Electric power transmission

• Electric power transmission
  

"Power line" redirects here. For the conservative U.S. blog, see Power Line. For the telecommunication technology, see Power line communication."Power grid" redirects here. For the board game, see Power Grid (board game).

Electric power transmission, a process in the delivery of electricity to consumers, is the bulk transfer of electrical power. Typically, power transmission is between the power plant and a substation near a populated area. Electricity distribution is the delivery from the substation to the consumers. Electric power transmission allows distant energy sources (such as hydroelectric power plants) to be connected to consumers in population centers, and may allow exploitation of low-grade fuel resources that would otherwise be too costly to transport to generating facilities.
Due to the large amount of power involved, transmission normally takes place at high voltage (110 kV or above). Electricity is usually transmitted over long distance through overhead power transmission lines. Underground power transmission is used only in densely populated areas due to its high cost of installation and maintenance, and because the high reactive power produces large charging currents and difficulties in voltage management.A power transmission system is sometimes referred to colloquially as a "grid"; however, for reasons of economy, the network is not a mathematical grid. Redundant paths and lines are provided so that power can be routed from any power plant to any load center, through a variety of routes, based on the economics of the transmission path and the cost of power. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line, which, due to system stability considerations, may be less than the physical or thermal limit of the line. Deregulation of electricity companies in many countries has led to renewed interest in reliable economic design of transmission networks. However, in some places the gaming of a deregulated energy system has led to disaster, such as that which occurred during the California electricity crisis of 2000 and 2001.

• AC power transmission
  

AC power transmission is the transmission of electric power by alternating current. Usually transmission lines use three phase AC current. Single phase AC current is sometimes used in a railway electrification system. In urban areas, trains may be powered by DC at 600 volts or so.Overhead conductors are not covered by insulation. The conductor material is nearly always an aluminum alloy, made into several strands and possibly reinforced with steel strands. Conductors are a commodity supplied by several companies worldwide. Improved conductor material and shapes are regularly used to allow increased capacity and modernize transmission circuits. Conductor sizes in overhead transmission work range in size from #6 American wire gauge (about 12 square millimetres) to 1,590,000 circular mils area (about 750 square millimetres), with varying resistance and current-carrying capacity. Thicker wires would lead to a relatively small increase in capacity due to the skin effect, that causes most of the current to flow close to the surface of the wire.Today, transmission-level voltages are usually considered to be 110 kV and above. Lower voltages such as 66 kV and 33 kV are usually considered sub-transmission voltages but are occasionally used on long lines with light loads. Voltages less than 33 kV are usually used for distribution. Voltages above 230 kV are considered extra high voltage and require different designs compared to equipment used at lower voltages.Overhead transmission lines are uninsulated wire, so design of these lines requires minimum clearances to be observed to maintain safety.


Bulk power transmission

Engineers design transmission networks to transport the energy as efficiently as feasible, while at the same time taking into account economic factors, network safety and redundancy. These networks use components such as power lines, cables, circuit breakers, switches and transformers.

A transmission substation decreases the voltage of electricity coming in allowing it to connect from long distance, high voltage transmission, to local, lower voltage, distribution. It also rerouts power to other transmission lines that serve local markets. The substation may also "reboost" power allowing it to travel greater distances from the power generation source along the high voltage transmission lines.This is the PacifiCorp Hale Substation, Orem, Utah.Transmission efficiency is improved by increasing the voltage using a step-up transformer, which reduces the current in the conductors, while keeping the power transmitted nearly equal to the power input. The reduced current flowing through the conductor reduces the losses in the conductor and since, according to Joule's Law, the losses are proportional to the square of the current, halving the current makes the transmission loss one quarter the original value.A transmission grid is a network of power stations, transmission circuits, and substations. Energy is usually transmitted within the grid with three-phase AC. DC systems require relatively costly conversion equipment which may be economically justified for particular projects. Single phase AC is used only for distribution to end users since it is not usable for large polyphase induction motors. In the 19th century two-phase transmission was used, but required either three wires with unequal currents or four wires. Higher order phase systems require more than three wires, but deliver marginal benefits.The capital cost of electric power stations is so high, and electric demand is so variable, that it is often cheaper to import some portion of the variable load than to generate it locally. Because nearby loads are often correlated (hot weather in the Southwest portion of the United States might cause many people there to turn on their air conditioners), imported electricity must often come from far away. Because of the economics of load balancing, transmission grids now span across countries and even large portions of continents. The web of interconnections between power producers and consumers ensures that power can flow even if a few links are inoperative.The unvarying (or slowly varying over many hours) portion of the electric demand is known as the "base load", and is generally served best by large facilities (and therefore efficient due to economies of scale) with low variable costs for fuel and operations, i.e. nuclear, coal, hydro. Renewables such as solar, wind, ocean/tidal, etc. are not considered "base load" but can still add power to the grid. Smaller- and higher-cost sources are then added as needed.Long-distance transmission of electricity (thousands of miles) is cheap and efficient, with costs of US$ 0.005 to 0.02 per kilowatt-hour (compared to annual averaged large producer costs of US$ 0.01 to US$ 0.025 per kilowatt-hour, retail rates upwards of US$ 0.10 per kilowatt-hour, and multiples of retail for instantaneous suppliers at unpredicted highest demand moments). Thus distant suppliers can be cheaper than local sources (e.g. New York City buys a lot of electricity from Canada). Multiple local sources (even if more expensive and infrequently used) can make the transmission grid more fault tolerant to weather and other disasters that can disconnect distant suppliers.Long distance transmission allows remote renewable energy resources to be used to displace fossil fuel consumption. Hydro and wind sources can't be moved closer to high population cities, and solar costs are lowest in remote areas where local power needs are the least. Connection costs alone can determine whether any particular renewable alternative is economically sensible. Costs can be prohibitive for transmission lines.

