three-phase electric power - wikipedia, the free encyclopedia

8/10/2019 Three-phase Electric Power - Wikipedia, The Free Encyclopedia

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Three-phase transformer with fourwire output for 208Y/120 volt

service: one wire for neutral, othersfor A, B and C phases

Three-phase electric powerFrom Wikipedia, the free encyclopedia

Three-phase electric poweris a common method of alternating-current electric power generation, transmission, and

distribution.[1]It is a type of polyphase system and is the most

common method usedby electrical grids worldwide to transferpower. It is also used to power large motors and other heavyloads. A three-phase system is usually more economical than anequivalentsingle-phase or two-phase system at the same line toground voltage because it uses less conductor material to

transmit electrical power.[2]The three-phase system wasindependently invented by Galileo Ferraris, Mikhail Dolivo-Dobrovolsky and Nikola Tesla in the late 1880s.

Contents

1 Principle

2 Generation and distribution

3 Transformer connections

4 Three-wire and four-wire circuits

5 Balanced circuits

5.1 Wye5.2 Delta

6 Single-phase loads

6.1 Unbalancedloads

6.2 Non-linear loads

7 Three-phase loads

8 Phase converters

8.1 Mechanical

8.2Non-mechanical

9 Alternatives to three-phase

10 Color codes

11 See also

12 Notes

13 References

Principle
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Three-phase electric powertransmission

In a balanced three-phase power supply system (by far, the most common type), three conductors eachcarry an alternating current of the same frequency and voltagerelative to a common reference (Typically such a reference isconnected to ground and often to a current-carrying conductorcalled the neutral) but with a phase difference of one third theperiod; hence the voltage on any conductor reaches its peak atone third of a cycle after one of the other conductors and one

third of a cycle before the third conductor. From any of the threeconductors, the peak voltage on the other two conductors isdelayed 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 inan electric motor and generate other phase arrangements usingtransformers (For instance, a two phase system using a Scott-Ttransformer).

With a perfectly balanced three phase supply the instantaneous voltage of any phase is exactly equal in

magnitude but opposite to the sum of the other two phases. This means that if the load on the threephases is balanced as well, the return path for the current in any phase conductor is the other two phaseconductors.

Hence, the sum of the currents in the three conductors is always zero and the current in each conductor isequal to and in the opposite direction as the sum of the currents in the other two. Thus, each conductoracts 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 supplycan 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.[3]

Three-phase systems may also utilize a fourth wire, particularly in low-voltage distribution. This is theneutral wire. The neutral allows three separate single-phase supplies to be provided at a constant voltageand 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 eachphase. Further up the supply chain in high-voltage distribution the currents are usually well balanced andit 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 or no current. With a balanced load, all the phase conductors carry the same current and so

can be the same size.

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 constantmagnitude, which simplifies the design of electric motors.
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Animation of three-phase current flow

Left: Elementary six-wire three-phase alternator, with each

phase using a separate pair of transmission wires.[4]Right:Elementary three-wire three-phase alternator, showing how

the phases can share only three wires.[5]

Most household loads are single-phase. In North American residences, three-phase power might feed amultiple-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 maybe 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 ofthe phases in the right order is required to ensure the intended direction of rotation of three-phasemotors. For example, pumps and fans may not work in reverse. Maintaining the identity of phases isrequired if there is any possibility two sources can be connected at the same time; a directinterconnection between two different phases is a short-circuit.

Generation and distribution

At the power station, an electrical generatorconverts mechanical power into a set ofthree AC electric currents, one from eachcoil (or winding) of the generator. The

windings are arranged such that the currentsvary sinusoidally at the same frequency butwith the peaks and troughs of their waveforms offset to provide threecomplementary currents with a phase

separation of one-third cycle (120 or 23radians). The generator frequency istypically 50 or 60 Hz, varying by country.

At the power station, transformers changethe voltage from generators to a levelsuitable for transmission minimizing losses.

After further voltage conversions in thetransmission network, the voltage is finallytransformed to the standard utilizationbefore power is supplied to customers.

