How to find currents in a three-phase circuit. Equipment phasing - basic concepts and definitions

A three-phase circuit consists of three main elements: a three-phase generator, a transmission line with all the necessary equipment, and receivers (consumers). The voltage between the line wire and the neutral (Ua, Ub, Uc) is called phase. The voltage between two line wires (UAB, UBC, UCA) 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:

14. Symmetrical and asymmetrical receivers in three-phase circuits, vector diagrams.

Vector diagram when connecting the receiver with a star in case of symmetrical load .

15. Current in the neutral wire in three-phase circuits. Neutral (zero working) wire - the wire, connecting the neutrals of electrical installations to each other in three-phase electrical networks. When connecting windings generator and power receiver according to the “star” scheme, phase voltage depends on the load connected to each phase. If, for example, a three-phase motor is connected, the load will be symmetrical and the voltage between the neutral points of the generator and the motor will be zero. However, if a different load is connected to each phase, the so-called neutral bias voltage, which will cause load voltage unbalance. In practice, this can lead to the fact that some consumers will have low voltage, and some will have high voltage. Undervoltage leads to incorrect operation of connected electrical installations, and overvoltage can, in addition, lead to damage to electrical equipment or the occurrence of fire. Connecting the neutral points of the generator and the power receiver with a neutral wire allows you to reduce the neutral bias voltage to almost zero and equalize the phase voltages at the power receiver. A little tension will be caused only resistance neutral wire.

15 Question Current in the neutral wire in three-phase circuits.

Three-phase circuits with a neutral wire are called four-wire circuits.

Usually the resistance of the wires is not taken into account /

Then phase e.g. receiver will be equal to phase. generator voltage. .

Given that the complex resistances are equal, the currents are determined

In accordance with 1 order Kirgoff current in neutr. wire

When symmet. eg

When carrying eg

The neutral wire equalizes the phase voltages.

16 Operating modes of a three-phase successor.

There are two types of connections: star and triangle. In turn, when connected in a star, the system can be three- or four-wire.

Star connection

In Fig. Figure 6 shows a three-phase system when the generator and load phases are connected in a star. Here wires AA', BB' and CC' are linear wires.

Linear called the wire connecting the beginning of the phases of the generator and receiver windings. The point at which the ends of the phases are connected into a common node is called neutral(in Fig. 6 N and N’ are the neutral points of the generator and load, respectively).

The wire connecting the neutral points of the generator and receiver is called neutral(shown in dotted line in Fig. 6). A three-phase system when connected in a star without a neutral wire is called three-wire, with neutral wire – four-wire.

All quantities related to phases are called phase variables, to the line - linear. As can be seen from the diagram in Fig. 6, when connected in a star, the linear currents and are equal to the corresponding phase currents. If there is a neutral wire, the current in the neutral wire . If the system of phase currents is symmetrical, then. Consequently, if the symmetry of the currents were guaranteed, then the neutral wire would not be needed. As will be shown below, the neutral wire ensures the maintenance of symmetry of voltages across the load when the load itself is unbalanced.

Since the voltage at the source is opposite to the direction of its EMF, the phase voltages of the generator (see Fig. 6) act from points A, B and C to the neutral point N; - phase load voltages.

Line voltages act between line wires. In accordance with Kirchhoff's second law for linear voltages, we can write

;

Note that it is always the sum of voltages along a closed circuit.

In Fig. Figure 7 shows a vector diagram for a symmetrical voltage system. As its analysis shows (the rays of phase voltages form the sides of isosceles triangles with angles at the base equal to 300), in this case

Usually in calculations it is taken . Then for the case direct phase rotation, (at reverse phase rotation phase shifts y and change places). Taking this into account, based on relations (1) ... (3), complexes of linear voltages can be determined. However, with voltage symmetry, these quantities are easily determined directly from the vector diagram in Fig. 7. Directing the real axis of the coordinate system along the vector (its initial phase is zero), we count the phase shifts of linear voltages with respect to this axis, and determine their modules in accordance with (4). So for linear voltages we get: ; .

Triangle connection

Due to the fact that a significant part of the receivers included in three-phase circuits are asymmetrical, it is very important in practice, for example, in circuits with lighting devices, to ensure the independence of the operating modes of individual phases. In addition to the four-wire circuit, three-wire circuits also have similar properties when the receiver phases are connected in a triangle. But the generator phases can also be connected into a triangle (see Fig. 8).

