A transformer is a device that converts voltage alternating current(increase or decrease). The transformer consists of several windings (two or more), which are wound on a common ferromagnetic core. If the transformer consists of only one winding, then it is called an autotransformer. Modern current transformers are: rod, armor or toroidal. All three types of transformers have similar characteristics and reliability, but differ from each other in the way they are manufactured.

In rod-type transformers, the winding is wound on the core, and in rod-type transformers, the winding is included in the core. In a rod-type transformer, the windings are clearly visible, and only the lower and upper parts are visible from the core. The core of the armored transformer hides almost the entire winding. The windings of a rod-type transformer are arranged horizontally, while this arrangement in an armored transformer can be either vertical or horizontal.

Regardless of the type of transformer, it includes the following three functional parts: the magnetic system of the transformer (magnetic core), windings, and the cooling system.

The principle of operation of the transformer

In a transformer, it is customary to distinguish between primary and secondary windings. Voltage is supplied to the primary winding, and the voltage is removed from the secondary. The operation of the transformer is based on Faraday's law (the law of electromagnetic induction): a time-varying magnetic flux through an area bounded by a circuit creates an electromotive force. The converse is also true: a changing electric current induces a changing magnetic field.

There are two windings in a transformer: primary and secondary. The primary winding receives power from an external source, and the voltage is removed from the secondary winding. The alternating current in the primary winding creates an alternating magnetic field in the magnetic circuit, which, in turn, creates a current in the secondary winding.

Transformer operating modes

There are three modes of operation of the transformer: idling, short circuit mode, working mode. The transformer is "idling", when the outputs from the secondary windings are not connected anywhere. If the transformer core is made of a soft magnetic material, then the no-load current shows what losses occur in the transformer due to remagnetization of the core and eddy currents.

In the short circuit mode, the outputs of the secondary winding are short-circuited, and a small voltage is applied to the primary winding, so that the short circuit current is equal to the rated current of the transformer. The value of losses (power) can be calculated if the voltage in the secondary winding is multiplied by the short-circuit current. Such a transformer mode finds its technical application in instrument transformers.

If a load is connected to the secondary winding, then a current arises in it, inducing a magnetic flux directed opposite to the magnetic flux in the primary winding. Now, in the primary winding, the EMF of the power supply and the EMF of the supply induction are not equal, so the current in the primary winding increases until the magnetic flux reaches its previous value.

For a transformer in active load mode, the equality is true:
U_2/U_1=N_2/N_1, where U2, U1 are the instantaneous voltages at the ends of the secondary and primary windings, and N1, N2 are the number of turns in the primary and secondary windings. If U2 > U1, the transformer is called step-up, otherwise we have a step-down transformer. Any transformer is usually characterized by the number k, where k is the transformation ratio.

Types of transformers

There are several types of transformers depending on their application and characteristics. For example, in electrical networks of settlements, industrial enterprises, power transformers are used, the main task of which is to lower the voltage in the network to the generally accepted one - 220 V.

If the transformer is designed to regulate current, it is called a current transformer, and if the device regulates voltage, then it is a voltage transformer. In conventional networks, single-phase transformers are used; in networks with three wires (phase, zero, ground), a three-phase transformer is needed.

Household transformer, 220V is intended for protection household appliances from voltage fluctuations.

The welding transformer is designed to separate the welding and power networks, to lower the voltage in the network to the value required for welding.

The oil transformer is intended for use in networks with voltages above 6,000 Volts. The design of the transformer includes: a magnetic circuit, windings, a tank, as well as covers with inputs. The magnetic circuit consists of 2 sheets of electrical steel, which are isolated from each other, the windings are usually made of aluminum or copper wire. Voltage regulation is performed using a branch that is connected to the switch.

There are two types of branch switching: switching under load - OLTC (regulation under load), as well as without load, after the transformer is disconnected from the external network (PBV, or switching without excitation). The second method of voltage regulation has become more widespread.

Speaking about the types of transformers, it is impossible not to talk about the electronic transformer. Electronic transformer is a specialized power source that is used to convert voltage 220V to 12 (24)V, at high power. An electronic transformer is much smaller than a conventional one, with the same load parameters.

Ideal transformer equations

In order to calculate the main characteristics of transformers, it is customary to use simple equations that every modern student knows. To do this, use the concept of an ideal transformer. An ideal transformer is such a transformer in which there is no energy loss for heating the windings and eddy currents. In an ideal transformer, the energy of the primary circuit is converted completely into the energy of the magnetic field, and then into the energy of the secondary winding. That is why we can write:
P1=I1*U1=P2=I2*U2,
where P1, P2 are the electric current powers in the primary and secondary windings, respectively.

Transformer core

The magnetic circuit is a plate of electrical steel, which concentrate the magnetic field of the transformer. A fully assembled system with parts holding the transformer together is the core of the transformer. The part of the magnetic circuit on which the windings are attached is called the core of the transformer. The part of the magnetic circuit that does not carry the winding and closes the magnetic circuit is called the yoke.