Most automotive alternators generate three

phase AC and rectify it to DC with a diodebridge.[6]

Transformer connections

A "delta" connected transformer winding isconnected between phases of a three-phasesystem. A "wye" ("star") transformer connects each winding from a phase wire to a common neutralpoint.

In an "open delta" or "V" system, only two 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.[7]In open delta, eachtransformer must carry current for its respective phases as well as current for the third phase, therefore
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Wye (Y) and Delta () circuits

A transformer for a "high-leg delta"system; (Assuming a 200 V, 3-Phase

supply) 200 V 3-phase motors wouldbe connected to L1, L2 and L3. 200V Single-phase load would beconnected between L1 and L2. Singlephase 100 V supplies (180 degrees"out of phase") would be obtainedbetween either L1 or L2 and theneutral (N). L3 (wild or high leg)will be 173.2 V with respect to theneutral.

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%).[8][9]

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 phaseto ground. Another variation is a "corner grounded" delta system, which is a closed delta that is

grounded at one of the junctions of transformers.[10]

Three-wire and four-wire circuits

There are two basic three-phase configurations:delta and wye (star). Asshown on the left, a deltaconfiguration requiresonly 3 wires for

transmission but a wye(star) configuration mayutilise a fourth wire. Thefourth wire, if present, isprovided as a Neutral andis normally Grounded.The '3-wire' and '4-wire'designations do not countthe ground wire usedabove many transmission

lines which is solely forfault protection and doesnot carry current undernon-fault conditions.

A four-wire system withsymmetrical voltagesbetween phase andneutral is obtained when

the neutral is connected to the "common star point" of all supply

windings. In such a system, all three phases will have the samemagnitude of voltage relative to the Neutral. Other non-symmetrical systems have been used.

The four-wire wye system is used when ground referenced voltages or the flexibility of more voltageselections are required. Faults on one phase to ground will cause a protection event (fuse or breakeropen) locally and not involve other phases or other connected equipment. An example of application islocal distribution in Europe (and elsewhere), where each customer may be only fed from one phase andthe neutral (which is common to the three phases). When a group of customers sharing the neutral drawunequal phase currents, the common neutral wire carries the currents resulting from these imbalances.Electrical engineers try to design the system so the loads are balanced as much as possible within

premises where 3-phase power is utilized. [11]These same principles apply to the wide scale distributionof power to individual premises. Hence, every effort is made by supply authorities to distribute all threephases over a large number of premises so that, on average, as nearly as possible a balanced load is seenat the point of supply. For domestic use, some countries such as the UK may supply one phase and
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Three phase AC generator connectedas a wye source to a wye connected

load.

neutral at a high current (up to 100A) to one property, while others such as Germany may supply 3phases and neutral to each customer, but at a lower fuse rating, typically 32A per phase, and 'shuffled' toavoid the effect that more load tends to be put on the first phase.

In North America, a high-leg delta supply is sometimes used, where one winding of a delta connectedtransformer feeding the load is center-tapped and that center tap is grounded and connected as a Neutral,as shown on the right. This setup produces three different voltages. If the voltage between the center tap(neutral) and each of the two adjacent phases is 120 V (100%), the voltage across any two phases is 240

V (200%) and the Neutral to "high leg" voltage is 208 V (173%).[7]

The reason for providing the delta connected supply is usually to power large motors requiring a rotatingfield. However, the premises concerned will also require the "normal" North American 120 V supplies,two of which are derived (180 degrees "out of phase") between the "Neutral" and either of the centertapped phase points.

Balanced circuits

In the perfectly balanced case all three lines share equivalent loads. Examining the circuits we can deriverelationships between line voltage and current, and load voltage and current for wye and delta connectedloads.

In a balanced system each line will produce equal voltage magnitudes at phase angles equally spacedfrom each other. With V1as our reference and V3lagging V2lagging V1, using angle notation, we

have:[12]

These Voltages feed into either a wye or delta connected load.