For a symmetric EMF system we have

Thus, in the absence of load in the generator phases in the circuit in Fig. 8 currents will be zero. However, if you swap the beginning and end of any of the phases, then a short circuit current will flow in the triangle. Therefore, for a triangle, the order of connecting the phases must be strictly observed: the beginning of one phase is connected to the end of another.

The diagram for connecting the phases of the generator and receiver into a triangle is shown in Fig. 9.

It is obvious that when connected in a triangle, the line voltages are equal to the corresponding phase voltages. According to Kirchhoff's first law, the connection between the linear and phase currents of the receiver is determined by the relations

Similarly, line currents can be expressed through the phase currents of the generator.

In Fig. Figure 10 shows a vector diagram of a symmetrical system of linear and phase currents. Its analysis shows that with current symmetry

In conclusion, we note that in addition to the considered star-star and delta-delta connections, star-delta and delta-star circuits are also used in practice.

Our gardening partnership installed a three-phase electric meter with a current transformer. The meter was new with all seals. However, when the load is completely switched off, the meter disk rotates slowly, that is, the meter is “self-propelled”. It is clear that the partnership did not want to pay for the energy recorded by the meter, which it did not actually use.

At first they decided that the meter was faulty. The meters were replaced several times, but the self-propelled gun remained. As a result, we came to a different conclusion - the meter is not to blame. We began to think what causes such a “self-propelled movement”? The factory instructions attached to the three-phase meter state: it is necessary to connect the meter to the network, observing the phase rotation sequence, so that phase A of the network is connected to the first terminal of the meter, phase B to the second, and phase C to the third terminal of the meter.

The phase sequence can be easily established using a phase indicator. There is always one at power plants, in electrical facilities of large factories, but where would it be in gardening societies? Our attempt to rent a phase indicator for a couple of days from a large institution was unsuccessful. We had to make our own “Device for determining the phase sequence”, with the help of which it was possible to determine this correct sequence. As a result, after eliminating the violation of the sequence of phase alternation, the “self-propelled” meter disappeared. Therefore, there was no longer any need to pay for energy unused by gardeners.

Device for determining the phase sequence in a three-phase network

So, the above-mentioned “Device for determining the phase sequence” is designed to determine the phase in which the voltage lags behind the voltage in the phase arbitrarily taken as the starting point. Knowledge of this lag is necessary for the correct connection to the network of devices in which the phase sequence must be observed, for example, three-phase four-wire (with zero) electricity meters.

The design of the device is quite simple (Fig. 1). On a base made of electrical insulating material, such as textolite, there are two wall-mounted electric sockets with conventional incandescent lighting lamps screwed into them, covered with transparent casings made from plastic containers for juices, water, etc. A capacitor and terminals for connecting wires are also fixed on the base.

Some terminals from the lamps and the capacitor are soldered (point O), the other ends of the wires are connected to terminals A, B and C (Fig. 2).

The principle of operation of the “Device for determining the phase sequence” is as follows. When connecting the “Device...” to a three-phase network, due to the presence of a capacitor in each phase, the voltage changes, which leads to different incandescence of the lamps. (In our case, phase B is connected to the capacitor.) By the amount of incandescence (brightness of the lamps) it is judged whether the remaining phases (wires) belong to phase A or phase C.

Federal Agency for Education State Educational Institution of Higher Professional Education "Ural State Technical University - UPI"

Electrical engineering: Three-phase electrical circuits

Tutorial

V.S. Proskuryakov, S.V. Sobolev, N.V. Khrulkova Department of Electrical Engineering and Electrotechnological Systems

Ekaterinburg 2007

1. Basic concepts and definitions

2. Obtaining a three-phase EMF system.

3. Methods for connecting phases in a three-phase circuit.

4. Three-phase source voltages.

5. Classification of receivers in a three-phase circuit.

6. Calculation of a three-phase circuit when connecting the receiver phases with a “Star”

7. Neutral wire value

8. Calculation of a three-phase circuit when connecting the phases of the receiver with a “triangle”

9. Three-phase circuit power

Three-phase electrical circuits.

1. Basic concepts and definitions

A three-phase circuit is a combination of three electrical circuits in which

source of energy.