In a transformer, the rods can be arranged in different ways, therefore, there are four types of magnetic circuits (magnetic systems): a flat magnetic system, a spatial magnetic system, a symmetrical magnetic system, and an asymmetric magnetic system.

Transformer winding

Now let's talk about the winding of the transformer. The main part of the winding is a coil, which wraps around the magnetic circuit once and in which a magnetic field is induced. Under the winding understand the sum of the turns, the EMF of the entire winding is equal to the sum of the EMF in each turn.

In power transformers, the winding usually consists of conductors having a square section. Such a conductor is also called residential in another way. The square conductor is used to make more efficient use of the space inside the core. Either paper or enamel varnish can be used as insulation for each core. Two cores can be interconnected, and have one insulation - this design is called a cable.

Windings are of the following types: main, regulating and auxiliary. The main winding is called, to which current is supplied or from which current is removed (primary and secondary windings). A winding with leads for regulating the voltage transformation ratio is called a regulating winding.

Application of transformers

From the school physics course, it is known that power losses in wires are directly proportional to the square of the current strength. Therefore, to transmit current over long distances, the voltage is increased, and before being supplied to the consumer, on the contrary, it is lowered. In the first case, step-up transformers are needed, and in the second, step-down ones. This is the main application of transformers.

Transformers are also used in power circuits for household appliances. For example, televisions use transformers with several windings (to power circuits, transistors, a kinescope, etc.).

  1. The transformer insulation is based on matrixless vacuum impregnation and operates in an environment with high air humidity and in a chemically aggressive atmosphere.
  2. Minimum release of combustion energy (for example, 43 kg for a 1600 kVA transformer corresponds to 1.1% of the weight). Other insulating materials are practically non-combustible, self-extinguishing and do not contain any toxic additives.
  3. Contamination resistance of the transformer due to self-cleaning convection winding discs.
  4. Long creepage on the surface of the winding disks, which create the effect of insulating barriers.
  5. Transformer resistance to thermal shock even at extremely low temperatures (-50°C).
  6. Ceramic spacer blocks (non-flammable) between winding discs.
  7. Insulation of conductors glass-silk.
  8. Safe operation of the transformer due to the special winding structure The effect of voltage on the insulation never exceeds the insulation voltage (max. 10 V). Partial discharges in insulation are physically impossible.
  9. Cooling of the transformer is provided by vertical and horizontal cooling channels, and the minimum thickness of the insulation makes it possible to operate the transformer with large short-term overloads in a protective IP 45 enclosure without forced cooling.
  10. The insulating cylinder is made of practically non-combustible and self-extinguishing material, reinforced with fiberglass.
  11. Low voltage winding from standard wire or foil; copper is used as the winding material.
  12. The dynamic resistance of the transformer to short circuits is provided by ceramic insulators.


transformers- electromagnetic static converters of electrical energy.Transformers are called electromagnetic devices that serve to convert alternating current of one voltage into alternating current of another voltage at the same frequency and to transfer electrical energy by electromagnetic means from one circuit to another.

The main purpose of transformers- change the AC voltage. Transformers are also used to convert the number of phases and frequency.

Current transformers are called devices designed to convert current of any value into a current that can be measured by normal devices, as well as to power various relays and windings of electromagnets. The number of turns of the secondary winding of the current transformer is w2 > w1.

A feature of current transformers is their operation in a mode close to a short circuit, since their secondary winding is always closed to a small resistance.

Voltage transformers called devices designed to convert high voltage alternating current into low voltage alternating current and supply parallel coils measuring instruments and relay. The principle of operation and device of voltage transformers is similar to the principle of operation of power transformers. Number of turns of the secondary winding w2< w1, так как все измерительные трансформаторы напряжения – понижающего типа.

A feature of the operation of a voltage measuring transformer is that its secondary winding is always closed to high resistance, and the transformer operates in a mode close to the idle mode, since the connected devices consume a small current.

The most widespread are power voltage transformers, which are produced by the electrical industry at a power of over a million kilovolt-amperes and for voltages up to 1150 - 1500 kV.

For the transmission and distribution of electrical energy, it is necessary to increase the voltage of turbogenerators and hydrogenerators installed at power plants from 16 - 24 kV to voltages of 110, 150, 220, 330, 500, 750 and 1150 kV used in transmission lines, and then lower it again to 35 ; ten; 6; 3; 0.66; 0.38 and 0.22 kV to use energy in industry, agriculture and everyday life.

Since multiple transformations take place in energy systems, the power of transformers is 7-10 times higher than the installed power of generators at power plants.

Power transformers are produced mainly at a frequency of 50 Hz.

Low power transformers are widely used in various electrical installations, information transmission and processing systems, navigation and other devices. The frequency range at which transformers can operate is from a few hertz to 105 Hz.

According to the number of phases, transformers are divided into single-phase, two-phase, three-phase and multi-phase. Power transformers are produced mainly in a three-phase version. For use in single-phase networks are available.

Classification of transformers according to the number and connection schemes of the windings

Transformers have two or more windings inductively coupled to each other. Windings that consume energy from the network are called primary. Windings that supply electrical energy to the consumer are called secondary.