Wye

For the wye case, all loads see their respective line voltages, and

so:[12]
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Three phase AC generator connectedas a wye source to a delta connectedload.

whereZtotalis the sum of line and load impedances (Ztotal= ZLN+ ZY), and is the phase of the

total impedance (Ztotal).

The phase angle difference between voltage and current of each phase is not necessarily 0 and isdependent on the type of load impedance, Zy. Inductive and capacitive loads will cause current to either

lag or lead the voltage. However, the relative phase angle between each pair of lines (1 to 2, 2 to 3,and 3

to 1) will still be 120 degrees.

By performing Kirchhoff's Current Law (KCL) on the neutral node, the three phase currents sum up tothe total current in the neutral line. In the balanced case:

Delta

In the delta circuit loads are connected across the lines and so

loads see line-to-line voltages:[12]

Further:

where is the phase of delta impedance (Z).

Relative angles are preserved, so I31lags I23lags I12by 120 degrees. Calculating line currents by usingKCL at each delta node gives:
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And similarly for each other line:

again, is the phase of delta impedance (Z).

Single-phase loads

Single-phase loads may be connected across any two phases, or a load can be connected from phase to

neutral.[13]Distributing single-phase loads among the phases of a three-phase system balances the loadand makes most economical use of conductors and transformers.

In a symmetrical three-phase four-wire, wye system, the three phase conductors have the same voltageto the system neutral. The voltage between line conductors is 3 times the phase conductor to neutralvoltage.

VL-L= 3VL-N[14]

The currents returning from the customers' premises to the supply transformer all share the neutral wire.If the loads are evenly distributed on all three phases, the sum of the returning currents in the neutralwire is approximately zero. Any unbalanced phase loading on the secondary side of the transformer will

use the transformer capacity inefficiently.If the supply neutral is broken, phase-to-neutral voltage is no longer maintained. Phases with higherrelative loading will experience reduced voltage and phases with lower relative loading will experienceelevated voltage, up to the phase-to-phase voltage.

A high-leg delta provides phase-to-neutral relationship of VL-L= 2VL-N, however, L-N load is

imposed on one phase.[7]A transformer manufacturer's page suggests that L-N loading to not exceed 5%

of transformer capacity.[15]

3 is 1.73, so if VL-Nwas defined as 100%, VL-Lwould be 100% 1.73 = 173%If VL-Lwas

set as 100%, then VL-N 57.7%

Unbalanced loads

When the currents on the three live wires of a three-phase system are not equal or are not at an exact120 phase angle, the power-loss is greater than for a perfectly balanced system. The method ofsymmetrical components is used to analyze unbalanced systems.

Non-linear loads
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With linear loads, the neutral only carries the current due to imbalance between the phases. Devices thatutilize rectifier-capacitor front-end such as switch-mode power supplies, computers, office equipmentand such produce third order harmonics that are in-phase on all the supply phases. Consequently, suchharmonic currents add in the neutral which can cause the neutral current to exceed the phase

current.[13][16]

Three-phase loads

An important class of three-phase load is the electric motor. A three-phase induction motor has a simpledesign, inherently high starting torque and high efficiency. Such motors are applied in industry for manyapplications. A three-phase motor is more compact and less costly than a single-phase motor of the samevoltage class and rating and single-phase AC motors above 10 HP (7.5 kW) are uncommon. Three-phasemotors also vibrate less and hence last longer than single-phase motors of the same power used underthe same conditions.

Line frequency flicker in light can be reduced by evenly spreading three phases across line frequencyoperated light sources so that illuminated area is provided light from all three phases. The effect of linefrequency flicker is detrimental to super slow motion cameras used in sports event broadcasting. Threephase lighting has been applied successfully at the 2008 Beijing Olympics to provide consistent light

level for each frame for SSM cameras.[17]Resistance heating loads such as electric boilers or spaceheating may be connected to three-phase systems. Electric lighting may also be similarly connected.

