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Three-phase electric power
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« on: August 19, 2016, 02:23:52 PM »

Three-phase electric power
From Wikipedia, the free encyclopedia

Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of poly-phase system and is the most common method used by electrical grids worldwide to transfer power. It is also used to power large motors and other heavy loads. A three-phase system is usually more economical than an equivalent single-phase or two-phase system at the same voltage because it uses less conductor material to transmit electrical power. The three-phase system was independently invented by Galileo Ferraris, Mikhail Dolivo-Dobrovolsky and Nikola Tesla in the late 1880s.

Three-phase electric power transmission
In a three-phase power supply system, three conductors each carry an alternating current (of the same frequency) but the phase of the voltage on each conductor is displaced from each of the other conductors by 120 degrees. (One third of a 360 degree "cycle".) Hence, 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 third conductor. Using the voltage on one conductor as the reference, the peak voltage on the other two conductors is delayed by one third and two thirds of one cycle respectively. This phase delay gives constant power transfer over each cycle. It also makes it possible to produce a rotating magnetic field in an electric motor.

With a three phase supply, at any instant, the potential of any phase is exactly equal to and the opposite of the combination (sum) of the other two phases. This means that - if the load on the three phases is "balanced" - the return path for the current in any phase conductor is the other two phase conductors.

Hence, the sum of the currents in the three conductors is always zero and the current in each conductor is equal to and in the opposite direction as the sum of the currents in the other two. Thus, each conductor acts as the return path for the currents from the other two.
While a single phase AC power supply requires two conductors (Go and Return), a three phase supply can transmit three times the power by using only one extra conductor. This means that a 50% increase in transmission cost yields a 200% increase in the power transmitted.

Three-phase systems may also utilize a fourth wire, particularly 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 groups of domestic properties which are each 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 supply chain in high-voltage distribution the currents are usually well balanced and it is therefore normal to omit the neutral conductor.

Three-phase supplies have properties that make them very 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. This makes it possible to reduce the size of the neutral conductor because it carries little to no current; all the phase conductors carry the same current and so can be the same size, for a balanced load.
   Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations.
   Three-phase systems can produce a rotating magnetic field with a specified direction and constant magnitude, which simplifies the design of electric motors.

Most household loads are single-phase. In North American residences, three-phase power might feed a multiple-unit apartment block, but the household loads are connected only as single phase. In lower-density areas, only a single phase might be used for distribution. Some large European appliances may be powered by three-phase power, such as electric stoves and clothes dryers.

Wiring for the three phases is typically identified by color codes which vary by country. Connection of the phases in the right order is required to ensure the intended direction of rotation of three-phase motors. For example, pumps and fans may not work in reverse. Maintaining the identity of phases is required if there is any possibility two sources can be connected at the same time; a direct interconnection between two different phases is a short-circuit.

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 vary sinusoidally 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 2π⁄3 radians). The generator frequency is typically 50 or 60 Hz, varying by country.

At the power station, transformers change the voltage from generators to a level suitable for transmission minimizing 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" connected transformer winding is connected between phases of a three-phase system. A "wye" ("star") transformer connects each winding from a phase wire to a common neutral point.

In an "open delta" or "V" system, only two sets of transformers are used. A closed delta system can operate as an open delta if one of the transformers has failed or needs to be removed. In open delta, each transformer must carry current for its respective phases as well as current for the third phase, therefore capacity is reduced to 87%. With one of three transformers missing and the remaining two at 87% efficiency, the capacity is 58% ((2/3) 87%).

Where a delta-fed system must be grounded for protection from surge voltages, a grounding transformer (usually a zigzag transformer) may be connected to allow ground fault currents to return from any phase to ground. Another variation is a "corner grounded" delta system, which is a closed delta that is grounded at one of the junctions of transformers.


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