Polyphase transformers have windings connected in a multi-beam star or polygon. Three-phase transformers have a connection in a three-beam star and a delta.

Step-up and step-down transformers

Depending on the ratio of voltages on the primary and secondary windings, transformers are divided into step-up and step-down. AT step-up transformer primary winding has a low voltage, and the secondary has a high voltage. AT step down transformer On the contrary, the secondary winding has a low voltage, and the primary has a high voltage.

Transformers with one primary and one secondary winding are called two-winding. Fairly widespread three winding transformers having three windings for each phase, for example two on the low voltage side, one on the high voltage side, or vice versa. Polyphase transformers may have multiple high and low voltage windings.

Classification of transformers by design

By design, power transformers are divided into two main types - oil and dry.

AT oil transformers The magnetic circuit with windings is placed in a tank filled with transformer oil, which is a good insulator and coolant.

In accordance with regulatory documents, the design features of the transformer are reflected in the designation of its type and cooling systems.

Transformer type:

  • Autotransformer (for single-phase O, for three-phase T) - A
  • Split low voltage winding - P
  • Protection of the liquid dielectric using a nitrogen blanket without an expander - Z
  • Cast resin version - L
  • Three winding transformer - T
  • Transformer with tap changer - N
  • Dry transformer with natural air cooling (usually the second letter in the type designation), or version for the own needs of power plants (usually the last letter in the type designation) - C
  • Cable entry - K
  • Flange input (for complete TS) - F

Cooling systems for dry transformers:

  • Natural air with open design - C
  • Natural air with protected design - SZ
  • Natural air with hermetic design - SG
  • Air with forced air circulation - SD

Cooling systems for oil transformers:

  • Natural air and oil circulation - M
  • Forced air circulation and natural oil circulation - D
  • Natural air circulation and forced oil circulation with non-directional oil flow - MC
  • Natural air circulation and forced oil circulation with directional oil flow - NMC
  • Forced air and oil circulation with non-directional oil flow - DC
  • Forced air and oil circulation with directional oil flow - NDC
  • Forced circulation of water and oil with non-directional oil flow - C
  • Forced circulation of water and oil with directional oil flow - NC

Cooling systems for transformers with non-flammable liquid dielectric:

  • Liquid dielectric cooling with forced air circulation - ND
  • Cooling by non-combustible liquid dielectric with forced air circulation and directed flow of liquid dielectric - NND

It is difficult for a person who is not familiar with electrics to imagine what a transformer is, where it is involved, the purpose of its design elements.

General information about the device

A transformer is a static electromagnetic device designed to convert a variable frequency current with one voltage into an alternating current with a different voltage, but with the same frequency, based on the phenomenon of electromagnetic induction.

Devices are used in all spheres of human activity: electric power industry, radio engineering, radio-electronic industry, household sphere.

Design

The device of the transformer assumes the presence of one or more individual coils (tape or wire) under a single magnetic flux, wound on a core made of a ferromagnet.

The most important structural parts are as follows:

  • winding;
  • frame;
  • magnetic circuit (core);
  • cooling system;
  • insulation system;
  • additional parts necessary for protective purposes, for installation, providing access to the output parts.

In devices, you can most often see two types of winding: the primary, which receives electric current from an external supply source, and the secondary, from which the voltage is removed.

The core provides improved reverse contact of the windings, has a reduced resistance to magnetic flux.

Some types of devices operating at ultra-high and high frequency are produced without a core.

The production of devices is established in three basic winding concepts:

  • armored;
  • toroidal;
  • rod.

The device of rod transformers implies the winding of the winding on the core is strictly horizontal. In armored devices, it is enclosed in a magnetic circuit, placed horizontally or vertically.

Reliability, performance, design and principle of operation of the transformer are accepted without any influence of the principle of its manufacture.

Principle of operation

The principle of operation of the transformer is based on the effect of mutual induction. The flow of a variable frequency current from a third-party power supplier to the inputs of the primary winding forms a magnetic field in the core with a variable flux passing through the secondary winding and inducing the formation of an electromotive force in it. Shorting on the secondary winding on the power receiver causes the passage of electric current through the receiver due to the influence of the electromotive force, at the same time, a load current is formed in the primary winding.

The purpose of the transformer is to move the converted electrical energy (without changing its frequency) to the secondary winding from the primary with a voltage suitable for the operation of consumers.

Classification by type

Power

An alternating current power transformer is a device used to transform electricity in supply networks and electrical installations of significant power.

The need for power plants is explained by a serious difference in the operating voltages of main power lines and urban networks that come to end consumers, which are required for the operation of machines and mechanisms powered by electricity.

Autotransformers

The device and principle of operation of the transformer in this design implies a direct connection of the primary and secondary windings, due to this, their electromagnetic and electrical contact. The windings of the devices have at least three leads, which differ in their voltage.

The main advantage of these devices should be called good efficiency, because not all power is converted - this is significant for small differences in input and output voltages. Minus - the non-isolation of the transformer circuits (lack of separation) among themselves.

