What physical phenomena are used in the operation of a transformer. Operating principle of transformers

Generators that are located at power plants produce a very powerful EMF. In practice, such tension is rarely needed. Therefore, such voltage must be converted.

Transformers

Devices called transformers are used to convert voltage. Transformers can either increase the voltage or decrease it. There are also stabilizing transformers that do not increase or decrease the voltage.

Consider the transformer design in the following figure.

picture

Transformer design and operation

The transformer consists of two coils with wire windings. These coils are placed on a steel core. The core is not monolithic, but is assembled from thin plates.

One of the windings is called the primary. Connect to this winding AC voltage, which comes from the generator and which needs to be converted. The other winding is called the secondary winding. A load is connected to it. Load is all the devices and devices that consume energy.

The following figure shows symbol transformer.

picture

The operation of a transformer is based on the phenomenon of electromagnetic induction. When alternating current passes through the primary winding, an alternating magnetic flux is created in the core. And since the core is common, the magnetic flux induces a current in the other coil.

There are N1 turns in the primary winding of the transformer, its total induced emf is equal to e1 = N1*e, where e is the instantaneous value of the induced emf in all turns. e is the same for all turns of both coils.

The secondary winding has N2 turns. EMF e2 = N2*e is induced in it.

Hence:

We neglect the winding resistance. Consequently, the values ​​of induced emf and voltage will be approximately equal in magnitude:

When the secondary winding circuit is open, there is no current is flowing, hence:

Instantaneous emf values ​​e1, e2 oscillate in one phase. Their ratio can be replaced by the ratio of the values ​​of the effective emfs: E1 and E2. And we replace the ratio of instantaneous voltage values ​​with effective voltage values. We get:

E1/E2 ≈U1/U2 ≈N1/N2 = K

K – transformation coefficient. At K>0 the transformer increases the voltage when K<0 – the transformer reduces the voltage. If a load is connected to the ends of the secondary winding, an alternating current will appear in the second circuit, which will cause another magnetic flux to appear in the core.

This magnetic flux will reduce the change in the magnetic flux of the core. For loaded transformer, the following formula will be valid:

U1/U2 ≈ I2/I1.

That is, when the voltage increases several times, we will reduce the current by the same amount.

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

1. A time-varying electric current creates a magnetic field (electromagnetism)

2. A change in the magnetic flux passing through the winding creates an EMF in this winding (electromagnetic induction)

The alternating current flowing in the primary winding creates an alternating magnetic flux in the magnetic core, changes in which, in turn, passing through the secondary winding, create an alternating EMF in it.

Rice. 1 Schematic structure of the transformer. 1 - primary winding, 2 - secondary

Faraday's law

The emf created in the secondary winding can be calculated using Faraday's law, which states that:

N2 - number of turns in the secondary winding,

Φ is the total magnetic flux through one turn of the winding. If the turns of the winding are located perpendicular to the magnetic field lines, then the flux will be proportional to the magnetic field B and the area S through which it passes.

The EMF created in the primary winding is, respectively:

U1 - instantaneous voltage value at the ends of the primary winding,

N1 is the number of turns in the primary winding.

Dividing the equation U2 by U1, we get the ratio:

Ideal Transformer Equations

If the secondary winding is connected to a load, then electrical energy will be transferred from the primary circuit to the secondary. Ideally, 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 is the instantaneous value of the power supplied to the transformer coming from the primary circuit,

P2 is the instantaneous value of the power converted by the transformer entering the secondary circuit.

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

Thus, we find that as the voltage at the ends of the secondary winding U2 increases, the secondary circuit current I2 decreases.

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 is connected to the ends of the secondary winding, its reduced value to the primary circuit will be . This rule also applies to the secondary circuit: .

