Three-phase electric power


Three-phase electric power

:"This article deals with where, how and why "three phase" is used. For information on the basic mathematics and principles of three phase, see three-phase." Three-phase electric power is a common method of alternating-current electric power transmission. [ William D. Stevenson, Jr. "Elements of Power System Analysis Third Edition", McGraw-Hill, New York (1975). ISBN 0070612854. Page 2.] It is a type of polyphase system mainly used to power large motors and other such devices. A three-phase system is generally more economical than others because it uses less conductor material to transmit electric power than equivalent single-phase or two-phase systems at the same voltage.Fact|date=October 2008

In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electrical current. This delay between "phases" has the effect of giving constant power transfer over each cycle of the current, and also makes it possible to produce a rotating magnetic field in an electric motor.

Three phase systems may or may not have a neutral wire. A neutral wire allows the three phase system to use a higher voltage while still supporting lower voltage single phase appliances. In high voltage distribution situations it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).

Three phase has properties that make it very desirable in electric power systems. First, 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 eliminate or reduce the size of the neutral conductor; all the phase conductors carry the same current and so can be the same size, for a balanced load. Second, power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations. Finally, three-phase systems can produce a magnetic field that rotates in a specified direction, which simplifies the design of electric motors. Three is the lowest phase order to exhibit all of these properties.

Most household loads are single phase. In North America and some other countries, three phase power generally does not enter homes. Even in areas where it does, it is typically split out at the main distribution board.

The three phases are typically indicated by colors which vary by country. See the table for more information.

Generation and distribution

At the power station, an electrical generator converts mechanical power into a set of alternating electric currents, one from each electromagnetic coil or winding of the generator. The currents are sinusoidal functions of time, all at the same frequency but offset in time to give different phases. In a three-phase system the phases are spaced equally, giving a phase separation of one-third cycle. The power frequency is typically 50 Hz in Asia, Europe, South America (except Brazil, Colombia and the Dominican Republic) and Australia, and 60 Hz in the U.S., Canada, Brazil, Colombia and the Dominican Republic (but see Mains power systems for more detail).

Generators output at a voltage that ranges from hundreds of volts to 30,000 volts. At the power station, transformers "step-up" this voltage to one more suitable for transmission.

After numerous further conversions in the transmission and distribution network the power is finally transformed to the standard mains voltage ("i.e." the "household" voltage). The power may already have been split into single phase at this point or it may still be three phase. Where the stepdown is three phase, the output of this transformer is usually star (Y) connected with the standard mains voltage (120 V in North America and 230 V in Europe and Australia) being the phase-neutral voltage. Another system commonly seen in North America is to have a delta connected secondary with a centre tap on one of the windings supplying the ground and neutral. This allows for 240 V three phase as well as three different single phase voltages (120 V between two of the phases and the neutral, 208 V between the third phase (known as a high leg) and neutral and 240 V between any two phases) to be made available from the same supply.

ingle-phase loads

Single-phase loads may be connected to a three-phase system, either by connecting across two live conductors (a phase-to-phase connection), or by connecting between a phase conductor and the system neutral, which is either connected to the center of the Y (star) secondary winding of the supply transformer, or is connected to the center of one winding of a delta transformer (Highleg Delta system) (see transformer and Split phase). Single-phase loads should be distributed evenly between the phases of the three-phase system for efficient use of the supply transformer and supply conductors.

The neutral point of a three phase system exists at the mathematical center of an equilateral triangle formed by the phase points, and the line-to-line voltage of a three-phase system is correspondingly sqrt3 times the line to neutral voltage. Where the line-to-neutral voltage is a standard utilization voltage (for example in a 240 V/415 V system), individual single-phase utility customers or loads may each be connected to a different phase of the supply. Where the line-to-neutral voltage is not a common utilization voltage, for example in a 347/600 V system, single-phase loads must be supplied by individual step-down transformers. In multiple-unit residential buildings in North America, lighting and convenience outlets can be connected line-to-neutral to give the 120 V distribution voltage (115V utilization voltage), and high-power loads such as cooking equipment, space heating, water heaters, or air conditioning can be connected across two phases to give 208 V. This practice is common enough that 208 V single-phase equipment is readily available in North America. Attempts to use the more common 120/240 V equipment intended for three-wire single-phase distribution may result in poor performance since 240 V heating equipment will only produce 75% of its rating when operated at 208 V.

Where three phase at low voltage is otherwise in use, it may still be split out into single phase service cables through joints in the supply network or it may be delivered to a master distribution board (breaker panel) at the customer's premises. Connecting an electrical circuit from one phase to the neutral generally supplies the country's standard single phase voltage (120 VAC or 230 VAC) to the circuit.