Each individual circuit included in a three-phase circuit is usually called a phase.

Thus, the term “phase” has two meanings in electrical engineering: the first is the argument of a sinusoidally varying quantity, the second is part of a multiphase system of electrical circuits.

A three-phase circuit is a special case of multiphase AC systems.

The wide distribution of three-phase circuits is explained by a number of their advantages compared to both single-phase and other multiphase circuits:

cost-effectiveness of energy production and transmission compared to single-phase circuits;

the possibility of relatively simply obtaining a circular rotating magnetic field necessary for a three-phase asynchronous motor;

the ability to obtain two operating voltages in one installation - phase and linear.

Each phase of a three-phase circuit has a standard name:

first phase – phase “A”; second phase – phase “B”; the third phase is phase “C”.

The beginnings and ends of each phase also have standard notations. The beginnings of the first, second and third phases are designated A, B, C, respectively, and the ends of the phases are designated X, Y, Z.

The main elements of a three-phase circuit are: a three-phase generator that converts mechanical energy into electrical energy; power lines; receivers (consumers), which can be either three-phase (for example, three-phase asynchronous motors) or single-phase (for example, incandescent lamps).

2. Obtaining a three-phase EMF system.

A three-phase generator simultaneously creates three EMFs, equal in magnitude and differing in phase by 1200.

The generation of a three-phase EMF system is based on the principle of electromagnetic induction used in a three-phase generator. A three-phase generator is a synchronous electrical machine. The simplest design of such a generator is shown in Fig. 3.1.

Rice. 3.1. Three-phase generator device diagram

A three-phase winding 2 is placed on the generator stator 1. Each phase of the three-phase stator winding is a combination of several coils with a certain number of turns located in the stator slots. In Fig. In Fig. 3.1, each phase is conventionally depicted as one turn. The three phases of the generator stator winding are rotated in space relative to each other by 1/3 of the circle, i.e. magnetic phase axes are rotated in space by an angle

2 3 π = 120°. The beginnings of the phases are designated by the letters A, B and C, and the ends by X, Y, Z.

Rotor 3 of the generator is a permanent electromagnet, excited by the direct current of field winding 4. The rotor creates a constant magnetic field, the lines of force of which are shown in dotted lines in Fig. 3.1. When the generator operates, this magnetic field rotates along with the rotor.

When the turbine rotates the rotor at a constant speed, the conductors of the stator winding intersect with the magnetic field lines. In this case, a sinusoidal EMF is induced in each phase.

The magnitude of this EMF is determined by the intensity of the rotor magnetic field and the number of turns in the winding.

The frequency of this EMF is determined by the rotor speed.

Since all phases of the stator winding are the same (have the same number of turns) and interact with the same magnetic field of the rotating rotor, the EMF of all phases have the same amplitude E m and frequency ω.

like the magnetic axes of the phases in

space rotated on

120°, the initial phases of their EMF differ by an angle

Let us take the initial phase of the EMF of phase A equal to zero, that is, ψ еА = 0

eA = Em sin ω t .

The EMF of phase B lags behind the EMF of phase A by

E m sin(ω t − 120) .

eB = Em sin ω t −

The EMF of phase C lags behind the EMF of phase B by another

E m sin(ω t − 240) .

eС = Em sin ω t −

The effective value of the EMF of all phases is the same:

						E m = E

A three-phase symmetrical EMF system can be represented by trigonometric functions, functions of a complex variable, graphs on time diagrams, vectors on vector diagrams.

The analytical representation by trigonometric functions is given in (3.1) – (3.3).

In complex form, the EMFs of the phases are represented by their complex effective values:

		− j 120		− j 2400

EA = Ee	E; E.B.		; EC = Ee

Graphs of instantaneous values of a three-phase symmetrical EMF system on a time diagram are shown in Fig. 3.2. They are three sinusoids, shifted relative to each other by 1/3 of the period.

Rice. 3.2. Graphs of instantaneous values of a three-phase symmetrical EMF system.

In the vector diagram, the phase EMFs are depicted by vectors of the same length, rotated relative to each other at an angle of 120° (Fig. 3.3a).

Rice. 3.3. Vector diagrams of EMF of three-phase symmetrical systems. (a – direct phase sequence; b – reverse phase sequence).

