Electricity distribution

Electricity distribution is the final stage in the delivery of electricity. It is generally considered to include medium-voltage (less than 50 kV) power lines, electrical substations and pole-mounted transformers, low-voltage (less than 1000 V) distribution wiring and sometimes electricity meters.

Description

History

In the early days of electricity generation, 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 at the time there was no known way of doing DC voltage conversion (other than inefficient motor-generator sets). The voltages had to be fairly low with such systems due to the fact that it is difficult and dangerous to distribute high voltages to small loads. The losses in a cable are proportional to the square of the current, the length of the cable, and the resistivity of the material, and are inversely proportional to cross-sectional area. Early transmission networks were already using copper, which is one of the best economically feasible conductors for this application. To reduce the current while keeping power transmission constant requires increasing the voltage which, as previously mentioned, was problematic. This meant in order to keep losses to a reasonable level the Edison (DC) system needed thick cables and local generators.

Introduction of alternating current

The adoption of alternating current (AC) for electricity generation following the War of Currents dramatically changed the situation. Power transformers, installed at substations, could be used to raise the voltage from the generators and reduce it to supply loads. Increasing the voltage reduced the current in the transmission and distribution lines and hence the size of conductors required and distribution losses incurred. This made it more economic to distribute power over long distances. The ability to transform to extra-high voltages enabled generators to be located far from loads with transmission systems to interconnect generating stations and distribution networks.

In North America, early distribution systems used a voltage of 2200 volts corner-grounded delta. Over time, this was gradually increased to 2400 volts. As cities grew, most 2400 volt systems were upgraded to 2400/4160 Y three-phase systems, which also benefited from better surge suppression due to the grounded neutral. Some city and suburban distribution systems continue to use this range of voltages, but most have been converted to 7200/12470Y, 7620/13200Y, 14400/24940Y, and 19920/34500Y.

European systems used higher voltages, generally 3300 volts to ground, in support of the 220/380Y volt power systems used in those countries. In the UK, urban systems progressed to 6.6 kV and then 11 kV (phase to phase), the most common distribution voltage. North American and European power distribution systems also differ in that North American systems tend to have a greater number of low-voltage step-down transformers located closer to customers' premises. For example, in the US a pole-mounted transformer in a suburban setting may supply only a single or a very few houses, whereas in the UK a typical urban or suburban low-voltage substation might be rated at 2 MW and supply a whole neighbourhood. This is because the higher voltage used in Europe (230 V vs 120 V) may be carried over a greater distance with acceptable power loss. An advantage of the North American setup is that failure or maintenance on a single transformer will only affect a few customers. Advantages of the UK setup are that fewer transformers are required; larger and more efficient transformers are used, and due to diversity there need be less spare capacity in the transformers, reducing power wastage.

Rural Electrification systems, in contrast to urban systems, tend to use higher voltages because of the longer distances covered by those distribution lines (see Rural Electrification Administration). 7200 volts is commonly used in the United States; 11 kV and 33 kV are common in the UK, New Zealand and Australia; 11 kV and 22 kV are common in South Africa. Other voltages are occasionally used in unusual situations or where a local utility simply has engineering practices that differ from the norm.

In New Zealand, Australia, Saskatchewan, Canada, and South Africa, single wire earth return systems (SWER) are used to electrify remote rural areas.

Distribution network configurations

Distribution networks are typically of two types, radial or interconnected (see spot network substations ^*). A radial network leaves the station and passes through the network area with no connection to any other supply. This is typical of long rural lines with isolated load areas. An interconnected network is generally found in more urban areas and will have multiple connections to other points of supply.

These points of connection are normally open but allow various configurations by closing and opening switches. The benefit of the interconnected model is that in the event of a fault or required maintenance a small area of network can be isolated and the remainder kept on supply.

Within these networks there may be a mix of overhead line construction utilising traditional poles and wires and increasingly underground construction with cables and indoor or cabinet substations. Distribution feeders emanating from a substation are generally controlled by a circuit breaker which will open when a fault is detected. Circuit Reclosers may be installed to further segregate the feeder thus minimising the impact of faults.

Longer feeders may experience voltage drop requiring capacitors or voltage regulators to be installed.

Characteristics of the supply given to customers are generally mandated by law and by contract between the supplier and customer. Variables include:

AC or DC - Virtually all public electricity supplies are AC today. Users of large amounts of DC power such as some electric railways, telephone exchanges and industrial processes such as aluminium smelting either operate their own generating equipment or have equipment to derive DC from the public AC supply
Voltage, including tolerance (usually + or - a certain percentage)
Frequency (50 Hz and 60 Hz are most common, 16 2/3 Hz for railways)
Maximum current that may be drawn
Phase configuration (single phase, split phase, polyphase including two phase and three phase)
Restrictions on power factor of connected load
Earthing arrangements - TT, TN-S, TN-C-S or TN-C
Maximum prospective short circuit current
Maximum level and frequency of occurrence of transients

See List of countries with mains power plugs, voltages and frequencies.

Electricity industry reform has led to the creation of electricity markets through the separation of contestable retailing from distribution, a natural monopoly and the separation of the monopoly transmission from generation. It also led to the development of new terminology to describe the distributor such as line company, wires business and network company.

References

Saskpower single-wire ground return electrification in 1949

External links

IEEE Power Engineering Society
IEEE Power Engineering Society Distribution Subcommittee
U.S. Department of Energy Electric Distribution website ^*

Description

History

Introduction of alternating current

Distribution network configurations

See also

References

External links

Further reading

Gourt Home::Gourt :: Electricity distribution

Description

History

Introduction of alternating current

Distribution network configurations

See also

References

External links

Further reading