Current transformers

This term is used to denote a device powered directly from an electricity supplier, used to reduce the primary electric current to suitable values ​​for those used in measuring and protective circuits, signaling, communications.

The primary winding of current transformers, the device of which provides for the absence of galvanic connections, is connected to a circuit with an alternating electric current to be determined, and electrical measuring instruments are connected to the secondary winding. The electric current flowing through it approximately corresponds to the current of the primary winding, divided by the transformation ratio.

Voltage transformers

The purpose of these devices is to reduce the voltage in measuring circuits, automation and relay protection. Such protective and electrical measuring circuits in devices for various purposes isolated from high voltage circuits.

Pulse

These types of transformers are needed to change short-term video pulses, which, as a rule, have a repetition in certain period with a significant duty cycle, with a change in their shape reduced to a minimum. The purpose of use is the transfer of an orthogonal electrical impulse with the steepest cutoff and front, a constant amplitude indicator.

The main requirement for devices of this type, is the absence of distortion when transferring the shape of the converted voltage pulses. The action of a voltage of some form on the input causes a voltage pulse of an identical shape to be obtained at the output, but probably with a different range or reversed polarity.

Dividing

What is an isolation transformer becomes clear from the definition itself - this is a device with a primary winding that is not electrically connected (i.e. separated) from the secondary.

There are two types of such devices:

  • power;
  • signal.

Power ones are used to improve the reliability of power networks in case of an unexpected synchronous connection with the ground and current-carrying parts, or non-current-carrying elements that have become energized due to insulation failure.

Signaling are used to ensure galvanic isolation electrical circuits.

Matching

How a transformer of this type works is also clear from its name. Matching devices are called devices that are used to match the resistance of individual elements of electrical circuits with a minimized change in the waveform. Also, devices of this type are used to exclude galvanic interactions between individual parts of the circuits.

peak transformers

The principle of operation of peak transformers is based on the transformation of the nature of the voltage, from the input sinusoidal to pulsed. The polarity after the transition changes after half a period.

dual choke

Its purpose, device and principle of operation, as a transformer, are absolutely identical to devices with a pair of similar windings, which, in this case, are absolutely the same, wound oppositely or in concert.

It is also common to see this name this device, like a counter inductive filter. This indicates the scope of the device - input voltage filtering in power supplies, audio equipment, digital devices.

Operating modes

Idling (XX)

This order of operation is implemented from the opening of the secondary network, after which the flow of electric current in it stops. An idle current flows in the primary winding, its constituent element is the magnetizing current.

When the secondary current is zero, the electromotive force of induction in the primary winding completely compensates for the voltage of the supply source, and therefore, if the load currents are lost, the current flowing through the primary winding corresponds in value to the magnetizing current.

The functional purpose of the idle operation of transformers is to determine their most important parameters:

  • transformation indicator;
  • losses in the magnetic circuit.

Load mode

The mode is characterized by the functioning of the device when voltage is applied to the inputs of the primary circuit and the load is connected in the secondary. The loading current goes through the "secondary", and in the primary - the total load current and the idle current. This mode of operation is considered to be predominant for the device.

The basic law of induction emf answers the question of how the transformer works in the main mode. The principle is as follows: applying a load to the secondary winding causes the formation of a magnetic flux in the secondary circuit, which forms a loading electric current in the core. It is directed in the direction opposite to its flow, created by the primary winding. In the primary circuit, the parity of the electromotive forces of the electricity supplier and induction is not observed; in the primary winding, the electric current is increased until the magnetic flux returns to its original value.

Short circuit (short circuit)

The transition of the device to this mode is carried out with a short circuit of the secondary circuit. Short circuit - a special type of load, the applied load - the resistance of the secondary winding - is the only one.

The principle of operation of the transformer in short-circuit mode is as follows: an insignificant AC voltage, the conclusions of the secondary are short-circuited. The input voltage is set in such a way that the value of the closing current corresponds to the value of the rated electric current of the device. The voltage value determines the energy losses attributable to the heating of the windings, as well as to the active resistance.

This mode is typical for measuring instruments.

Based on the variety of devices and types of purpose of transformers, it is safe to say that today they are indispensable devices used almost everywhere, which ensure stability and achieve the required voltage values ​​for the consumer, both civil networks and industrial networks.

Maybe someone thinks that a transformer is something between a transformer and a terminator. This article is intended to destroy such ideas.

A transformer is a static electromagnetic device designed to convert an alternating electric current of one voltage and a certain frequency into an electric current of another voltage and the same frequency.

The operation of any transformer is based on a phenomenon discovered by Faraday.

Purpose of transformers

Different types of transformers are used in almost all power supply circuits for electrical appliances and in the transmission of electricity over long distances.

Power plants generate a current of relatively low voltage - 220 , 380 , 660 B. Transformers, increasing the voltage to values ​​​​of the order thousand kilovolts, can significantly reduce losses in the transmission of electricity over long distances, and at the same time reduce the cross-sectional area of ​​power transmission lines.

Just before getting to the consumer (for example, to a regular home outlet), the current passes through a step-down transformer. This is how we get our usual 220 Volt.