Since the EMF induced in the stator windings have the same amplitudes and are shifted in phase relative to each other by the same angle of 120°, the resulting three-phase EMF system is symmetrical.

It should be noted that the alternation in time of phase EMF depends on the direction of rotation of the generator rotor relative to the three-phase stator winding. When the rotor rotates clockwise, as shown in Fig. 3.1, the resulting symmetrical three-phase EMF system has direct alternation (A – B – C) (Fig. 3.3a). When the rotor rotates counterclockwise, a symmetrical three-phase EMF system is also formed. However, the alternation of phase EMFs will change over time. This alternation is called reverse (A – C – B) (Fig. 3.3b).

The alternation of phase EMF is important to consider when analyzing three-phase circuits and devices. For example, the phase sequence determines the direction of rotation of three-phase motors, etc. To practically determine the phase sequence, special devices are used - phase indicators.

By default, when constructing three-phase circuits and analyzing them, the direct alternation of phase EMFs of a three-phase source is assumed.

In the diagrams, the stator winding of the generator is shown as shown in Fig. 3.4a using the accepted designations for the beginnings and ends of phases.

In the equivalent circuit, a three-phase source is represented by three ideal EMF sources (Fig. 3.4b)

Rice. 3.4. Conventional image of a generator stator winding.

The conditional positive direction of the EMF in each phase is taken to be the direction from the end of the phase to the beginning.

3. Methods of connecting phases in a three-phase circuit.

To build a three-phase circuit, a separate electricity receiver or one phase of a three-phase receiver is connected to each phase of a three-phase source.

Fig. 3.5 Diagram of an unconnected three-phase circuit.

Here the three-phase source is represented by three ideal emf sources E&A, E&B, E&C. The three phases of the receiver are represented as conditionally ideal

elements with total complex resistances Z a, Z b, Z c. Each receiver phase is connected to the corresponding source phase, as shown in Fig. 3.5. In this case, three electrical circuits are formed, structurally united by one three-phase source, i.e. three-phase circuit. In this circuit, the three phases are connected only structurally and have no electrical connection with each other (not electrically connected to each other). Such a circuit is called an unconnected three-phase circuit and is practically not used.

In practice, the three phases of a three-phase circuit are interconnected (electrically connected).

There are various ways to connect the phases of three-phase sources and three-phase consumers of electricity. The most common are star and delta connections. At the same time, the method of connecting source phases and consumer phases in three-phase systems may be different. The source phases are usually connected by a star, the consumer phases are connected either by a star or a delta.

When the phases of a generator (or transformer) winding are connected with a star, their ends X, Y and Z are connected to one common point N, called the neutral point (or neutral) (Fig. 3.6). The ends of the receiver phases x, y, z are also connected to one point n (neutral point of the receiver). This connection is called a star connection.

Rice. 3.6. Scheme of connecting the phases of the source and receiver into a star.

Wires A-a, B-b and C-c connecting the beginning of the phases of the generator and receiver are called line wires (line wire A, line wire B, line wire C). The N-n wire connecting point N of the generator to point n of the receiver is called the neutral wire.

Here, as before, each phase represents an electrical circuit in which the receiver is connected to the corresponding phase of the source through a neutral wire and one of the linear wires (dotted line in Fig. 3.6). However, unlike an unconnected three-phase circuit, the transmission line uses fewer wires. This determines one of the advantages of three-phase circuits - the efficiency of energy transmission.

When connecting the phases of a three-phase power supply with a triangle (Fig. 3.12), the end X of one phase is connected to the beginning B of the second phase, the end Y of the second phase is connected to the beginning C of the third phase, the end of the third phase Z is connected to the beginning of the first phase A. The beginnings of phases A, B and C are connected using three wires to the three phases of the receiver, also connected in a delta manner.

Rice. 3.7. Connection diagram of the source and receiver phases in a triangle

Here, too, each phase represents an electrical circuit in which the receiver is connected to the corresponding phase of the source through two linear wires (dotted line in Fig. 3.7). However, a transmission line uses even fewer wires. This makes power transmission even more economical

With the "triangle" connection method, the phases of the receiver are named by two symbols in accordance with the linear wires to which this phase is connected: phase "ab", phase "bc", phase "ca". Phase parameters indicate

corresponding indices: Z ab, Z bc, Z ca

A three-phase source connected in a star connection creates two three-phase voltage systems of different magnitudes. In this case, a distinction is made between phase voltages and linear voltages.

