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Frequently asked questions

Alternating current is a form of electricity in which the current alternates in direction (and the voltage alternates in polarity) at a frequency defined by the generator (usually between 50 and 60 times per second, ie, 50 - 60 hertz).

AC was adopted for power transmission in the early days of electricity supply because it had two major advantages over direct current (DC): its voltage could be stepped up or down according to need using transformers and it could be interrupted more easily than DC.

This is electrical current that does not alternate (see alternating current), the electrons flow through the circuit in one direction.

As a result, DC does not generate reactive power (see Reactive Power). This means that, in a DC system, only real (or active) power is transmitted, making better use of the system’s capacity. The transmission of DC current has very low losses.

It is a concept that describes the loss of power in a system resulting from the production of electric and magnetic fields in it.

Today’s electrical transmission systems are almost exclusively based on alternating current (AC), but the development of high-voltage direct current (HVDC) technology has made it possible to build a DC grid that can handle bulk power flows over long distances. Power from such DC grids can be fed into the AC networks as needed.

A technology developed by Hitachi Energy in the 1950s to move large amounts of power over substantial distances - typically by overhead transmission lines, but also by way of submarine cables. Another important aspect of HVDC lines is that they can never be overloaded. Because HVDC transmits only active (real) power, no line capacity is wasted on transmitting reactive power. This means that the same power can be transmitted over fewer (or smaller) transmission lines than would be required using AC, and less land is needed to accommodate the lines. HVDC induces minimal magnetic fields, so the power lines may be built safely closer to human habitation.

An adaptation of classic HVDC, developed by Hitachi Energy in the 1990s. It can be used to transmit electricity in lower power ranges (tens of megawatts) to an upper range of 1,100 megawatt (MW) (±320 kilovolts).HVDC Light offers the same benefits as traditional HVDC systems, but also provides more secure power control (superior to classic HVDC) and quick power restoration in the event of a blackout. It is the only technology available that allows long-distance underground high-voltage transmission.

Special equipment is needed to convert electricity from alternating current (AC) to direct current (DC), or vice versa. HVDC converter stations use power electronic devices called thyristors to make these conversions.

A thyristor is a semiconductor device used in HVDC installations, as a high-speed, high-power switch, capable of turning power supplies of many megawatts on within a split second. Thyristors are a component used in inverters and rectifiers. (See also Inverter and Rectifier).

A semiconductor is a material whose electrical properties can be significantly influenced by physical factors (mostly electrical conditions, but also pressure, temperature, light, etc). This means that a semiconductor will behave either as an insulator or a conductor of electricity, depending on the conditions to which it is exposed.

An electrical device for converting direct current (DC) into alternating current (AC).

An electrical device used to convert alternating current (AC) into direct current (DC).

Devices that interrupt high currents to protect electrical equipment from damage caused by current surges, eg, from a short circuit or a lightning strike. (On a much smaller scale, they are used as an alternative to fuses in the home.)

Equipment used to control, protect, and regulate the flow of electrical power in a transmission or distribution network. It is often located in substations, but can be associated with any electrical equipment that might need to be isolated for fault correction (eg, if a voltage drop occurred in one part of the grid, it might be necessary to shut off the affected section to prevent the fault spreading), or for maintenance purposes. The main components of switchgear are circuit breakers, which interrupt high-voltage current to protect electrical equipment from excessive current.

Transmission is the movement of power at high voltage (above ca. 50 kV), usually over long distances. Raising the voltage allows power to be transmitted more efficiently, ie, with lower losses. Distribution is the transport of electricity at medium voltage (between ca. 1 and 50 kV) over shorter distances to industrial, commercial and residential areas.