The operation of a transformer is based on the phenomenon of electromagnetic induction. One of the windings, called the primary winding, is supplied with voltage from an external source. The alternating current flowing through the primary winding creates an alternating magnetic flux in the magnetic core, shifted in phase, with a sinusoidal current, by 90° relative to the voltage in the primary winding. As a result of electromagnetic induction, an alternating magnetic flux in the magnetic circuit creates in all windings, including the primary, 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. When the secondary windings are not connected to anything (no-load mode), the induced emf in the primary winding almost completely compensates for the voltage of the power source, so the current through the primary winding is small and is determined mainly by its inductive reactance. The induction voltage on the secondary windings in no-load mode is determined by the ratio of the number of turns of the corresponding winding w2 to the number of turns of the primary winding w1:

When the secondary winding is connected to a load, current begins to flow through it. This current also creates a magnetic flux in the magnetic circuit, and it is directed opposite to the magnetic flux created by the primary winding. As a result, the compensation of the induced emf and the emf of the power source is disrupted in the primary winding, which leads to an increase in the current in the primary winding until the magnetic flux reaches almost the same value. In this mode, the ratio of the currents of the primary and secondary windings is equal to the inverse ratio of the number of turns of the windings

the stress ratio to a first approximation also remains the same. As a result, the power consumed from the source in the primary winding circuit is almost completely transferred to the secondary.

Schematically, the above can be depicted as follows:

U1 → I1 → I1w1 → Ф → ε2 → I2

The instantaneous magnetic flux in the magnetic core of the transformer is determined by the time integral of the instantaneous value of the emf in the primary winding, and in the case of a sinusoidal voltage, it is shifted in phase by 90° with respect to the emf. The EMF induced in the secondary windings is proportional to the first derivative of the magnetic flux, and for any current shape it coincides in phase and shape with the EMF in the primary winding.

Transformers- electromagnetic static converters electrical energy. Transformers are electromagnetic devices used to transform alternating current one voltage into alternating current of another voltage at the same frequency and for the transfer of electrical energy electromagnetically from one circuit to another.

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

Current transformers are devices designed to convert current of any magnitude into a current acceptable for measurements with normal instruments, as well as to power various relays and electromagnet windings. The number of turns of the secondary winding of the current transformer ω2 > ω1.

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

Voltage transformers are devices designed to convert high-voltage alternating current into low-voltage alternating current and power parallel coils of measuring instruments and relays. The principle of operation and design of voltage transformers is similar to the principle of operation of power transformers. Number of turns of the secondary winding ω2

The peculiarity of the operation of a voltage measuring transformer is that its secondary winding is always short-circuited to a high resistance, and the transformer operates in a mode close to the no-load mode, since the connected devices consume insignificant 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 in 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 them again to 35 ; 10; 6; 3; 0.66; 0.38 and 0.22 kV to use energy in industry, agriculture and everyday life.

Since energy systems undergo multiple transformations, 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 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.

Based on the number of phases, transformers are divided into single-phase, two-phase, three-phase and multiphase. Power transformers are produced mainly in three-phase versions. For use in single-phase networks, they are produced.

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

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

Multiphase transformers have windings connected into a multi-beam star or polygon. Three-phase transformers have a three-pointed star and delta connection.

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. IN step-up transformer The primary winding has low voltage and the secondary winding has high voltage. IN step-down transformer On the contrary, the secondary winding has a low voltage, and the primary winding has a high voltage.

Transformers having one primary and one secondary winding are called two-winding. Quite widespread three-winding transformers having three windings per phase, for example two on the low voltage side, one on the high voltage side, or vice versa. Multiphase transformers may have several high and low voltage windings.

Classification of transformers by design

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

IN oil transformers The magnetic core with windings is located in a tank filled with transformer oil, which is a good insulator and cooling agent.