Rectifiers may use a three-phase source to produce a six-pulse DC output.[18]The output of suchrectifiers is much smoother than rectified single phase and, unlike single-phase, does not drop to zerobetween pulses. Such rectifiers may be used for battery charging, electrolysis processes such asaluminium production or for operation of DC motors. "Zig-zag" transformers may make the equivalent

of six-phase full-wave rectification, twelve pulses per cycle, and this method is occasionally employedto reduce the cost of the filtering components, while improving the quality of the resulting DC.

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

In many European countries electric stoves are usually designed for a three-phase feed. However, theindividual heating units are often connected between phase and neutral to allow for connection to a

single-phase circuit e.g. if within an older domestic property a three-phase feed is not yet available.[19]

Other usual three-phase loads in the domestic field are tankless water heating systems and storageheater.

Phase converters

Phase converters are used when three-phase equipment needs to be operated on a single-phase powersource. They are used when three-phase power is not available or cost is not justifiable. Such convertersmay also allow the frequency to be varied (resynthesis) allowing speed control. Some railway

locomotives use a single-phase source to drive three-phase motors fed through an electronic drive.[20]

Mechanical

One method to generate three-phase power from a single-phase source is the rotary phase converter,essentially a three-phase motor with special starting arrangements and power factor correction thatproduces balanced three-phase voltages. When properly designed, these rotary converters can allow
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system for frequency control.

Monocyclic powerwas 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 fraction (usually

1/4 of the line voltage) winding that 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 typically would use six phases or twelve phases. High-phase-order transmission lines allow

transfer of slightly less than proportionately higher power through a given volume without theexpense of a high-voltage direct current (HVDC) converter at each end of the line. However, they

require correspondingly more pieces of equipment.

Color codes

Conductors of a three-phase system are usually identified by a color code, to allow for balanced loadingand to assure the correct phase rotation for 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. Forexample, in the U.S. and Canada, different color codes are used for grounded (earthed) and ungroundedsystems.

Country L1 L2 L3 Neutral

Ground /

protective

earth

Australia and New Zealand as perAS/NZS 3000:2007 Figure 3.2 (or as per

IEC 60446 as approved by AS:3000)

Red(orbrown)[note 1]

White (orblack)[note 1]

(prev.yellow)

Dark blue(or

grey)[note 1]

Black (orblue)[note 1]

Green/yellowstriped(green on

very oldinstallations)

Canada (mandatory)[22] Red Black BlueWhite orGrey

Green or barecopper

Canada (isolated three-phaseinstallations)[23]

Orange Brown Yellow White Green

European Union and all countries whouse European CENELEC standards April2004 (IEC 60446), Hong Kong from

July 2007, Singapore from March 2009

Brown Black Grey BlueGreen/yellowstriped[note 2]

Older European (prior to IEC 60446,

Green/yellowstriped(green on
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varied by country [note 3] Red Yellow Blue Black installationsbefore c.1970)

UK beforeApril 2006, Hong Kongbefore April 2009, South Africa,Malaysia, Singapore before February

2011

Red Yellow Blue Black

Green/yellowstriped(green oninstallations

before c.1970)

India and Pakistan Red Yellow Blue Black Green/yellowstriped, orgreen

Former USSR (Russia, Ukraine,Kazakhstan) and People's Republic ofChina (per GB 50303-2002 Section15.2.2)

Yellow Green Red Sky blueGreen/yellowstriped

Norway Black White/Grey Brown Blue

Yellow/greenstriped, oldermay be onlyyellow orbare copper

United States (common practice)[note 4] Black Red BlueWhite, orgrey

Green,green/yellowstriped,[note 5]

or a barecopper wire

United States (alternative practice)[note 6] Brown

Orange(deltasystems), orviolet (wyesystems)

Yellow Grey, orwhite

Green

See also

Three-phase AC railway electrification

Charging station

Frequency converter

Industrial & multiphase power plugs & sockets

International Electrotechnical Exhibition

JohnHopkinson

Y-transform

Notes

1. ^ abcdIn Australia and New Zealand, active conductors can be any color except green/yellow, green, yellow,

blackor light blue. Yellow is no longer permitted in the 2007 revision of wiring code ASNZS 3000. European
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References

co orco es are use or a or ex ca es suc as ex ens on ea s, app ance ea s e c. an are equa y

permitted for use in building wiring per AS/NZS 3000:2007.

