Three phase power supply system

A three-phase power supply system is a special case of multiphase systems of electric circuits of alternating current , in which sinusoidal emfs of the same frequency created by a common source operate, shifted relative to each other in time by a certain phase angle . In a three-phase system, this angle is 2π / 3 (120 °).

The multi-wire (six-wire) three-phase AC system was invented by Nikola Tesla . A significant contribution to the development of three-phase systems was made by M.O. Dolivo-Dobrovolsky , who first proposed three- and four-wire AC transmission systems, revealed a number of advantages of low-conductivity three-phase systems in relation to other systems and conducted a number of experiments with an asynchronous electric motor .

Content

Description

Each of the existing EMFs is in its phase of the periodic process, therefore it is often called simply a “phase”. Also called "phases" are conductors - carriers of these EMFs. In three-phase systems, the shear angle is 120 degrees. Phase conductors are indicated in the Russian Federation by Latin letters L with a digital index of 1 ... 3, or A, B and C ^[1] .

Common phase wire designations:

Russia, EC (above 1000 V)	Russia, EU (below 1000 V)	Germany	Denmark
BUT	L1	L1	R
B	L2	L2	S
C	L3	L3	T

Animated image of current flow along a symmetrical three-phase circuit with a star connection

Vector diagram of phase currents. Symmetric mode.

Graphical representation of phase currents versus time

Benefits

A possible layout of a three-phase network in multi-apartment residential buildings

Profitability.
- Cost-effective transmission of electricity over long distances.
- Less material consumption of 3-phase transformers.
- Less material consumption of power cables, since at the same power consumption currents in phases are reduced (compared to single-phase circuits).
The balance of the system. This property is one of the most important, since in an unbalanced system an uneven mechanical load arises on the power generating unit , which significantly reduces its service life.
The ability to easily obtain a circular rotating magnetic field necessary for the operation of an electric motor and a number of other electrical devices. 3-phase current motors (asynchronous and synchronous) are simpler than single-phase or 2-phase direct current motors and have high efficiency rates.
The ability to obtain in one installation two operating voltages - phase and linear, and two power levels when connected to a "star" or "triangle".
The possibility of a sharp reduction in the flicker and stroboscopic effect of luminaires on fluorescent lamps by placing three lamps (or groups of lamps) powered by different phases in one lamp.

Due to these advantages, three-phase systems are the most common in modern electric power industry.

Connection diagrams for three-phase circuits

Star

A star is such a connection when the ends of the phases of the generator windings (G) are connected to one common point, called the neutral point or neutral . The ends of the phases of the consumer windings (M) are also connected to a common point.

The wires connecting the beginning of the phases of the generator and the consumer are called linear . The wire connecting the two neutrals is called neutral .

A three-phase circuit having a neutral wire is called a four-wire circuit. If there is no neutral wire - three-wire.

If the resistances Z a , Z b , Z c of the consumer are equal to each other, then such a load is called symmetrical .

Linear and Phase Values

The voltage between the phase conductor and the neutral (U a , U b , U c ) is called phase. The voltage between the two phase conductors (U AB , U BC , U CA ) is called linear. To connect the windings with a star, with a symmetrical load, the relationship between linear and phase currents and voltages is valid:

$I_{L}=I_{F};\qquad U_{L}={\sqrt {3}}\times {U_{F}}$ ${\ displaystyle I_ {L} = I_ {F}; \ qquad U_ {L} = {\ sqrt {3}} \ times {U_ {F}}}$

It is easy to show that the line voltage is phase shifted by $\pi /6$ ${\ displaystyle \ pi / 6}$ relatively phase:

$u_{L}^{ab}=u_{F}^{a}-u_{F}^{b}=U_{F}[\cos(\omega t)-\cos(\omega t-2\pi /3)]=2U_{F}\sin(-\pi /3)\sin(\omega t-\pi /3)={\sqrt {3}}U_{F}\cos(\omega t+\pi -\pi /3-\pi /2)$ ${\ displaystyle u_ {L} ^ {ab} = u_ {F} ^ {a} -u_ {F} ^ {b} = U_ {F} [\ cos (\ omega t) - \ cos (\ omega t- 2 \ pi / 3)] = 2U_ {F} \ sin (- \ pi / 3) \ sin (\ omega t- \ pi / 3) = {\ sqrt {3}} U_ {F} \ cos (\ omega t + \ pi - \ pi / 3- \ pi / 2)}$

$u_{L}={\sqrt {3}}U_{F}\cos(\omega t+\pi /6)$ ${\ displaystyle u_ {L} = {\ sqrt {3}} U_ {F} \ cos (\ omega t + \ pi / 6)}$

