Electricity distribution

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11kV/400V-230V transformer in an older suburb of Wellington, New Zealand
11kV/400V-230V transformer in an older suburb of Wellington, New Zealand

Electricity distribution is the penultimate stage in the delivery (before retail) of electricity to end users. 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.


[edit] Description

[edit] History

In the early days of electricity generation to about 1900, 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 were used (on the order of 100 volts) since that was a practical voltage for incandescent lamps, which were then the primary electrical load. The low voltage also required less insulation to be safely distributed within buildings.

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 and copper required for a given quantity of power transmitted would require a higher transmission voltage, but no convenient efficient method existed to change the voltage level 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 of the farthest customer to avoid the need for excessively large and expensive conductors.

[edit] 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 economical to distribute power over long distances. Generators (such as hydroelectric sites) could be located far from the loads.

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 4160/2400 volt, three-phase systems. 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 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 close to customers' premises. For example, in the US a pole-mounted transformer in a suburban setting may supply 1-3 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 (380 V vs 230 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 the transformers may be fewer, larger and more efficient, and due to diversity there need be less spare capacity in the transformers, reducing power wastage. In North American city areas with many customers per unit area, network distribution will be used, with multiple transformers and low-voltage busses interconnected over several city blocks.

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, 12470 and 25000 volt distribution is common 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 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 still used in distribution due to the economy, efficiency and reliabilty of transformers. High-voltage DC is used for transmission of large blocks of power over long distances, or for interconnecting adjacent AC networks, but not for distribution to customers.

[edit] 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 normal 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 the operating utility linemen carefully 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 utilizing traditional utility poles and wires and, increasingly, underground construction with cables and indoor or cabinet substations. However, underground distribution can cost as much as 11 times as much as overhead construction. In part to reduce this cost, underground power lines are sometimes colocated with other utility lines in what are called Common utility ducts. Distribution feeders emanating from a substation are generally controlled by a circuit breaker or fuse which will open when a fault is detected. Automatic Circuit Reclosers may be installed to further segregate the feeder thus minimising the impact of faults.

Long feeders experience voltage drop requiring capacitors or voltage regulators to be installed, and the phase physical relationship to be interchanged.

Characteristics of the supply given to customers are generally mandated by contract between the supplier and customer. Deviations from the normal usage pattern usually invoke monthly surcharges. 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 or have adjacent dedicated generating equipment, or use rectifiers to derive DC from the public AC supply
  • Voltage, including tolerance (usually +10 or -15 percentage)
  • Frequency, commonly 50 & 60 Hz, 16-2/3 Hz for some railways and, in a few older industrial and mining locations, 25 Hz
  • Phase configuration (single phase, polyphase including two phase and three phase)
  • Maximum demand (usually measured as the largest amount of power delivered within a 15 or 30 minute period during a billing period)
  • Load Factor, expressed as a ratio of average load to peak load over a period of time. Load factor indicates the degree of effective utilization of equipment (and capital investment) of distribution line or system.
  • 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.

[edit] Modern Distribution Systems

The modern distribution system begins as the primary circuit leaves the sub-station and ends as the secondary service enters the customers meter socket. A variety of methods, materials, and equipment are used among the various utility companies across the U.S., but the end result is similar. First, the energy leaves the sub-station in a primary circuit, usually with all three phases.

The most common type of primary is known as a wye configuration (so named because of the shape of a "Y".) The wye configuration includes 3 phases (represented by the three outer parts of the "Y") and a neutral (represented by the center of the "Y".) The neutral is grounded both at the substation and at every power pole. In a typical 12470Y/7200 volt system, the pole mount transformer's primary winding is rated for 7200 volts and is connected across one phase of power and the neutral. The primary and secondary (low voltage) neutrals are bonded (connected) together to provide a path to blow the primary fuse if any fault occurs that allows primary voltage to enter the secondary lines. An example of this type of fault would be a primary phase falling across the secondary lines. Another example would be some type of fault in the transformer itself.

Electric distribution substations transform power from transmission voltage to the lower voltage used for local distribution to homes and businesses.
Electric distribution substations transform power from transmission voltage to the lower voltage used for local distribution to homes and businesses.

The other type of primary configuration is known as delta, this method is older and less common. Delta is so named because of the shape of the Greek letter delta, a triangle. Delta has only 3 phases and no neutral. In delta there is only a single voltage, between two phases (phase to phase), while in wye there are two voltages, between two phases and between a phase and neutral (phase to neutral). Wye primary is safer because if one phase becomes grounded, that is makes connection to the ground through a person, tree, or other object, it should trip out the fused cutout similer to a household circuit breaker tripping. In delta, if a phase makes connection to ground it will continue to function normally. It takes two or three phases to make connection to ground before the fused cutouts will open the circuit.

[edit] Economic and Political

In the United States, Electric industry "deregulation" reform, started in the mid-1990s, has led to the creation of electricity markets through the elimination of the former natural monopoly of generation, transmission, and distribution. As a consequence, electricity has become more of a commodity. The separation has also led to the development of new terminology to describe the business units, e.g. line company, wires business and network company.

[edit] See also

[edit] References

[edit] External links

[edit] Further reading

  • Brown, R. E., Electric Power Distribution Reliability, Marcel Dekker, Inc., 2002.
  • Burke, J., Power Distribution Engineering, Marcel Dekker, Inc., 1994.
  • Hoffman, P., Scheer, R., Marchionini, B., Distributed Energy Resources: A Key Element of Grid Modernization DE - March/April 2004 [2]
  • Short, T. A. Electric Power Distribution Handbook, CRC Press, 2004.
  • Westinghouse Electric Corporation, Distribution Systems, vol. 3, 1965.
  • Westinghouse Electric Corporation, Electric power transmission patents; Tesla polyphase system. (Transmission of power; polyphase system; Tesla patents)
  • Willis, H. L., Power Distribution Planning Reference Book, Marcel Dekker, Inc., 2nd ed., 2004.
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