The most common type of transformer is power transformers . They are designed to convert voltage in electrical circuits. In addition to power transformers, various electronic devices use:

  • pulse transformers;
  • power transformers;
  • current transformers.

The principle of operation of the transformer

Transformers are single-phase and multi-phase, with one, two or more windings. Consider the scheme and principle of operation of the transformer using the example of the simplest single-phase transformer.

What is a transformer made of? In the simplest case, from one metal core and two windings . The windings are not electrically connected to each other and are insulated wires.

One winding (it is called primary ) is connected to an AC power source. The second winding is called secondary , is connected to the final consumer of current.


When a transformer is connected to an AC source, an alternating current flows in the turns of its primary winding. I1 . This creates a magnetic flux F , which permeates both windings and induces an emf in them.

It happens that the secondary winding is not under load. This mode of operation of the transformer is called idle mode. Accordingly, if the secondary winding is connected to any consumer, a current flows through it I2 , arising under the influence of EMF.

The magnitude of the EMF that occurs in the windings directly depends on the number of turns of each winding. The ratio of the EMF induced in the primary and secondary windings is called the transformation ratio and is equal to the ratio of the number of turns of the respective windings.

By selecting the number of turns on the windings, it is possible to increase or decrease the voltage on the current consumer from the secondary winding.

Ideal Transformer

An ideal transformer is a transformer in which there is no energy loss. In such a transformer, the current energy in the primary winding is completely converted first into the energy of the magnetic field, and then into the energy of the secondary winding.

Of course, such a transformer does not exist in nature. However, in the case when heat losses can be neglected, it is convenient to use the formula for an ideal transformer in calculations, according to which the current powers in the primary and secondary windings are equal.

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Energy losses in the transformer

The efficiency of transformers is quite high. However, energy losses occur in the winding and core, causing the temperature to rise during the operation of the transformer. For small power transformers, this is not a problem, and all the heat goes to environment- natural air cooling is used. Such transformers are called dry.

In more powerful transformers air cooling is insufficient and oil cooling is applied. In this case, the transformer is placed in a mineral oil tank through which heat is transferred to the tank walls and dissipated into the environment. In high-power transformers, exhaust pipes are additionally used - if the oil boils, the resulting gases need an outlet.


Of course, transformers are not as simple as it might seem at first glance - after all, we briefly reviewed the principle of the transformer. An electrical engineering test with transformer calculation tasks can suddenly become a real problem. always ready to help in solving any problems with your studies! Contact Zaochnik and learn easily!

With the invention of the transformer, there was a technical interest in alternating current. Russian electrical engineer Mikhail Osipovich Dolivo-Dobrovolsky in 1889 proposed a three-phase alternating current system with three wires (a three-phase alternating current system with six wires was invented by Nikola Tesla, US patent No. , built the first three-phase asynchronous motor with a squirrel-cage squirrel-cage winding and a three-phase winding on the rotor (three-phase asynchronous motor invented by Nikola Tesla, US patent No. with three rods of the magnetic circuit located in the same plane. At the electrical exhibition in Frankfurt am Main in 1891, Dolivo-Dobrovolsky demonstrated an experimental three-phase high-voltage power transmission with a length of 175 km. The three-phase generator had a power of 230 kW at a voltage of 95 V.

In the early 1900s, the English metallurgist researcher Robert Hadfield conducted a series of experiments to determine the effect of additives on the properties of iron. Only a few years later he managed to supply customers with the first ton of transformer steel with silicon additives.

The next major leap in core technology was made in the early 1930s, when the American metallurgist Norman P. Gross established that, under the combined effect of rolling and heating, silicon steel developed extraordinary magnetic properties in the rolling direction: magnetic saturation increased by 50 %, hysteresis losses were reduced by 4 times, and the magnetic permeability increased by 5 times.

Basic principles of the transformer

The operation of a transformer is based on two basic principles:

  1. A time-varying electric current creates a time-varying magnetic field (electromagnetism)
  2. A change in the magnetic flux passing through the winding creates an EMF in this winding (electromagnetic induction)

On one of the windings, called primary winding voltage is applied from an external source. The alternating current flowing through the primary winding creates an alternating magnetic flux in the magnetic circuit. As a result of electromagnetic induction, an alternating magnetic flux in the magnetic circuit creates in all windings, including the primary one, an induction EMF proportional to the first derivative of the magnetic flux, with a sinusoidal current shifted 90 ° in the opposite direction with respect to the magnetic flux.

In some transformers operating at high or ultra-high frequencies, the magnetic circuit may be absent.

Faraday's Law

The EMF generated in the secondary winding can be calculated from Faraday's law, which states that:

U 2- Voltage on the secondary winding, N 2 - number of turns in the secondary winding, Φ - total magnetic flux, through one turn of the winding. If the turns of the winding are perpendicular to the lines of the magnetic field, then the flux will be proportional to the magnetic field B and square S through which it passes.

EMF generated in the primary winding, respectively:

U 1- instantaneous value of voltage at the ends of the primary winding, N 1 is the number of turns in the primary winding.