The power transmission grid is organized so that each phase carries the same magnitude of current out of the major parts of the transmission system. The currents returning from the customers' premises to the last supply transformer all share the neutral wire, but the three-phase system ensures that the sum of the returning currents is approximately zero. The primary side of that supply transformer commonly uses a delta winding, and no neutral is needed in the high voltage side of the network. Any unbalanced phase loading on the secondary side of the transformer will use the transformer capacity inefficiently, but equal current will be drawn from the phases feeding the primary delta winding, leaving the high voltage network unaffected.

If the supply neutral of a three-phase system with line-to-neutral connected loads is broken, generally the voltage balance on the loads will no longer be maintained. The now-virtual neutral point will tend to drift toward the most heavily loaded phase, causing undervoltage conditions on that phase only. Correspondingly, the lightly-loaded phases may approach the line-to-line voltage, which exceeds the line-to-neutral voltage by a factor of √3, causing overheating and failure of many types of loads. For example, if several houses are connected to a common three-phase transformer, each house might be connected to one of the three phases. If the neutral connection is broken at the transformer or on the distribution line somewhere upstream of the transformer, all equipment in a house might be damaged due to overvoltage. This type of failure event can be difficult to troubleshoot if the drifting neutral effect is not understood. With inductive and/or capacitive loads, all phases can suffer damage as the reactive current moves across abnormal paths in the unbalanced system, especially if resonance conditions occur. For this reason, neutral connections are a critical part of a power distribution network and must be made as reliable as any of the phase connections.

Independent (or nearly so) three phase systems are sometimes interconnected using DC transmission (with the requisite transformation equipment) in order to isolate certain potential electrical transients from propagating from one system to another.

Three-phase loads

[
magnetic field of a three-phase motor.] The most important class of three-phase load is the electric motor. A three phase induction motor has a simple design, inherently high starting torque, and high efficiency. Such motors are applied in industry for pumps, fans, blowers, compressors, conveyor drives, and many other kinds of motor-driven equipment. A three-phase motor will be more compact and less costly than a single-phase motor of the same voltage class and rating; and single-phase AC motors above 10 HP (7.5 kW) are uncommon. Three phase motors will also vibrate less and hence last longer than single phase motor of the same power used under the same conditions.

Large air conditioning, etc. equipment use three-phase motors for reasons of efficiency, economy and longevity.

Resistance heating loads such as electric boilers or space heating may be connected to three-phase systems. Electric lighting may also be similarly connected. These types of loads do not require the revolving magnetic field characteristic of three-phase motors but take advantage of the higher voltage and power level usually associated with three-phase distribution. Fluorescent lighting systems also benefit from reduced flicker if adjacent fixtures are powered from different phases.

Large rectifier systems may have three-phase inputs; the resulting DC current is easier to filter (smooth) than the output of a single-phase rectifier. Such rectifiers may be used for battery charging, electrolysis processes such as aluminum production, or for operation of DC motors.

An interesting example of a three-phase load is the electric arc furnace used in steelmaking and in refining of ores.

In much of Europe stoves are designed for a three phase feed. Usually the individual heating units are connected between phase and neutral to allow for connection to a single phase supply. In many areas of Europe, single phase power is the only source available.

Phase converters

Occasionally the advantages of three-phase motors make it worthwhile to convert single-phase power to three phase. Small customers, such as residential or farm properties may not have access to a three-phase supply, or may not want to pay for the extra cost of a three-phase service, but may still wish to use three-phase equipment. Such converters may also allow the frequency to be varied allowing speed control. Some locomotives are moving to multi-phase motors driven by such systems even though the incoming supply to a locomotive is nearly always either DC or single phase AC.

Because single-phase power goes to zero at each moment that the voltage crosses zero but three-phase delivers power continuously, any such converter must have a way to store energy for the necessary fraction of a second.

One method for using three-phase equipment on a single-phase supply is with a rotary phase converter, essentially a three-phase motor with special starting arrangements and power factor correction that produces balanced three-phase voltages. When properly designed these rotary converters can allow satisfactory operation of three-phase equipment such as machine tools on a single phase supply. In such a device, the energy storage is performed by the mechanical inertia (flywheel effect) of the rotating components. An external flywheel is sometimes found on one or both ends of the shaft.

A second method that was popular in the 1940s and 1950s was a method that was called the transformer method. In that time period capacitors were more expensive relative to transformers. So an autotransformer was used to apply more power through fewer capacitors. This method performs well and does have supporters, even today. The usage of the name transformer method separated it from another common method, the static converter, as both methods have no moving parts, which separates them from the rotary converters.

Another method often attempted is with a device referred to as a static phase converter. This method of running three phase equipment is commonly attempted with motor loads though it only supplies ⅔ power and can cause the motor loads to run hot and in some cases overheat. This method will not work when sensitive circuitry is involved such as CNC devices, or in induction and rectifier type loads.

