Power Transmission Line

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.

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. Overhead 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.

Electric power can also be transmitted by underground power cables instead of overhead power lines. This is a more expensive option, as the life-cycle cost of an underground power cable is a multiple of the overhead power line. However, they can assist the transmission of power across:

* Densely populated urban areas
* Areas where land is unavailable or planning consent is difficult
* Rivers and other natural obstacles
* Land with outstanding natural or environmental heritage
* Areas of significant or prestigious infrastructural development
* Land whose value must be maintained for future urban expansion and rural development

Compared to overhead lines, underground cables emit no electric field, can be engineered to emit no magnetic fields, have better power loss characteristics, and can absorb emergency power loads. They also need merely a narrower strip of about 10 metres to install, whereas the lack of cable insulation requires an overhead line to be installed on a strip of about 200 metres wide to be kept permanently clear for safety, maintenance and repair. Those advantages can in some cases justify the higher investment cost.

Most high-voltage underground cables for power transmission that are currently sold on the market are insulated by a sheath of cross linked polyethylene (XLPE). Some cable may have a lead jacket in conjunction with XLPE insulation to allow for fiber optics to be seamlessly integrated within the cable. In the past underground power cables used to be insulated with oil and paper and ran in a rigid steel pipe, or a semi-rigid aluminium or lead jacket or sheath. The oil was kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation. There are still many of those oil-and-paper insulated cables in use worldwide.ny particular renewable alternative is economically sensible. Costs can be prohibitive for transmission.


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 AC, 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 I^2R 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 I^2R 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 , and in the UK at 7.4% in 1998.

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