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 a liquid dielectric using a nitrogen blanket without an expander - Z
• Version with cast insulation - L
• Three winding transformer - T
• Transformer with on-load tap-changer - N
• Dry transformer with natural air cooled(usually the second letter in the type designation), or execution for the own needs of power plants (usually last letter in the type designation) - C
• Cable entry - K
• Flange input (for complete transformer substations) - F

Cooling systems for dry-type transformers:

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

Oil transformer cooling systems:

• Natural circulation of air and oil - 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 directed oil flow - NMC
• Forced circulation of air and oil with non-directional oil flow - DC
• Forced circulation of air and oil with directed oil flow - NDC
• Forced circulation of water and oil with non-directional oil flow - C
• Forced circulation of water and oil with directed oil flow - NC

Cooling systems for transformers with non-flammable liquid dielectric:

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

Content:

In electrical engineering, quite often there is a need to measure quantities with large values. To solve this problem, current transformers are used, the purpose and operating principle of which makes it possible to carry out any measurements. For this purpose, the primary winding of the device is connected in series to a circuit with alternating current, the value of which must be measured. The secondary winding is connected to measuring instruments. There is a certain proportion between the currents in the primary and secondary windings. All transformers of this type are highly accurate. Their design includes two or more secondary windings, to which protective devices, measuring instruments and metering devices are connected.

What is a current transformer?

Current transformers are devices in which the secondary current used for measurements is in proportion to the primary current coming from electrical network.

The primary winding is connected to the circuit in series with the current conductor. The secondary winding is connected to any load in the form of measuring instruments and. A proportional relationship arises between the currents of both windings, corresponding to the number of turns. In high voltage transformer devices, insulation between the windings is carried out based on the full operating voltage. As a rule, one end of the secondary winding is grounded, so the winding and ground potentials will be approximately the same.

All current transformers are designed to perform two main functions: measurement and protection. Some devices may combine both functions.

• Instrument transformers transmit the received information to connected measuring instruments. They are installed in high voltage circuits in which it is impossible to directly connect measuring instruments. Therefore, only the secondary winding of the transformer is connected to counters, current windings of wattmeters and other metering devices. As a result, the transformer converts alternating current, even a very high value, into alternating current with indicators that are most acceptable for the use of conventional measuring instruments. At the same time, the isolation of measuring instruments from high-voltage circuits is ensured, and the electrical safety of operating personnel is increased.
• Protective transformer devices primarily transmit the received measurement information to control and protection devices. With the help of protective transformers, alternating current of any value is converted into alternating current with the most suitable value, providing power to relay protection devices. At the same time, relays that are accessible to personnel are isolated from high voltage circuits.

Purpose of transformers

Current transformers belong to the category of special auxiliary devices used in conjunction with various measuring devices and relays in alternating current circuits. Main function Such transformers are transforming any current value to values ​​that are most convenient for carrying out measurements, providing power to disconnecting devices and relay windings. Due to the insulation of the devices, service personnel are reliably protected from high voltage electric shock.

Current transformers are designed for high voltage electrical circuits where there is no possibility direct connection measuring instruments. Their main purpose is to transmit received data on electric current to measuring devices connected to the secondary winding.

An important function of transformers is state control electric current in the circuit to which they are connected. During connection to the power relay, constant checks of the networks are performed, the presence and condition of grounding. When the current reaches an emergency value, protection is activated, turning off all equipment in use.

Principle of operation

The operating principle of current transformers is based on. Voltage from external network enters the power primary winding with a certain number of turns and overcomes it impedance. This leads to the appearance of a magnetic flux around the coil, captured by the magnetic circuit. This magnetic flux is located perpendicular to the direction of the current. Due to this, losses of electric current during the conversion process will be minimal.

When the turns of the secondary winding, located perpendicularly, intersect, activation occurs by magnetic flux electromotive force. Under the influence of the EMF, a current appears that is forced to overcome the total resistance of the coil and the output load. At the same time, a voltage drop is observed at the output of the secondary winding.