Figure 3.8 shows the equivalent circuit of a three-phase star-connected source connected to a power line.

Fig.3.8. Three-phase source equivalent circuit

Phase voltage U Ф - the voltage between the beginning and end of a phase or between a line wire and a neutral (U & A, U & B, U & C). For probation

positive directions of phase voltages take directions from the beginning to the end of the phases.

Line voltage (U L) - the voltage between linear wires or between the beginnings of phases (U & AB, U & BC, U & CA). Conditionally positive

the directions of linear voltages are taken from points corresponding to the first index, to points corresponding to the second index (that is, from points with a higher potential to points with a lower one) (Fig. 3.8).

A story about the installation of electrical equipment, namely two oil transformers, caught my eye. The work was completed successfully. As a result, there was the following power supply scheme. Actually the transformers themselves, input switches, sectional disconnectors, two bus sections. According to the installers, the commissioning work was completed successfully. We started switching on both transformers for parallel operation and got it. Naturally, the installers claimed that they had checked the phase rotation from both sources and everything matched. But not a word was said about phasing. But in vain! Now let's take a closer look at what went wrong.

What is phase alternation?

As you know, in a three-phase network there are three opposite phases. Conventionally, they are designated as A, B and C. Remembering the theory, we can say that the phase sinusoids are shifted relative to each other by 120 degrees. So, there can be six different alternation orders in total, and they are all divided into two types - direct and reverse. The following order is considered direct alternation - ABC, BCA and CAB. The reverse order will be CBA, BAC and DIA, respectively.

To check the order of phase alternation, you can use a device such as a phase indicator. We have already talked about that. Let's look specifically at the sequence of checking with the FU 2 device.

How to check?

The device itself (shown in the photo below) consists of three windings and a disk that rotates during testing. It has black marks that alternate with white ones. This is done for ease of reading the result. The device operates on the principle of an asynchronous motor.

So, we connect three wires from a three-phase voltage source to the device terminals. Press the button on the device, which is located on the side wall. We will see that the disk begins to rotate. If it rotates in the direction of the arrow drawn on the device, it means that the phase sequence is direct and corresponds to one of the order options ABC, BCA or CAB. When the disk rotates in the opposite direction of the arrow, we can talk about reverse alternation. In this case, one of these three options is possible - CBA, BAC or DIA.

If we return to the story with the installers, then all they did was just determine the sequence of phases. Yes, in both cases the order was the same. However, it was still necessary to check the phasing. And it cannot be done using a phase indicator. When turned on, opposite phases were connected. To find out where A, B and C are conditionally, you had to use a multimeter or.

A multimeter measures the voltage between the power phases and if it is zero, then the phases are the same. If the voltage corresponds to the linear voltage, then they are different. This is the simplest and most effective way. You can find out more about this in our article. You can, of course, use an oscilloscope and look at the oscillogram which phase lags behind which by 120 degrees, but this is impractical. Firstly, this makes the technique an order of magnitude more complicated, and secondly, such a device costs a lot of money.

The video below clearly shows how to check phase rotation:

When should order be considered?

It is necessary to check the phase rotation when operating three-phase AC motors. The order of the phases will change the direction of rotation of the motor, which is sometimes very important, especially if there are many mechanisms on the site that use motors.

It is also important to take into account the order of the phases when connecting a CA4 induction type electric meter. If the order is reversed, a phenomenon such as spontaneous movement of the disk on the counter is possible. New electronic meters, of course, are insensitive to phase rotation, but a corresponding image will appear on their indicator.

If you have an electrical power cable with which you need to connect a three-phase power supply, and you need phasing control, it can be done without special devices. Often the cores inside the cable differ in the color of the insulation, which greatly simplifies the “dialing” process. So, to find out where phase A, B or C is conditionally located, you only need. At both ends we will see veins of the same color. We will accept them as the same. You can learn more about it from our article.

Device for determining the phase sequence in a three-phase network

Some terminals from the lamps and the capacitor are soldered (point O), the other ends of the wires are connected to terminals A, B and C (Fig. 2).

Hello, dear guests and regular readers of the Electrician's Notes website.

A few days ago, an acquaintance called me asking me to look into the situation.

He had a team of electricians working at his site.