2. ^Theinternational standard green-yellow marking of protective-earth conductors was introduced to reduce the

riskof confusion by color blind installers. About 7% to 10% of men cannot clearly distinguish between red

andgreen, which is a particular concern in older schemes where red marks a live conductor and green marks

protective earth or safety ground.

3. ^InEurope, there still exist many installations with older colors but, since the early 1970s, all new

installations use green/yellow earth according to IEC 60446. (E.g. Phase/Neutral+Earth German: black/grey +red France green/red + White Russia: Red/ Grey + Black; Switzerland: Red/ Grey +Yellow or yellow & red

Denmark: White/Black + Red

4. ^See Paul Cook: Harmonised colours and alphanumeric marking (http://electrical.theiet.org/wiring-

regulations/cable-colours/harmonised.cfm?type=pdf). IEE Wiring Matters, Spring 2006.

5. ^Inthe U.S., a green/yellow striped wire may indicate an isolated ground. 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.

6. ^Since 1975, the U.S. National Electric Code has not specified coloring of phase conductors. It is commonpractice in many regions to identify 120/208 (wye) conductors as black, red, and blue, and 277/480 (wye or

delta) conductors as brown, orange, yellow. In a 120/240 delta system with a 208v high leg, the high leg

(typically B phase) is always marked orange, commonly A phase is black and C phase is either red or blue.

Local regulations may amend the N.E.C. The U.S. National Electric Code has color requirements for

grounded conductors, ground, and grounded-delta 3-phase systems which result in one ungrounded leg having

a higher voltage potential to ground than the other two ungrounded legs.

1. ^William D. Stevenson, Jr.Elements of Power System Analysis Third Edition, McGraw-Hill, New York

(1975). ISBN 0-07-061285-4, p. 2

2. ^http://www.allaboutcircuits.com/vol_2/chpt_10/2.html

3. ^Cotton, H,Electrical Technology, 6th Ed., Pitman, London, 1950, p. 268

4. ^Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed., 1917, vol. 4, Ch. 46: Alternating Currents, p.

1026, fig. 1260.

5. ^Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed., 1917, vol. 4, Ch. 46: Alternating Currents, p.

1026, fig. 1261.6. ^http://www.rle.mit.edu/per/conferencepapers/cpconvergence00p583.pdf

7. ^ abcFowler, Nick (2011).Electrician's Calculations Manual 2nd Edition (http://books.google.com/books?

id=9QBWHJdPGM0C). McGraw-Hill. pp. 35. ISBN 9780071770170.

8. ^McGraw-Hill (1920).Power51 (17) http://books.google.com/books?

id=u91QAAAAYAAJ&pg=PA673&lpg=PA673|url=missing title (help). Retrieved 21 December 2012.

9. ^H. W. Beaty, D.G.Fink (ed) Standard Handbook for Electrical Engineers Fifteenth Edition,McGraw-Hill,

2007ISBN 0-07-144146-8, p. 1011

10. ^Schneider (http://static.schneider-electric.us/docs/Circuit%20Protection/Molded%20Case%20Circuit%20Breakers/2700DB0202.pdf)

11. ^http://www.rapid-tech.com.au/Fluke-2_Saving%20energy%20through%20load%20balancing.pdf

