Three-phase electric power (abbreviated 3ϕ) is the most widely used form of alternating current (AC) for electricity generation, transmission, and distribution.William D. Stevenson, Jr. Elements of Power System Analysis Third Edition, McGraw-Hill, New York (1975). , p. 2 It is a type of polyphase system that uses three wires (or four, if a neutral return is included; not counting any protective conductor) and is the standard method by which electrical grids deliver power around the world.
In a three-phase system, each of the three phases is offset by 120 degrees of phase shift relative to the others. This arrangement produces a more constant flow of power compared with single-phase systems, making it especially efficient for transmitting electricity over long distances and for powering heavy loads such as industrial machinery. Because it is an AC system, voltages can be easily increased or decreased with transformers, allowing high-voltage transmission and low-voltage distribution with minimal loss.
Three-phase circuits are also more economical: a three-wire system can transmit more power than a two-wire single-phase system of the same phase-to-phase voltage while using less conductor material. Beyond transmission, three-phase power is commonly used to run large induction motors, other electric motors, and heavy industrial loads, while smaller devices and household equipment often rely on single-phase circuits derived from the same network.
Three-phase electrical power was first developed in the 1880s by several inventors and has remained the backbone of modern electrical systems ever since.
Terminology The conductors between a voltage source and a load, excluding those serving neutral, mid-point and protective functions, are called line conductors.
The voltage between any two line conductors is called line-to-line voltage, while the voltage measured between any line conductor and a neutral conductor is called line-to-neutral voltage, per section 195 of IEC 60050. Other sections define variations of these terms, with section 601 having phase to phase voltage and phase to neutral voltage, and section 141 having polyphase line-to-line voltage and polyphase line-to-neutral voltage. Some electrical textbooks contain the terms line voltage (equivalent to line-to-line) and phase voltage (equivalent to line-to-neutral).
System voltage is often expressed in the form X/Y, as found in IEC 60038, with X being line-to-neutral voltage, and Y being line-to-line voltage. For example, in countries with nominal 230 V power (line-to-neutral), where line-to-line voltage is 400 V, this may be expressed as 230/400 V.
Transformer windings consist of either delta (often symbolised as 'Δ') or star (often symbolised as 'Y') configurations, per IEC 60050 terminology. The star arrangement is more commonly known as wye in the US.
History Polyphase power systems were independently invented by Galileo Ferraris, Mikhail Dolivo-Dobrovolsky, Jonas Wenström, John Hopkinson, William Stanley Jr., and Nikola Tesla in the late 1880s.
Three phase power evolved out of electric motor development. In 1885, Galileo Ferraris was doing research on rotating magnetic fields. Ferraris experimented with different types of asynchronous electric motors. The research and his studies resulted in the development of an alternator, which may be thought of as an alternating-current motor operating in reverse, so as to convert mechanical (rotating) power into electric power (as alternating current). On 11 March 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.
Two months later Nikola Tesla gained for a three-phase electric motor design, application filed October 12, 1887. Figure 13 of this patent shows that Tesla envisaged his three-phase motor being powered from the generator via six wires.
These alternators operated by creating systems of alternating currents displaced from one another in phase by definite amounts, and depended on rotating magnetic fields for their operation. The resulting source of polyphase power soon found widespread acceptance. The invention of the polyphase alternator is key in the history of electrification, as is the power transformer. These inventions enabled power to be transmitted by wires economically over considerable distances. Polyphase power enabled the use of water-power (via hydroelectric generating plants in large dams) in remote places, thereby allowing the mechanical energy of the falling water to be converted to electricity, which then could be fed to an electric motor at any location where mechanical work needed to be done. This versatility sparked the growth of power-transmission network grids on continents around the globe.
Mikhail Dolivo-Dobrovolsky developed a three-phase electrical generator and a three-phase electric motor in 1888 and studied star and delta connections. His three-phase three-wire transmission system was displayed in 1891 in Germany at the International Electrotechnical Exhibition, where Dolivo-Dobrovolsky used the system to transmit electric power at the distance of 176 km (110 miles) with 75% efficiency. In 1891 he also created a three-phase transformer and short-circuited (squirrel-cage) induction motor.Gerhard Neidhöfer: Michael von Dolivo-Dobrowolsky und der Drehstrom. Geschichte der Elektrotechnik VDE-Buchreihe, Volume 9, VDE VERLAG, Berlin Offenbach, . He designed the world's first three-phase hydroelectric power plant in 1891. Inventor Jonas Wenström received in 1890 a Swedish patent on a similar three-phase system. The possibility of transferring electrical power from a waterfall at a distance was explored at the Grängesberg mine. A fall at Hällsjön, Smedjebackens kommun, where a small iron work had been located, was selected. In 1893, a three-phase system was used to transfer a distance of 15 km (10 miles), becoming the first commercial application.