Three-phase power

To connect the windings with a star, with a symmetrical load, the power of a three-phase network is $P=3U_{F}I_{F}cos\varphi =3{\frac {U_{L}}{\sqrt {3}}}I_{L}cos\varphi ={\sqrt {3}}U_{L}I_{L}cos\varphi$ ${\ displaystyle P = 3U_ {F} I_ {F} cos \ varphi = 3 {\ frac {U_ {L}} {\ sqrt {3}}} I_ {L} cos \ varphi = {\ sqrt {3}} U_ {L} I_ {L} cos \ varphi}$

Consequences of burning off (breaking) the zero wire in three-phase networks

Existing types of line voltage protection, which can be found on sale in electrical stores

Tires for distribution of neutral wires (blue) and ground wires (green)

With a symmetrical load in a three-phase system, power supply to the consumer by linear voltage is possible even in the absence of a neutral wire . Despite this, when the load is supplied with phase voltage, when the load on the phases is not strictly symmetrical, the presence of a neutral wire is required. When it breaks or a significant increase in resistance (poor contact ) occurs the so-called phase imbalance , as a result of which the connected load, calculated on the phase voltage, can turn out to be an arbitrary voltage in the range from zero to linear (the specific value depends on the load distribution over the phases at the moment zero wire breakage). This is often the cause of the failure of consumer electronics in apartment buildings , which can lead to fires. Undervoltage can also cause equipment failure.

The Problem of Harmonics Multiples of the Third

Modern technology is increasingly equipped with switching network power supplies . A pulsed source without a power factor corrector consumes current in narrow pulses near the peaks of the sinusoid of the supply voltage at the charging intervals of the capacitor of the input rectifier . A large number of such power sources in the network creates an increased current of the third harmonic of the supply voltage. The harmonics currents that are multiples of the third, instead of mutual compensation, are mathematically summed in the neutral conductor (even with a symmetrical load distribution) and can lead to its overload even without exceeding the allowable power consumption in phases. Such a problem exists, in particular, in office buildings with a large number of simultaneously operating office equipment. The solution to the problem of the third harmonic is the use of a power factor corrector (passive or active) as part of the scheme of produced switching power supplies. The requirements of IEC 1000-3-2 standard impose restrictions on the harmonic components of the load current of devices with a power of 50 watts or more. In Russia, the number of harmonic components of the load current is standardized by the standards GOST R 54149-2010, GOST 32144-2013 (from 1.07.2014), OST 45.188-2001.

Triangle

A triangle is such a connection when the end of the first phase connects to the beginning of the second phase, the end of the second phase to the beginning of the third, and the end of the third phase connects to the beginning of the first.

The relationship between linear and phase currents and voltages

To connect the windings with a triangle, with a symmetrical load, the relationship between linear and phase currents and voltages is valid:

$I_{L}={\sqrt {3}}\times {I_{F}};\qquad U_{L}=U_{F}$ {\ displaystyle I_ {L} = {\ sqrt {3}} \ times {I_ {F}}; \ qquad U_ {L} = U_ {F}}

Three-phase current power when connected by a triangle

To connect the windings with a triangle, with a symmetrical load, the three-phase current power is:

$P=3U_{F}I_{F}cos\varphi =3U_{L}{\frac {I_{L}}{\sqrt {3}}}cos\varphi ={\sqrt {3}}U_{L}I_{L}cos\varphi$ ${\ displaystyle P = 3U_ {F} I_ {F} cos \ varphi = 3U_ {L} {\ frac {I_ {L}} {\ sqrt {3}}} cos \ varphi = {\ sqrt {3}} U_ {L} I_ {L} cos \ varphi}$

Common Voltage Standards

A country	frequency Hz	Voltage (phase / linear), Volt
Russia	fifty	220/230 ^[2] (domestic networks) 380/660, 400/690, 380, 400, 220/380, 3000, 6000, 10000 (industrial networks)
EU countries	fifty	230/400, 400/690 (industrial networks) 660 450
Japan	50 (60)	120/208
USA	60	120/208, 277/480 240 (triangle only)

Marking

Conductors belonging to different phases are marked with different colors. Different colors also mark the neutral and protective conductors. This is done to ensure proper protection against electric shock, as well as for the convenience of maintenance, installation and repair of electrical installations and electrical equipment - phasing (phase rotation, that is, the sequence of currents flowing in phases) is important, since the direction of rotation of three-phase motors depends on it , the correct operation of controlled three - phase rectifiers and some other devices. In different countries, the marking of conductors has its own differences, however, many countries adhere to the general principles of color marking of conductors set forth in the standard of the International Electrotechnical Commission IEC 60445: 2010.

Three phase double circuit power line

Phase Colors

Each phase in a three-phase system has its own color. It varies by country. The colors of the international standard IEC 60446 ( IEC 60445 ) are used.