12. ^ abcJ. Duncan Glover; Mulukutla S. Sarma; Thomas J. Overbye (April 2011). Power System Analysis &
http://en.wikipedia.org/wiki/Special:BookSources/978-1-111-42579-1http://en.wikipedia.org/wiki/Special:BookSources/978-1-111-42579-1http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://books.google.com/books?id=HrtXToLEbVoChttp://www.rapid-tech.com.au/Fluke-2_Saving%20energy%20through%20load%20balancing.pdfhttp://static.schneider-electric.us/docs/Circuit%20Protection/Molded%20Case%20Circuit%20Breakers/2700DB0202.pdfhttp://en.wikipedia.org/wiki/Special:BookSources/0071441468http://en.wikipedia.org/wiki/Help:CS1_errors#bare_url_missing_titlehttp://books.google.com/books?id=u91QAAAAYAAJ&pg=PA673&lpg=PA673http://en.wikipedia.org/wiki/Special:BookSources/9780071770170http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://books.google.com/books?id=9QBWHJdPGM0Chttp://www.rle.mit.edu/per/conferencepapers/cpconvergence00p583.pdfhttp://en.wikipedia.org/wiki/Hawkins_Electrical_Guidehttp://en.wikipedia.org/wiki/Hawkins_Electrical_Guidehttp://www.allaboutcircuits.com/vol_2/chpt_10/2.htmlhttp://en.wikipedia.org/wiki/Special:BookSources/0070612854http://en.wikipedia.org/wiki/Isolated_groundhttp://electrical.theiet.org/wiring-regulations/cable-colours/harmonised.cfm?type=pdfhttp://en.wikipedia.org/wiki/IEC_60446http://en.wikipedia.org/wiki/Color_blindness


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esgn p: oo s.goog e.com oo s = r o o . engage earn ng. pp. . - -

111-42579-1.

13. ^ abLowenstein, Michael. "The 3rd Harmonic Blocking Filter: A Well Established Approach to Harmonic

Current Mitigation" (http://www.iaei.org/blogpost/890108/159506/The-3rd-Harmonic-Blocking-Filter-A-

Well-Established-Approach-to-Harmonic-Current-Mitigation). IAEI Magazine. Retrieved 24 November 2012.

14. ^Theboy electrician by J W Sims M.I.E.E. (Page 98)

15. ^Federal pacific (http://www.federalpacific.com/university/transbasics/chapter3.html)

16. ^Enjeti, Prasad. "Harmonics in Low Voltage Three-Phase Four-Wire Electric Distribution Systems and

Filtering Solutions"

(http://www.pserc.wisc.edu/documents/general_information/presentations/pserc_seminars/pserc_seminars0/enj

eti_slides.pdf). Texas A&M University Power Electronics and Power Quality Laboratory. Retrieved 24

November 2012.

17. ^Hui,Sun. "Sports Lighting Design Considerations For The Beijing 2008 Olympic Games"

(http://www.geappliances.com/email/olympic_specifier/downloads/SportsLightinginBeijing2008OlympicGame

s.pdf). GE Lighting. Retrieved 18 December 2012.

18. ^IEEE (http://www.ewh.ieee.org/soc/es/Nov1998/08/CONVERTR.HTM)19. ^"British and European practices for domestic appliances compared" (http://books.google.co.uk/books?

id=O3gnAAAAMAAJ&q=%22full+size+cookers+for+Europe+have+to+be+connected+to+a+three-

phase+star+connected+supply%22&dq=%22full+size+cookers+for+Europe+have+to+be+connected+to+a+thre

e-

phase+star+connected+supply%22&hl=en&sa=X&ei=fSipUKXYCfCk0AX4zYDoBA&ved=0CDAQ6AEwA

A),Electrical Times, volume 148, page 691, 1965.

20. ^Japan Railway & Transport Review. No. 58: 58. Oct 2011 http://www.jrtr.net/jrtr58/pdf/51-60web.pdf

|url=missing title (help).

21. ^Brittain, J. E. (2007). "Electrical Engineering Hall of Fame: Charles F. Scott".Proceedings of the IEEE95

(4):836839. doi:10.1109/JPROC.2006.892488 (http://dx.doi.org/10.1109%2FJPROC.2006.892488).

22. ^Canadian Electrical Code Part I, 23rd Edition, (2002) ISBN 1-55324-690-X, rule 4-036 (3)

23. ^Canadian Electrical Code23th edition 2002, rule 24-208(c)
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