They were installing two 10/0.4 (kV) power oil transformers with a capacity of 400 (kVA). Busbars of sections 1 and 2 of 0.4 (kV) were fed from each transformer. An intersectional circuit breaker was provided between the busbars of sections 1 and 2.

Here is a photo of two sections with a voltage of 400 (V).

During commissioning, we decided to try to switch on both transformers for parallel operation. When turned on, an incident occurred in which the protection was triggered on two input circuit breakers at once.

They began to figure it out. The conditions for switching on transformers for parallel operation were met, but not all. We came to the conclusion that the phasing of the tires of two sections 400 (B) was not observed. The installation team assures that the preliminary phasing was carried out correctly. A little later it turned out that they carried out phasing using an FU-2 phase indicator on each section, and in both cases the device showed a direct sequence of phases.

Phase indicator FU-2

The order of phase rotation (phase sequence) in a three-phase voltage system can be checked using a portable induction phase indicator type FU-2. This is what he looks like.

For example, in the CA4-I678 meter, with the reverse sequence of phases, the disk begins to “self-propel”. In modern electronic meters such as SET-4TM and PSCh-4TM, when the phase sequence is reversed, a notification is displayed on the screen.

P.S. In the following articles we will talk about the correct phasing. Subscribe to the site news so as not to miss new articles.

Often, when servicing electrical equipment, it is necessary to check phase rotation and perform phasing. This is most often used when coordinating the operation of transformers. In our article we will describe the phase rotation in a 3-phase network, the necessary tools and methods for correct phasing.

Introductory story

Let's imagine installing two oil transformers. Electricians carried out successful commissioning of transformers, input switches, busbars and sectional dividers. But when they tried to run the transformers in parallel, a short circuit occurred. The electricians said that they checked the phase rotation and everything was in order. But apparently no one took into account the phasing, which led to such an error. Let's take a closer look at the essence of the problem in this case.

What is phase rotation

A three-phase network has three phases, designated A, B and C. If we recall physics, this means that the sinusoids of the phases are shifted by 120˚ from each other. In total, there are six types of alternation orders, which in turn can be divided into two groups - direct and reverse. Direct alternations look like ABC, BSA and SAV, and reverse ones look like SVA, BAC and ASV. To check the phase rotation, use a device - a phase indicator.

What is needed to check the phases

The phase indicator (see figure below) consists of three windings and a disk, which will rotate during testing. To make it easier to recognize the result, black and white marks are applied to the disc. The FU works in the same way as an asynchronous motor.

If we connect three wires to the terminals, we will see that the disk will begin to rotate. If it rotates clockwise, this means direct phase alternation (ABC, BCA or CAB). If the disk rotates counterclockwise, this means reverse phase alternation (CBA, BAC or ACB).

Let's return to our story with the electricians; they checked the phase rotation, which coincided in one and the other case. It was necessary to perform phasing, and here we could not do without a phase indicator (PI). Electricians connected opposite phases at startup, and in order to find out exactly where A, B and C were, they had to use a multimeter or oscilloscope.

The multimeter device measures the voltage between the phases of different power sources; reaching zero means that the phases are the same. Otherwise, line voltage will mean that the phases are opposite. This method is the fastest and easiest, but you can also use an oscilloscope, which will show which phase lags behind the other by 120˚.

In what cases is the order taken into account?

Checking phase rotation is necessary when using three-phase AC motors. The direction of rotation of the motor depends on the order of the phases; this is a very important condition, especially when several mechanisms use motors.

Another case when it is necessary to pay attention to phase rotation is when working with a CA4 induction type electric meter. When the order is reversed, spontaneous rotation of the disk on the counter sometimes occurs. Modern meters are not so sensitive to phase rotation, but they will also display the corresponding data on the indicator.

Sometimes phasing control can be performed without special instruments. This is if the connection of a three-phase power supply network is carried out using which is possible at the Yugtelekabel company. If the conductors inside the cable differ in color, then the dialing is carried out much faster. Sometimes you just need to remove the outer insulation of the cable to understand which phase is located (A, B or C). If the wires at both ends are the same color, then they are the same.

You should not always rely on color coding; not all manufacturers adhere to such trends; sometimes you can find different colors at different ends of the cable. Therefore, it is better to use the wire ringer.