Dividing the equation U 2 on the U 1, we get the ratio:

Ideal transformer equations

An ideal transformer is a transformer that has no energy losses for heating the windings and winding leakage fluxes. In an ideal transformer, all lines of force pass through all the turns of both windings, and since the changing magnetic field generates the same EMF in each turn, the total EMF induced in the winding is proportional to its total number of turns. Such a transformer transforms all incoming energy from the primary circuit into a magnetic field and then into the energy of the secondary circuit. In this case, the incoming energy is equal to the converted energy:

P1- instantaneous value of the power supplied to the transformer, coming from the primary circuit, P2- the instantaneous value of the power converted by the transformer entering the secondary circuit.

Combining this equation with the ratio of voltages at the ends of the windings, we get the equation for an ideal transformer:

Thus, we obtain that with increasing voltage at the ends of the secondary winding U 2, the current of the secondary circuit decreases I 2.

To convert the resistance of one circuit to the resistance of another, you need to multiply the value by the square of the ratio. For example, resistance Z2 connected to the ends of the secondary winding, its reduced value to the primary circuit will be . This rule is also valid for the secondary circuit: .

Transformer operating modes

Short circuit mode

In short circuit mode, a small alternating voltage is applied to the primary winding of the transformer, the secondary winding leads are short-circuited. The input voltage is set so that the short-circuit current is equal to the rated (calculated) current of the transformer. Under such conditions, the value of the short circuit voltage characterizes the losses in the transformer windings, the losses in the ohmic resistance. The power loss can be calculated by multiplying the short circuit voltage by the short circuit current.

This mode is widely used in measuring current transformers.

Loaded mode

When a load is connected to the secondary winding, a current arises in the secondary circuit, which creates a magnetic flux in the magnetic circuit, directed opposite to the magnetic flux created by the primary winding. As a result, the equality of the induction EMF and the EMF of the power source is violated in the primary circuit, which leads to an increase in the current in the primary winding until the magnetic flux reaches almost the same value.

Schematically, the transformation process can be depicted as follows:

To do this, consider the response of the system to a sinusoidal signal u 1=U 1 e-jω t(ω=2π f, where f is the frequency of the signal, j is the imaginary unit). Then i 1=I 1 e-jω t etc., reducing the exponential factors, we get

U 1=-jω L1 I 1-jω L 12 I 2+I 1 R1

Jω L2 I 2-jω L 12 I 1+I 2 R2 =-I 2 Z n

The method of complex amplitudes allows us to investigate not only a purely active, but also an arbitrary load, while it is enough to replace the load resistance R n its impedance Z n. From the resulting linear equations, you can easily express the current through the load, using Ohm's law - the voltage across the load, etc.

T-shaped transformer equivalent circuit.

The part of the magnetic system of the transformer that does not carry the main windings and serves to close the magnetic circuit is called - yoke

Depending on the spatial arrangement of the rods, there are:

  1. Flat magnet system- a magnetic system in which the longitudinal axes of all rods and yokes are located in the same plane
  2. Spatial magnetic system- a magnetic system in which the longitudinal axes of the rods or yokes, or the rods and yokes are located in different planes
  3. Symmetrical magnetic system- a magnetic system in which all rods have the same shape, design and dimensions, and the relative position of any rod with respect to all yokes is the same for all rods
  4. Unsymmetrical magnetic system- a magnetic system in which individual rods may differ from other rods in shape, design or dimensions, or the relative position of any rod in relation to other rods or yokes may differ from the location of any other rod

windings

The main element of the winding is coil- an electrical conductor, or a series of such conductors connected in parallel (stranded core), once wrapping around a part of the transformer magnetic system, the electric current of which, together with the currents of other such conductors and other parts of the transformer, creates a magnetic field of the transformer and in which an electromotive force is induced under the action of this magnetic field .

Winding- a set of turns forming electrical circuit, which summarizes the EMF induced in the turns. In a three-phase transformer, a winding usually means a set of windings of the same voltage of three phases connected to each other.

The cross section of the winding conductor in power transformers is usually square in shape for the most effective use available space (to increase the fill factor in the core window). With an increase in the cross-sectional area of ​​the conductor, it can be divided into two or more parallel conductive elements in order to reduce eddy current losses in the winding and facilitate the operation of the winding. A conductive element of a square shape is called residential.

Each core is insulated with either paper winding or enamel lacquer. Two individually insulated and parallel-connected cores can sometimes have a common paper insulation. Two such insulated cores in a common paper insulation are called a cable.

A special kind of winding conductor is a continuously transposed cable. This cable consists of strands insulated with two layers of enamel lacquer, located axially to each other, as shown in the figure. A continuously transposed cable is obtained by moving the outer strand of one layer to the next layer at a constant pitch and applying a common outer insulation.

The paper winding of the cable is made of thin (several tens of micrometers) paper strips several centimeters wide, wound around the core. The paper is wrapped in several layers to obtain the required overall thickness.