Classification of current transformers

All current transformers can be classified depending on their features and technical characteristics:

1. By appointment. Devices can be measuring, protective or intermediate. The last option is used when turning on measuring instruments in current circuits relay protection and other similar circuits. In addition, there are laboratory current transformers that are characterized by high accuracy and a variety of .
2. By installation type. There are transformer devices for outdoor and indoor installation, overhead and portable. Some types of devices can be built into cars, electrical devices and other equipment.
3. According to the design of the primary winding. Devices are divided into single-turn or rod, multi-turn or coil, and also bus, for example, TSh-0.66.
4. Internal and external installation of transformers involves pass-through and support methods for installing these devices.
5. Transformer insulation can be dry, using bakelite, porcelain, and other materials. In addition, conventional and capacitor paper-oil insulation is used. Some designs use compound filling.
6. Depending on the number of transformation stages, devices can be one- or two-stage, that is, cascade.
7. The rated operating voltage of transformers can be up to 1000 V or more than 1000 V.

All characteristic classification features are present in the current and consist of certain.

Parameters and characteristics

Each current transformer has individual parameters and technical characteristics, which determine the scope of application of these devices.

Rated current. Allows the device to operate for a long time without overheating. Such transformers have a significant heating reserve, and normal operation is possible with overloads of up to 20%.

Rated voltage. Its value should provide normal work transformer. It is this indicator that affects the quality of insulation between the windings, one of which is at high voltage and the other is grounded.

Transformation ratio. It is the ratio between the currents in the primary and secondary windings and is determined by a special formula. Its actual value will differ from the nominal value due to certain losses during the transformation process.

Current error. Occurs in a transformer under the influence of magnetizing current. Absolute value The primary and secondary currents differ from each other by precisely this amount. The magnetizing current leads to the creation of a magnetic flux in the core. As it increases, the current error of the transformer also increases.

. Determines the normal operation of the device in its accuracy class. It is measured in Ohms and in some cases can be replaced by such a concept as rated power. The current value is strictly standardized, so the power value of the transformer completely depends only on the load.

Nominal limiting factor. It represents the multiple of the primary current to its rated value. The error of this multiplicity can reach up to 10%. During calculations, the load itself and its power factors must be rated.

Maximum secondary current ratio. Presented as the ratio of the maximum secondary current and its rated value when the effective secondary load is rated. The maximum multiplicity is related to the degree of saturation of the magnetic circuit, at which the primary current continues to increase, but the value of the secondary current does not change.

Possible malfunctions of current transformers

A current transformer connected to a load sometimes experiences malfunctions and even emergency situations. As a rule, this is associated with violations electrical resistance insulation of windings, reduction of their conductivity under the influence of elevated temperatures. Negative influence caused by accidental mechanical impacts or poorly executed installation.

During equipment operation, insulation damage most often occurs, causing interturn short circuits of the windings, which significantly reduces the transmitted power. Leakage currents can appear as a result of randomly created circuits, up to the occurrence of a short circuit.

In order to prevent emergency situations, specialists periodically check the entire operating circuit using thermal imagers. This makes it possible to promptly eliminate contact defects and reduce equipment overheating. The most complex tests and inspections are carried out in special laboratories.

Purpose of the transformer. A transformer is a static electromagnetic device that converts alternating current of one voltage into alternating current of another voltage of the same frequency.

Transformers can significantly increase the voltage produced by AC sources installed on power stations, and carry out the transmission of electricity over long distances at high voltages (110, 220, 500, 750 and 1150 kV). Thanks to this, energy losses in wires are greatly reduced and it is possible to significantly reduce the cross-sectional area of ​​power transmission line wires.

In places where electricity is consumed, the high voltage supplied from high-voltage power lines is again reduced by transformers to relatively low values ​​(127, 220, 380 and 660 V) at which electrical consumers installed in factories, factories, depots and residential buildings operate. On e. p.s. AC transformers are used to reduce the voltage supplied from contact network to traction motors and auxiliary circuits.

In addition to transformers used in power transmission and distribution systems, industry produces transformers: traction (for electrical power supply), for rectifier units, laboratory transformers with voltage regulation, for powering radio equipment, etc. All these transformers are called power transformers.

Transformers are also used to connect electrical measuring instruments to high voltage circuits (they are called measuring instruments), for electric welding and other purposes. Trance-

Formers are single-phase and three-phase, two- and multi-winding.