Principle
In a symmetric three-phase power supply system, three conductors each carry an alternating current of the same frequency and voltage amplitude relative to a common reference, but with a phase difference of one third of a cycle (i.e., 120 degrees out of phase) between each. The common reference is usually connected to ground and often to a current-carrying conductor called the neutral. Due to the phase difference, the voltage on any conductor reaches its peak at one third of a cycle after one of the other conductors and one third of a cycle before the remaining conductor. This phase delay gives constant power transfer to a balanced linear load. It also makes it possible to produce a rotating magnetic field in an electric motor and generate other phase arrangements using transformers (for instance, a two-phase system using a Scott-T transformer). The amplitude of the voltage difference between two phases is times the amplitude of the voltage of the individual phases.
The symmetric three-phase systems described here are simply referred to as three-phase systems because, although it is possible to design and implement asymmetric three-phase power systems (i.e., with unequal voltages or phase shifts), they are not used in practice because they lack the most important advantages of symmetric systems.
In a three-phase system feeding a balanced and linear load, the sum of the instantaneous currents of the three conductors is zero. In other words, the current in each conductor is equal in magnitude to the sum of the currents in the other two, but with the opposite sign. The return path for the current in any phase conductor is the other two phase conductors.
Constant power transfer is possible with any number of phases greater than one. However, two-phase systems do not have neutral-current cancellation and thus use conductors less efficiently, and more than three phases complicates infrastructure unnecessarily. Additionally, in some practical generators and motors, two phases can result in a less smooth (pulsating) torque.
Three-phase systems may have a fourth wire, common in low-voltage distribution. This is the neutral wire. The neutral allows three separate single-phase supplies to be provided at a constant voltage and is commonly used for supplying multiple single-phase loads. The connections are arranged so that, as far as possible in each group, equal power is drawn from each phase. Further up the distribution system, the currents are usually well balanced. Transformers may be wired to have a four-wire secondary and a three-wire primary, while allowing unbalanced loads and the associated secondary-side neutral currents.
Phase sequence
Wiring for three phases is typically identified by colors that vary by country and voltage. The phases must be connected in the correct order to achieve the intended direction of rotation of three-phase motors. For example, pumps and fans do not work as intended in reverse. Maintaining the identity of phases is required if two sources could be connected at the same time. A direct connection between two different phases is a short circuit and leads to flow of unbalanced current.
Advantages and disadvantages As compared to a single-phase AC power supply that uses two current-carrying conductors with no neutral, a three-phase supply with no neutral and the same phase-to-phase voltage can transmit the same power by using just 0.75 times as much conductor material.
Three-phase supplies have properties that make them desirable in electric power distribution systems: The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load, which allows a reduction of the size of the neutral conductor because it carries little or no current. With a balanced load, all the phase conductors carry the same current and so can have the same size. Power transfer into a linear balanced load is constant, which, in motor/generator applications, helps to reduce vibrations. Three-phase systems can produce a rotating magnetic field with a specified direction and constant magnitude, which simplifies the design of electric motors, as no starting circuit is required.
However, most loads are single-phase. In North America, single-family houses and individual apartments are supplied one phase from the power grid and use a split-phase system to the panelboard from which most branch circuits will carry 120 V. Circuits designed for higher powered devices such as stoves, dryers, or outlets for electric vehicles carry 240 V.
In Europe, three-phase power is normally delivered to the panelboard and further to higher powered devices.
Generation and distribution
At the power station, an electrical generator converts mechanical power into a set of three AC electric currents, one from each coil (or winding) of the generator. The windings are arranged such that the currents are at the same frequency but with the peaks and troughs of their wave forms offset to provide three complementary currents with a phase separation of one-third cycle (120° or radians). The generator frequency is typically 50 or 60 Hz, depending on the country.
At the power station, transformers change the voltage from generators to a level suitable for transmission in order to minimize losses.
After further voltage conversions in the transmission network, the voltage is finally transformed to the standard utilization before power is supplied to customers.
Most automotive alternators generate three-phase AC and rectify it to DC with a diode bridge.
Transformer connections A delta-configured transformer winding connects phases of a three-phase system directly from one to another. A star-configured transformer connects each phase winding to a common central (neutral) point.
A single three-phase transformer can be used, or three single-phase transformers.