How does a voltage transformer work? Electrical transformers. Operating principle and operating modes

The name "transformer" comes from the Latin word "transformare", which means "to transform, transform." This is precisely its essence - transformation by magnetic induction alternating current one voltage into alternating current of another voltage but similar frequency. The main purpose of the transformer is to use it in electrical networks and power supplies for various devices.

Device and principle of operation

A transformer is a device for converting alternating current and voltage, having no moving parts.

The transformer device consists of one or more separate wire, sometimes tape, coils (windings), which are covered by a single magnetic flux. Coils are usually wound around a core (magnetic core). It is usually made of ferromagnetic material.

The figure schematically shows the operating principle of the transformer.

The figure shows that the primary winding is connected to the AC source, and the other (secondary) is connected to the load. In this case, an alternating current flows in the turns of the primary winding, its value is I1. And both coils are surrounded by a magnetic flux F, which produces an electromotive force in them.

If the secondary winding is without load, then this mode of operation of the converter is called “idling”. When the secondary coil is under load, a current I2 arises in it under the action of an electromotive force.

The output voltage depends directly on how many turns there are on the coils, and the current strength depends on the diameter (section) of the wire. In other words, if both coils have an equal number of turns, then the output voltage will be equal to the input voltage. And if you wind 2 times more turns on the secondary coil, then the output voltage will become 2 times higher than the input.

The resulting current also depends on the diameter of the winding wire. For example, with a large load and a small diameter of the wire, overheating of the winding, disruption of the integrity of the insulation, and even complete failure of the transformer can occur.

To avoid such situations, tables have been compiled for calculating the converter and selecting the wire diameter for a given output voltage.

Classification by type

Transformers are usually classified according to several criteria: by purpose, by installation method, by type of insulation, by voltage used, etc. Let's consider the most common types of devices.

Power converters

This type of device is used to supply and receive electrical energy to and from power lines with voltages up to 1150 kW. Hence the name - power. These devices operate on low frequencies- about 50−60 Hz. Their design features are that they can contain several windings, which are located on an armored core made of electrical steel. Moreover, low voltage coils can be powered in parallel.

This device is called a split-winding transformer. Typically, power transformers are placed in a container with transformer oil, and the most powerful units are cooled active system. For installation at substations and power plants, three-phase devices with a power of up to 4 thousand kVA are used. They are most widespread, since losses in them are reduced by 15% compared to single-phase ones.

Autotransformers (LATR)

This is a special type of low-frequency device. In it, the secondary winding is simultaneously part of the primary and vice versa. That is, the coils are connected not only magnetically, but also electrically. Different voltages are obtained from the same winding, if several conclusions are made. By using fewer wires, the cost of the device is reduced. However, there is no galvanic isolation of the windings, and this is a significant drawback.

Autotransformers have found application in high-voltage networks and in automatic control installations for starting AC motors. It is advisable to use them at low transformation ratios. LATR is used to regulate voltage in laboratory conditions.

Current transformers

In such devices, the primary winding is connected directly to the current source, and the secondary winding is connected to devices with low internal resistance. These may be protective or measuring devices. The most common type of current transformer is the measuring one.

It consists of a core made of laminated silicon cold-rolled electrical steel, with one or more separate secondary windings wound on it. While the primary one can simply be a bus or a wire with a measured current passed through the window of the magnetic circuit. For example, current clamps operate on this principle. The main characteristic of transformer current is the transformation ratio.

Such converters are safe and therefore have found application in current measurement and in relay protection circuits.

Pulse converters

IN modern world pulse systems have almost completely replaced heavy low-frequency transformers. Typically, a pulsed device is made on a ferrite core of various shapes and sizes:

• ring;
• kernel;
• cup;
• in the form of the letter W;
• U-shaped.

The superiority of such devices is beyond doubt - they are capable of operating at frequencies up to 500 kHz or more.

Since this is a high-frequency device, its dimensions decrease significantly with increasing frequency. A smaller amount of wire is consumed on the winding, and to obtain a high-frequency current in the first circuit, it is enough to simply connect a field-effect or bipolar transistor.

There are many more types of transformers: isolation, matching, peak transformers, dual choke, etc. All of them are widely used in modern industry.

Area of ​​application of devices

Today, it is perhaps difficult to imagine an area of ​​science and technology where transformers are not used. They are widely used for the following purposes:

Based on the variety of devices and types of purposes of transformers, it can be argued that today they are irreplaceable, devices used almost everywhere, thanks to which stability and achievement of the voltage values ​​required by the consumer are ensured for both civil networks and industrial networks.