The principle of operation of the transformer. The operation of a transformer is based on the phenomenon of electromagnetic induction. The simplest transformer consists of a steel magnetic core 2 (Fig. 212) and two windings 1 and 3 located on it. The windings are made of insulated wire and are not electrically connected. Electrical energy is supplied to one of the windings from an alternating current source. This winding is called primary. To another winding called secondary, connect consumers (directly or through a rectifier).

When a transformer is connected to an alternating current source (electrical network), an alternating current i 1 flows in the turns of its primary winding, forming an alternating magnetic flux F. This flux passes through the magnetic core of the transformer and, penetrating the turns of the primary and secondary windings, induces alternating e. d.s. e 1 and e 2. If any receiver is connected to the secondary winding, then under the influence of e. d.s. e 2 current i 2 passes through its circuit.

The emf induced in each turn of the primary and secondary windings of the transformer, according to the law of electromagnetic induction, depends on the magnetic flux passing through the turn and the rate of its change. The magnetic flux of each transformer is a certain value depending on the voltage and frequency of alternating current in the source to which the transformer is connected. The rate of change of magnetic flux is also constant; it is determined by the frequency of change of alternating current. Consequently, the same e is induced in each turn of the primary and secondary windings. d.s. As a result attitude effective values e. d.s. E 1 and E 2 induced in the primary and secondary windings of the transformer will be equal to the ratio of the number of turns? 1 and? 2 of these windings, i.e.

E 1 / E 2 = ? 1 / ? 2.

Attitude e. d.s. E in high voltage windings to e. d.s. E nn low voltage windings (or the ratio of the numbers of their turns) is called transformation ratio,

n = E in / E nn = ? vn / ? nn.

The transformation coefficient is always greater than one. If we neglect the voltage drops in the primary and secondary windings of the transformer (in transformers of medium and high power they usually do not exceed 2-5% of the rated voltage values ​​U 1 and U 2), then we can assume that the ratio of the voltage U 1 of the primary winding to the voltage U 2 of the secondary winding is approximately equal to the ratio of the numbers of their turns, i.e.

U 1 / U 2 ? ? 1 / ? 2

Thus, by selecting the required ratio between the numbers of turns of the primary and secondary windings, you can increase or decrease the voltage at the receiver connected to the secondary winding. If it is necessary to obtain a voltage on the secondary winding that is greater than that supplied to the primary, then step-up transformers are used, in which the number of turns in the secondary winding is greater than in the primary.

In step-down transformers, on the contrary, the number of turns of the secondary winding is less than in the primary.

Transformer cannot convert voltage direct current. When its primary winding is connected to a DC network, a magnetic flux that is constant in magnitude and direction is created in the transformer, which cannot induce e. d.s. in the primary and secondary windings. Therefore, there will be no transfer of electrical energy from the primary winding to the secondary.

When the primary winding of a transformer is connected to an alternating current network, a certain current passes through this winding, called no-load current. When the load is turned on, current begins to flow through the secondary winding of the transformer, and the current passing through the primary winding also increases. How more load transformer, i.e. the electrical power and current i 2 given by its secondary winding to the receivers connected to it, the greater the electrical power and current i 1 coming from the network to the primary winding.

Due to the fact that power losses in a transformer are usually small, it can be approximately assumed that the powers in the primary and secondary windings are the same. In this case, we can assume that the currents in the windings of the transformer are approximately inversely proportional to the voltages: I 1 /I 2? U 2 /U 1 or that the currents in the windings of the transformer are inversely proportional to the number of turns of the primary and secondary windings: I 1 /I 2? ? 2/? 1 . This means that in a step-up transformer, the current in the secondary winding is less than in the primary (as many times as the voltage U 2 is greater than the voltage U 1), and in a step-down transformer, the current in the secondary winding is greater than in the primary. Therefore, in transformers, the higher voltage windings are made of thinner wires than the lower voltage windings.