Disc winding

Windings are divided according to:

  1. Appointment
    • Main- transformer windings to which the energy of the converted alternating current is supplied or from which the energy of the converted alternating current is removed.
    • Regulatory- with a low winding current and a not too wide regulation range, taps can be provided in the winding to regulate the voltage transformation ratio.
    • Auxiliary- windings intended, for example, for supplying an auxiliary network with a power significantly less than the rated power of the transformer, to compensate for the third harmonic magnetic field, to bias the magnetic system with direct current, etc.
  2. Execution
    • Ordinary winding- the turns of the winding are located in the axial direction along the entire length of the winding. Subsequent turns are wound tightly to each other, leaving no intermediate space.
    • screw winding- the helical winding can be a variant of the multilayer winding with distances between each winding turn or lead.
    • Disc winding- disk winding consists of a number of disks connected in series. In each disc, the coils are wound radially in a helical pattern inward and outward on adjacent discs.
    • foil winding- foil windings are made of a wide copper or aluminum sheet with a thickness from tenths of a millimeter to several millimeters.

Schemes and groups for connecting the windings of three-phase transformers

There are three main ways to connect the phase windings of each side of a three-phase transformer:

  • Y-connection ("star"), where each winding is connected at one end to a common point, called neutral. There is a "star" with a conclusion from a common point (designation Y 0 or Y n) and without it (Y)
  • Δ-connection ("delta"), where three phase windings are connected in series
  • Z-connection ("zigzag"). At this method connection, each phase winding consists of two identical parts placed on different rods of the magnetic circuit and connected in series, opposite. The resulting three phase windings are connected at a common point, similar to a "star". Usually a "zigzag" is used with a branch from a common point (Z 0)

Both the primary and secondary windings of a transformer can be connected by any of the three ways shown above, in any combination. The specific method and combination is determined by the purpose of the transformer.

Y-connection is usually used for windings operating under high voltage. This is due to many reasons:

The windings of a three-phase autotransformer can only be connected in a "star";

When instead of one heavy-duty three-phase transformer, three single-phase autotransformers are used, it is impossible to connect them in any other way;

When the secondary winding of the transformer feeds the high-voltage line, the presence of a grounded neutral reduces overvoltages during lightning strikes. Without neutral grounding, it is impossible to operate the differential protection of the line, in terms of leakage to earth. In this case, the primary windings of all receiving transformers on this line should not have a grounded neutral;

The design of voltage regulators (tap switches) is greatly simplified. Placing winding taps from the "neutral" end ensures the minimum number of contact groups. The requirements for switch insulation are reduced, as it operates at minimum voltage relative to earth;

This compound is the most technologically advanced and the least metal-intensive.

Delta connection is used in transformers where one winding is already connected in star, especially with the neutral terminal.

The operation of the still widespread transformers with the Y / Y 0 scheme is justified if the load on its phases is the same (three-phase motor, three-phase electric furnace, strictly calculated street lighting, etc.). If the load is unbalanced (domestic and other single-phase), then the magnetic flux in the core is out of balance, and the uncompensated magnetic flux (the so-called "zero sequence flux") closes through the cover and tank, causing them to heat up and vibrate. The primary winding cannot compensate for this flow, because its end is connected to a virtual neutral not connected to the generator. The output voltages will be distorted (there will be "phase imbalance"). For a single-phase load, such a transformer is essentially an open-core choke, and its impedance is high. The current of a single-phase short circuit will be greatly underestimated compared to the calculated one (for a three-phase short circuit), which makes the operation of protective equipment unreliable.

If the primary winding is connected in a triangle (transformer with Δ/Y 0 circuit), then the windings of each rod have two leads both to the load and to the generator, and the primary winding can magnetize each rod separately, without affecting the other two and without violating magnetic balance. The single-phase resistance of such a transformer will be close to the calculated one, the voltage imbalance is practically eliminated.

On the other hand, with a triangle winding, the design of the tap switch (high voltage contacts) becomes more complicated.

The connection of the winding with a triangle allows the third and multiple harmonics of the current to circulate inside the ring formed by three series-connected windings. Closing the third harmonic currents is necessary to reduce the resistance of the transformer to non-sinusoidal load currents (non-linear load) and maintain its voltage sinusoidal. The third current harmonic in all three phases has the same direction, these currents cannot circulate in a winding connected by a star with an isolated neutral.

The lack of ternary sinusoidal currents in the magnetizing current can lead to significant distortion of the induced voltage, in cases where the core has 5 rods, or it is made in armored version. A delta-connected transformer winding will eliminate this disturbance, since a delta-connected winding will dampen the harmonic currents. Sometimes transformers provide for the presence of a tertiary Δ-connected winding, provided not for charging, but to prevent voltage distortion and a decrease in zero-sequence impedance. Such windings are called compensation. Distribution transformers intended for charging, between phase and neutral on the primary side, are usually equipped with a delta winding. However, the current in the delta winding can be very low to achieve the minimum power rating, and the required winding conductor size is extremely inconvenient for factory fabrication. In such cases, the high-voltage winding can be connected in a star, and the secondary winding in a zigzag. The zero-sequence currents circulating in the two taps of a zigzag winding will balance each other, the zero-sequence impedance of the secondary side is mainly determined by the stray magnetic field between the two branches of the windings, and is expressed as a very small number.