Grid input

At the generating plants the energy is produced at a relatively low voltage of up to 30 kV (Grigsby, 2001, p. 4-4), then stepped up by the power station transformer to a higher voltage (115 kV to 765 kV AC, ± 250-500 kV DC, varying by country) for transmission over long distances to grid exit points (substations).

Losses

Transmitting electricity at high voltage reduces the fraction of energy lost to Joule heating. For a given amount of power, a higher voltage reduces the current and thus the resistive losses in the conductor. For example, raising the voltage by a factor of 10 reduces the current by a corresponding factor of 10 and therefore the losses by a factor of 100, provided the same sized conductors are used in both cases. Even if the conductor size is reduced x10 to match the lower current the losses are still reduced x10. Long distance transmission is typically done with overhead lines at voltages of 115 to 1,200 kV. However, at extremely high voltages, more than 2,000 kV between conductor and ground, corona discharge losses are so large that they can offset the lower resistance loss in the line conductors.Transmission and distribution losses in the USA were estimated at 7.2% in 1995 [2], and in the UK at 7.4% in 1998. [3]As of 1980, the longest cost-effective distance for electricity was 4,000 miles (7,000 km), although all present transmission lines are considerably shorter. (see Present Limits of High-Voltage Transmission)In an alternating current transmission line, the inductance and capacitance of the line conductors can be significant. The currents that flow in these components of transmission line impedance constitute reactive power, which transmits no energy to the load. Reactive current flow causes extra losses in the transmission circuit. The ratio of real power (transmitted to the load) to apparent power is the power factor. As reactive current increases, the reactive power increases and the power factor decreases. For systems with low power factors, losses are higher than for systems with high power factors. Utilities add capacitor banks and other components throughout the system — such as phase-shifting transformers, static VAR compensators, and flexible AC transmission systems (FACTS) — to control reactive power flow for reduction of losses and stabilization of system voltage.
Electrical power is always partially lost by transmission. This applies to short distances such as between components on a printed circuit board as well as to cross country high voltage lines. The major component of power loss is due to ohmic losses in the conductors and is equal to the product of the resistance of the wire and the square of the current: For a system which delivers a power, P, at unity power factor at a particular voltage, V, the current flowing through the cables is given by . Thus, the power lost in the lines, Therefore, the power lost is proportional to the resistance and inversely proportional to the square of the voltage. A higher transmission voltage reduces the current and thus the power lost during transmission.In addition, a low resistance is desirable in the cable. While copper cable could be used, aluminium alloy is preferred due to its much better conductivity to weight ratio making it lighter to support, as well as its lower cost. The aluminium is normally mechanically supported on a steel core.

HVDC

High voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is required to be transmitted over very long distances, it can be more economical to transmit using direct current instead of alternating current. For a long transmission line, the value of the smaller losses, and reduced construction cost of a DC line, can offset the additional cost of converter stations at each end of the line. Also, at high AC voltages significant (although economically acceptable) amounts of energy are lost due to corona discharge, the capacitance between phases or, in the case of buried cables, between phases and the soil or water in which the cable is buried.HVDC links are sometimes used to stabilize against control problems with the AC electricity flow. In other words, to transmit AC power as AC when needed in either direction between Seattle and Boston would require the (highly challenging) continuous real-time adjustment of the relative phase of the two electrical grids. With HVDC instead the interconnection would: (1) Convert AC in Seattle into HVDC. (2) Use HVDC for the three thousand miles of cross country transmission. Then (3) convert the HVDC to locally synchronized AC in Boston, and optionally in other cooperating cities along the transmission route. One prominent example of such a transmission line is the Pacific DC Intertie located in the Western United States.

Grid exit

At the substations, transformers are again used to step the voltage down to a lower voltage for distribution to commercial and residential users. This distribution is accomplished with a combination of sub-transmission (33 kV to 115 kV, varying by country and customer requirements) and distribution (3.3 to 25 kV). Finally, at the point of use, the energy is transformed to low voltage (100 to 600 V, varying by country and customer requirements).

Limitations

The amount of power that can be sent over a transmission line is limited. The origins of the limits vary depending on the length of the line. For a short line, the heating of conductors due to line losses sets a "thermal" limit. If too much current is drawn, conductors may sag too close to the ground, or conductors and equipment may be damaged by overheating. For intermediate-length lines on the order of 100 km (60 miles), the limit is set by the voltage drop in the line. For longer AC lines, system stability sets the limit to the power that can be transferred. Approximately, the power flowing over an AC line is proportional to the sine of the phase angle between the receiving and transmitting ends. Since this angle varies depending on system loading and generation, it is undesirable for the angle to approach 90 degrees. Very approximately, the allowable product of line length and maximum load is proportional to the square of the system voltage. Series capacitors or phase-shifting transformers are used on long lines to improve stability. High-voltage direct current lines are restricted only by thermal and voltage drop limits, since the phase angle is not material to their operation.

Communications

Operators of long transmission lines require reliable communications for control of the power grid and, often, associated generation and distribution facilities. Fault-sensing protection relays at each end of the line must communicate to monitor the flow of power into and out of the protected line section so that faulted conductors or equipment can be quickly de-energized and the balance of the system restored. Protection of the transmission line from short circuits and other faults is usually so critical that common carrier telecommunications are insufficiently reliable. In remote areas a common carrier may not be available at all.