge Westinghouse respectively) was known as the War of Currents. At the conclusion of their campaigning, AC became the dominant form of transmission of power. Power transformers, installed at power stations, could be used to raise the voltage from the generators, and transformers at local substations could reduce voltage to supply loads. Increasing the voltage reduced the current in the transmission and distribution lines and hence the size of conductors and distribution losses. This made it more economical to distribute power over long distances. Generators (such as hydroelectric sites) could be located far from the loads.
Variations
North American and European power distribution systems differ in that North American systems tend to have a greater number of low-voltage step-down transformers located close to customers’ premises. For example, in the US a pole-mounted transformer in a suburban setting may supply 7-11 houses, whereas in the UK a typical urban or suburban low-voltage substation would normally be rated between 315 kVA and 1 MVA and supply a whole neighbourhood. This is because the higher domestic voltage used in Europe (415 V vs 230 V) may be carried over a greater distance with acceptable power loss. An advantage of the North American system is that failure or maintenance on a single transformer will only affect a few customers. Advantages of the UK system are that the transformers are fewer in number, larger and more efficient, and due to the diversity of many loads there need be less spare capacity in the transformers, reducing waste. In North American city areas with many customers per unit area, network distribution may be used, with multiple transformers interconnected with low voltage distribution buses over several city blocks.
Rural electrification systems, in contrast to urban systems, tend to use higher distribution voltages because of the longer distances covered by distribution lines (see Rural Electrification Administration). 7.2, 12.47, 25, and 34.5 kV distribution is common in the United States; 11 kV and 33 kV are common in the UK, Australia and New Zealand; 11 kV and 22 kV are common in South Africa. Other voltages are occasionally used.
In New Zealand, Australia, Saskatchewan, Canada, and South Africa, single wire earth return systems (SWER) are used to electrify remote rural areas.
While power electronics now allow for conversion between DC voltage levels, AC is preferred in distribution due to the economy, efficiency and reliability of transformers. High-voltage DC is used for transmission of large blocks of power over long distances, for transmission over submarine cables for medium distances or for interconnecting adjacent AC networks, but not for local distribution to customers. Electric power is normally generated at 11-25kV in a power station. To transmit power over long distances, it is then stepped-up to higher voltages as necessary: 400kV, 330kV, 275kV, 220kV, 132kV, 110kV and 66kV are common in UK, Ireland, Australia and New Zealand, while 765kV, 500kV, 345kV, 230kV, 138kV, 115kV and 69kV are common in North America. Power is carried through this transmission network of high voltage lines for hundreds of kilometres and delivers the power as an interconnected power pool called the ‘electric grid’. This grid is then connected to load centres (cities) through a sub-transmission network of lines at voltages from 33kV up to 230kV or more. These lines terminate at substations, where the voltage is further stepped-down to 25kV or less for power distribution to customers through a distribution network of local lines at these lower voltages. A ‘grid’ does not actually enable power to flow with no loss from one end to the other – it may be hundreds of kilometers long, but the power flows inside the grid are typically much shorter than that, and it would be very inefficient to treat the ‘grid’ as a long-distance transmission carrier. The ‘grid’ really performs a ‘balancing’ function – enabling local power generators across a country to synchronise their power outputs and thus readily share generated power with their neighbours.
Modern distribution systems
The modern distribution system begins as the primary circuit leaves the sub-station and ends as the secondary service enters the customer’s meter socket by way of a service drop. Distribution circuits serve many customers. The voltage used is appropriate for the shorter distance and varies from 2,300 to about 35,000 volts depending on utility standard practice, distance, and load to be served. Distribution circuits are fed from a transformer located in an electrical substation, where the voltage is reduced from the high values used for power transmission.
Conductors for distribution may be carried on overhead pole lines, or in densely populated areas, buried underground. Urban and suburban distribution is done with three-phase systems to serve both residential, commercial, and industrial loads. Distribution in rural areas may be only single-phase if it is not economical to install three-phase power for relatively few and small customers.
Only large consumers are fed directly from distribution voltages; most utility customers are connected to a transformer, which reduces the distribution voltage to the relatively low voltage used by lighting and interior wiring systems. The transformer may be pole-mounted or set on the ground in a protective enclosure. In rural areas a pole-mount transformer may serve only one customer, but in more built-up areas multiple customers may be connected. In very dense city areas, a secondary network may be formed with many transformers feeding into a common bus at the utilization voltage. Each customer has a service drop connection and a meter for billing. (Some very small loads, such as yard lights, may be too small to meter and so are charged only a monthly rate.)
A ground connection to local earth is normally provided for the customer’s system as well as for the equipment owned by the utility. The purpose of connecting the customer’s system to ground is to limit the voltage that may develop if high voltage conductors fall down onto lower-voltage conductors which are usually mounted lower to the ground, or if a failure occurs within a distribution transformer. If all conductive objects are bonded to the same earth grounding system, the risk of electric shock is minimized. However, multiple connections between the utility ground and customer ground can lead to stray voltage problems; customer piping, swimming pools or other equipment may develop objectionable voltages. These problems may be difficult to resolve since they often originate from places other than the customer’s premises.
SOURCE
dianfp19 