By using the connection of a pair of windings in different ways, it is possible to achieve different degrees of bias voltage between the sides of the transformer.

  1. Only transformers with the same angular error between the primary and secondary voltages can operate in parallel.
  2. Poles with the same polarity on the high and low voltage sides must be connected in parallel.
  3. Transformers should have approximately the same voltage ratio.
  4. The short circuit impedance voltage must be the same, within ±10%.
  5. The power ratio of the transformers should not deviate more than 1:3.
  6. The switches for the number of turns should be in positions that give the voltage gain as close as possible.

In other words, this means that the most similar transformers should be used. Identical models of transformers are the best option. Deviations from the above requirements are possible with the use of relevant knowledge.

Frequency

Transformer Voltage Regulation

Depending on the load of the electrical network, its voltage changes. For the normal operation of consumer electrical consumers, it is necessary that the voltage does not deviate from the specified level by more than the permissible limits, and therefore apply various ways voltage regulation in the network.

Troubleshooting

Type of malfunction Cause
Overheat Overload
Overheat Low oil level
Overheat Closures
Overheat Insufficient cooling
Breakdown Overload
Breakdown Oil contamination
Breakdown Low oil level
Breakdown Turn insulation aging
cliff Poor solder quality
cliff Strong electromechanical deformations during short circuit
Increased hum The weakening of the pressing of the laminated magnetic circuit
Increased hum Overload
Increased hum
Increased hum short circuit in the winding
The appearance of air in the gas relay (with thermosiphon filter) The thermosiphon filter is plugged, air enters the gas relay through the plug

Overvoltage transformer

Types of surges

During use, transformers may be subjected to voltages in excess of their operating parameters. These surges are classified according to their duration into two groups:

  • Momentary overvoltage- power frequency voltage of relative duration ranging from less than 1 second to several hours.
  • Transient overvoltage- short-term overvoltage ranging from nanoseconds to several milliseconds. The rise time can range from a few nanoseconds to a few milliseconds. Transient overvoltage can be oscillatory and non-oscillatory. They usually have unidirectional action.

The transformer can also be subjected to a combination of transient and transient overvoltages. Transient overvoltages can immediately follow transient overvoltages.

Overvoltages are classified into two main groups, characterizing their origin:

  • Overvoltages caused by atmospheric influences. Most often, transient overvoltages occur due to lightning near high-voltage transmission lines connected to a transformer, but sometimes a lightning impulse can strike a transformer or the transmission line itself. The peak voltage value depends on the lightning impulse current and is a statistical variable. Lightning impulse currents over 100 kA have been registered. In accordance with measurements made on high-voltage power lines, in 50% of cases, the peak value of lightning impulse currents is in the range from 10 to 20 kA. The distance between the transformer and the impact point of the lightning impulse affects the rise time of the impulse that strikes the transformer, the shorter the distance to the transformer, the shorter the time.
  • Overvoltages generated inside the power system. This group covers both short-term and transient overvoltages resulting from changes in the operating and maintenance conditions of the power system. These changes may be caused by a violation of the switching process or a breakdown. Temporary overvoltages are caused by ground faults, load shedding, or low frequency resonance phenomena. Transient overvoltages occur when the system is frequently disconnected from or connected to. They can also occur when the external insulation ignites. When switching a reactive load, the transient voltage can rise up to 6-7 p.u. due to numerous interruptions of the transient current in the circuit breaker with a pulse rise time of up to a few fractions of microseconds.

The ability of the transformer to withstand surges

Transformers must pass certain dielectric strength tests before leaving the factory. Passing these tests indicate the likelihood uninterrupted operation transformer.

Tests are described in international and national standards. Tested transformers confirm high operational reliability.

Additional condition high degree reliability is to ensure acceptable overvoltage limits, since the transformer during operation can be subjected to more serious overvoltages compared to test test conditions.

It is necessary to emphasize the extreme importance of planning and accounting for all types of overvoltages that may occur in the power system. For normal execution given condition need to understand the origin various types overvoltages. The magnitude of the different types of overvoltages is a statistical variable. The ability of the insulation to withstand surges is also a statistical variable.

see also

Transformer in Wiktionary
Transformer at Wikimedia Commons
  • Integrated transformer test bench

Notes

  1. Kharlamova T. E. History of science and technology. Power industry. Textbook. St. Petersburg: SZTU, 2006. 126 p.
  2. Kislitsyn A. L. Transformers: Textbook for the course "Electromechanics" .- Ulyanovsk: UlGTU, 2001. - 76 with ISBN 5-89146-202-8
  3. Power transformers: the main milestones in the development of c.t. n. Savintsev Yu.M. Available on 01/25/2010
  4. Power transformer: stages of evolution. D.t. n., prof. Popov G. V. at transform.ru. Available on 02.08.2008
  5. History of the transformer on energoportal.ru. Available on 02.08.2008
  6. winders Power Transformer Principles and Applications. - P. 20–21.