8.1.Basic concepts and definitions

Electrical equipment of three-phase current (synchronous compensators, transformers, power transmission lines) is subject to mandatory phasing before the first connection to the network, as well as after repairs, during which the order and rotation of phases could be violated.

In general, phasing consists of checking the phase coincidence of the voltage of each of the three phases of the switched-on electrical installation with the corresponding phases of the network voltage.

Phasing involves three significantly different operations. The first of them consists of checking and comparing the order of the phases of the switched-on electrical installation and network. The second operation consists of checking the phase coincidence of voltages of the same name, i.e., the absence of an angular shift between them. Finally, the third operation consists of checking the identity (color) of the phases whose connection is supposed to be performed. The purpose of this operation is to check the correct connection between all elements of the electrical installation, i.e., ultimately, the correct supply of conductive parts to the switching device.

Phase. A three-phase voltage system is understood as a set of three symmetrical voltages, the amplitudes of which are equal in value and shifted (the amplitude of a sinusoid of one voltage relative to the preceding amplitude of a sinusoid of another voltage) by the same phase angle (Fig. 8.1, a).

Thus, the angle that characterizes a certain stage of a periodically changing parameter (in this case, voltage) is called the phase angle or simply phase. When considering two (or more) sinusoidally varying voltages of the same frequency together, if their zero (or amplitude) values do not occur simultaneously, they are said to be out of phase. The shift is always determined between identical phases. Phases are indicated in capital letters A, B, C. Three-phase systems are also represented by rotating vectors (Fig. 8.1, b).

In practice, a phase of a three-phase system is also understood as a separate section of a three-phase circuit through which the same current passes, shifted relative to the other two in phase. Based on this, the winding of a generator, transformer, motor, or three-phase line wire is called a phase in order to emphasize that they belong to a specific section of the three-phase circuit. To recognize equipment phases, colored marks in the form of circles, stripes, etc. are applied to equipment casings, busbars, supports and structures. Elements of equipment belonging to a phase A, painted yellow, phases V-v green and phase C-to red. Accordingly, the phases are often called yellow, green and red: g, h, k.

Thus, depending on the issue under consideration, a phase is either an angle characterizing the state of a sinusoidally varying quantity at each moment of time, or a section of a three-phase circuit, i.e., a single-phase circuit that is part of a three-phase circuit.

The order of the phases. Three-phase voltage and current systems may differ from each other in the order of the phases. If the phases (eg mains) follow each other in order A, B, C - this is the so-called direct phase order (see § 7.3). If the phases follow each other in order A, C, B - This is the reverse order of the phases.

The order of the phases is checked with an induction phase indicator of type I-517 or a phase indicator of the FU-2 type with a similar design. The phase indicator is connected to the voltage system being tested. The terminals of the device are marked, i.e. indicated by letters A,V, S. If the phases of the network coincide with the markings of the device, the phase indicator disk will rotate in the direction indicated by the arrow on the device casing. This rotation of the disk corresponds to the direct order of the network phases. Rotating the disk in the opposite direction indicates the reverse order of the phases. Obtaining the direct order of phases from the reverse is done by changing the positions of any two phases of the electrical installation.

Sometimes instead of the term “phase sequence” they say “phase sequence”. To avoid confusion, we agree to use the term “phase rotation” only when it is related to the concept of a phase as a section of a three-phase circuit.

Phase rotation. So, by phase alternation we should understand the order in which the phases of a three-phase circuit (windings and terminals of electrical machines, line wires, etc.) are located in space, if you start bypassing them each time from the same point (point) and carry out in the same direction, for example, from top to bottom, clockwise, etc. Based on this definition, they talk about alternating designations for the terminals of electrical machines and transformers, the colors of wires and busbars.

Phase coincidence. When phasing three-phase circuits, there are various options for alternating the designations of inputs on the switching device and supplying voltages of different phases to these inputs (Fig. 8.2, a, b). Options in which the order of phases does not match, or the order of alternation of phases of the electrical installation and the network, when the switch is turned on, leads to a short circuit.

At the same time, the only possible option is when both coincide. A short circuit between the connected parts (electrical installation and network) is excluded here.

By phase coincidence during phasing, this is precisely the option understood, when the same voltages are supplied to the switch inputs, belonging in pairs to the same phase, and the designations (colors) of the switch inputs are consistent with the designation of the voltage phases (Fig. 8.2, c).