Some devices are made which create an imitation three-phase from three-wire single phase supplies. This is done by creating a third "subphase" between the two live conductors, resulting in a phase separation of 180° − 90° = 90°. Many three-phase devices will run on this configuration, but at lower efficiency.

Variable-frequency drives (also known as solid-state inverters) are used to provide precise speed and torque control of three phase motors. Some models can be powered by a single phase supply. VFDs work by converting the supply voltage to DC and then converting the DC to a suitable three phase source for the motor.

Digital phase converters are similar to variable frequency drives but are designed for fixed frequency operation from a single-phase source. Similar to a variable frequency drive, they use a microprocessor to control solid state power switching components to maintain balanced three-phase voltages.

Alternatives to three-phase

* Three-wire single-phase distribution is used when three phase power is not available, and allows double the normal utilization voltage to be supplied for high-power loads.

* Two-phase electric power, like three phase, gives constant power transfer to a linear load. For loads which connect each phase to neutral, assuming the load is the same power draw, the two wire system has a neutral current which is greater than neutral current in a three phase system. Also motors are not entirely linear, which means that despite the theory, motors running on three phase tend to run smoother than those on two phase. The generators at Niagara Falls installed in 1895 were the largest generators in the world at the time and were two-phase machines. True two-phase power distribution is essentially obsolete. Special purpose systems may use a two-phase system for control. Two-phase power may be obtained from a three-phase system using an arrangement of transformers called a Scott-T transformer.

* "Monocyclic power" was a name for an asymmetrical modified two-phase power system used by General Electric around 1897, championed by Charles Proteus Steinmetz and Elihu Thomson. This system was devised to avoid patent infringement. In this system, a generator was wound with a full-voltage single phase winding intended for lighting loads, and with a small (usually ¼ of the line voltage) winding which produced a voltage in quadrature with the main windings. The intention was to use this "power wire" additional winding to provide starting torque for induction motors, with the main winding providing power for lighting loads. After the expiration of the Westinghouse patents on symmetrical two-phase and three-phase power distribution systems, the monocyclic system fell out of use; it was difficult to analyze and did not last long enough for satisfactory energy metering to be developed.

* High phase order systems for power transmission have been built and tested. Such transmission lines use six or 12 phases and design practices characteristic of extra-high voltage transmission lines. High-phase order transmission lines may allow transfer of more power through a given transmission line right-of-way without the expense of a high-voltage direct current (HVDC) converter at each end of the line.

Color codes

Conductors of a three phase system are usually identified by a color code, to allow for balanced loading and to assure the correct phase rotation for induction motors. Colors used may adhere to International Standard IEC 60446, older standards, or to no standard at all, and may vary even within a single installation. For example, in the U.S. and Canada, different color codes are used for grounded (earthed) and un-grounded systems.

fnb|1 Since 1975, the U.S. National Electric Code has not specified coloring of phase conductors. It is common practice in many regions to identify 120/208Y conductors as Black/Red/Blue. Local regulations may amend the N.E.C.

fnb|2 The U.S. National Electric Code does not specify coloring of phase conductors. It is common practice in many regions to identify 277/480Y conductors as Brown/Orange/Yellow. Local practice may amend the N.E.C. The US N.E.C. rule 517.160 (5) states these colors are to be used for isolated power systems in health care facilities. Color of conductors does not identify voltage of a circuit, because there is no formal standard. fnb|3 In Australia and New Zealand, any colour is permitted except green/yellow, green or yellow. Yellow is no longer permitted in the 2007 revision of wiring code ASNZS 3000.

Note that in the U.S. a green/yellow striped wire may indicate an Isolated groundFact|date=August 2007. In most countries today, green/yellow striped wire may only be used for protective earth (safety ground), and may never be unconnected or used for any other purpose.

fnb|4 The international standard green-yellow marking of protective-earth conductors was introduced to reduce the risk of confusion by color blind installers. About 7% to 10% of men cannot clearly distinguish between red and green, which is a particular concern in older schemes were red marks a live conductor and green marks protective earth (U.S. terminology: safety ground).

fnb|5 In Europe there still exist installations with older colors for protective earth, but since the early 1970s, all new installations use green/yellow according to IEC 60446.

fnb|6 See Paul Cook: [http://www.iee.org/Publish/WireRegs/IEE_Harmonized_colours.pdf Harmonised colours and alphanumeric marking] . IEE Wiring Matters, Spring 2006.

See also

* Frequency converter
* Industrial & multiphase power plugs & sockets
* Y-Δ transform
* International Electro-Technical Exhibition - 1891
* People
** Nikola Tesla
** John Hopkinson
** Mikhail Dolivo-Dobrovolsky

References


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