A country	L1	L2	L3	Neutral / zero	Earth / protective earth
Russia, Belarus, Ukraine, Kazakhstan (until 2009), China	White	The black	Red	Blue	Yellow / Green (Striped)
The European Union and all countries that use the European standard CENELEC since April 2004 ( IEC 60446 ), Hong Kong since July 2007, Singapore since March 2009, Ukraine, Kazakhstan since 2009, Argentina, Russia since 2009	Brown	The black	Gray	Blue	Yellow / Green (Striped) ^[3]
European Union until April 2004 ^[4]	Red	Yellow	Blue	The black	Yellow / Green (Striped) (green in pre-1970 installations)
India, Pakistan, United Kingdom until April 2006, Hong Kong until April 2009, South Africa, Malaysia, Singapore until February 2011	Red	Yellow	Blue	The black	Yellow / Green (Striped) (green in pre-1970 installations)
Australia and New Zealand	Red (or brown) ^[5]	White or black) (formerly yellow)	Dark blue (or gray)	Black (or blue)	Yellow / Green (Striped) (green in very old installations)
Canada (required) ^[6]	Red	The black	Blue	White or gray	Green or copper colors
Canada (in isolated three-phase installations) ^[7]	Orange	Brown	Yellow	White	Green
USA (alternative practice) ^[8]	Brown	Orange ( triangle system), or purple ( star system)	Yellow	Gray or white	Green
USA (common practice) ^[9]	The black	Red	Blue	White or gray	Green, yellow / green (striped), ^[10] or copper wire
Norway	The black	White gray	Brown	Blue	Yellow / green (striped), in older installations only yellow or copper colors can be found

Notes

↑ GOST 2.709-89 in force in the Russian Federation prescribes the designation of phase conductors of three-phase alternating current: L1, L2, L3, and at the same time allows the designation A, B, C.
↑ According to GOST 29322-2014
↑ The yellow-green marking has been adopted as the international standard for protection against electric shock by color blind people . Between 7% and 10% of people cannot recognize red and green.
↑ There are still many installations in Europe with the old color scheme of the early 1970s. New installations use yellow / green grounding buses in accordance with IEC 60446 . (Phase / zero + earth; Germany: black / gray + red; France green / red + white; Russia: red / gray + black; Switzerland: redd / gray + yellow or yellow and red; Denmark: white / black + red
↑ In Australia and New Zealand, phases can be any color, but not yellow-green, green, yellow, black, or blue.
↑ Canadian Electrical Code Part I, 23rd Edition, (2002) ISBN 1-55324-690-X , rule 4-036 (3)
↑ Canadian Electrical Code 23rd edition of 2002, rules 24-208 (c)
↑ Beginning in 1975 in the USA, the National Electric Code did not have special phase designations. According to established practice, for the connection, the star 120/208 phases were marked in black, red and blue, and when connected, the star or triangle 277/480 phases were indicated by brown, orange and yellow. In a 120/240 system, the triangle with the highest voltage of 208 volts (usually phase B) was always indicated in orange, the overall phase A was black, and phase C was red or blue.
↑ See Paul Cook: Harmonised colors and alphanumeric marking . IEE Wiring Matters, Spring 2006.
↑ In the United States, a yellow-green (striped) wire may indicate an isolated ground ^{[ unknown term ]} . Today, in most countries, yellow-green (striped) wires are used for protective grounding and cannot be disconnected and used for other purposes.

Links

[1] GOST 2.709-89 in force in the Russian Federation prescribes the designation of phase conductors of three-phase alternating current: L1, L2, L3, and at the same time allows the designation A, B, C.

[2] According to GOST 29322-2014

[3] The yellow-green marking has been adopted as the international standard for protection against electric shock by color blind people . Between 7% and 10% of people cannot recognize red and green.

[4] There are still many installations in Europe with the old color scheme of the early 1970s. New installations use yellow / green grounding buses in accordance with IEC 60446 . (Phase / zero + earth; Germany: black / gray + red; France green / red + white; Russia: red / gray + black; Switzerland: redd / gray + yellow or yellow and red; Denmark: white / black + red

[5] In Australia and New Zealand, phases can be any color, but not yellow-green, green, yellow, black, or blue.

[6] Canadian Electrical Code Part I, 23rd Edition, (2002) ISBN 1-55324-690-X , rule 4-036 (3)

[7] Canadian Electrical Code 23rd edition of 2002, rules 24-208 (c)

[8] Beginning in 1975 in the USA, the National Electric Code did not have special phase designations. According to established practice, for the connection, the star 120/208 phases were marked in black, red and blue, and when connected, the star or triangle 277/480 phases were indicated by brown, orange and yellow. In a 120/240 system, the triangle with the highest voltage of 208 volts (usually phase B) was always indicated in orange, the overall phase A was black, and phase C was red or blue.

[9] See Paul Cook: Harmonised colors and alphanumeric marking . IEE Wiring Matters, Spring 2006.

[10] In the United States, a yellow-green (striped) wire may indicate an isolated ground ^{[ unknown term ]} . Today, in most countries, yellow-green (striped) wires are used for protective grounding and cannot be disconnected and used for other purposes.