Question 1. What does a transformer consist of?
Answer. The simplest transformer consists of a closed magnetic circuit and two windings in the form of cylindrical coils.
One of the windings is connected to a source of alternating sinusoidal current with voltage u 1 and is called the primary winding. The load of the transformer is connected to the other winding. This winding is called secondary
winding

Question 2. How is energy transferred from one winding to another?
Answer. The transfer of energy from one winding to another is carried out by electromagnetic induction. Alternating sinusoidal current i 1 flowing through the primary winding of the transformer excites an alternating magnetic flux in the magnetic circuit F s, which penetrates the turns of both windings and induces EMF
And
with amplitudes proportional to the number of turns w 1 And w 2. When connected to the secondary winding of the load in it under the influence EMF e 2 an alternating sinusoidal current occurs i 2 and some tension is established u 2.
There is no electrical connection between the primary and secondary windings of the transformer and energy is transferred to the secondary winding through a magnetic field excited in the core.

Question 3. What is the secondary winding of the transformer in relation to the load?
Answer. In relation to the load, the secondary winding of the transformer is a source of electrical energy with EMF e 2. Neglecting losses in the transformer windings, we can assume that the supply voltage U 1 ≈ E 1, and the load voltage U 2 ≈ E 2.

Question 4. What is the transformation ratio?
Answer. Because EMF windings are proportional to the number of turns, then the ratio of the supply voltage of the transformer and the load is also determined by the ratio of the number of turns of the windings, i.e.
U 1 /U 2 ≈ E 1 /E 2 ≈ w 1 /w 2 = k.
Magnitude k called the transformation ratio.

Question 5. Which transformer is called a step-down transformer?
Answer. If the number of turns of the secondary winding is less than the number of turns of the primary w 2< w 1 , That k> 1 and the voltage in the load will be less than the voltage at the transformer input. Such a transformer is called a step-down transformer.

Question 6. Which transformer is called a step-up transformer?
Answer. If the number of turns of the secondary winding more number turns of primary w 2 > w 1, That k < 1 и напряжение в нагрузке будет больше напряжения на входе трансформатора. Такой трансформатор называется повышающим.

Question 7. Which winding of the transformer is called the high voltage winding (HV)?
Answer. Winding connected to the network with more high voltage, is called the high voltage winding (HV). The second winding is called the winding low voltage(NN).

Question 8. Which transformers are called “dry”?
Answer. Transformers in which heat is removed by air flow are called “dry” transformers.

Question 9. Which transformers are called “oil”?
Answer. In cases where the air flow cannot remove thermal energy in such a way as to ensure limitation
winding insulation temperatures are at an acceptable level; a liquid medium is used for cooling, immersing the transformer in a tank with special transformer oil, which simultaneously acts as a coolant and electrical insulation. Such transformers are called “oil transformers”.

Question 10. How are transformers designated on electrical diagrams?
Answer.


The figure shows the symbols of single-phase two-winding (1, 2, 3) and multi-winding (7, 8) transformers, as well as three-phase transformers (12, 13, 14, 15, 16). The designations of single-phase (4, 5) and three-phase (9, 10) autotransformers and voltage (6) and current (11) instrument transformers are also shown here.

Question 11. What determines the operating conditions and properties of a transformer?
Answer. The operating conditions and properties of the transformer are determined by a system of parameters called nominal, i.e. values ​​of quantities corresponding to the design operating mode of the transformer. They are indicated in the reference data and on the plate attached to the product.

Question 12. How does the operating frequency of a transformer affect its weight and dimensions?
Answer. Increasing the operating frequency of the transformer allows, other things being equal, to significantly reduce the weight and dimensions of the product. Indeed, the voltage of the primary winding is approximately equal to the EMF induced in it by the magnetic flux in the core Φ c, and the total power, for example, of a single-phase transformer is equal to

where and are the specified nominal values ​​of induction in the core and current density in the winding, and S c ∼ l 2 And S i – cross section core and total cross section w 1 winding turns. Therefore, increasing the power frequency f allows you to proportionally reduce the cross-section of the core with the same transformer power, i.e. square its linear dimensions l.

Question 13. What is the transformer magnetic circuit used for?
Answer. The magnetic core of the transformer serves to increase the mutual induction of the windings and, in general, is not a necessary design element. When working on high frequencies ah, when losses in a ferromagnet become unacceptably large, and also when it is necessary to obtain linear characteristics, transformers without a core are used, the so-called. air transformers. However, in the vast majority of cases, the magnetic core is one of the three main elements of the transformer. By design, magnetic cores of transformers are divided into core and armored.

Question 14. What conditions must the design of the transformer windings satisfy?
Answer. The design of transformer windings must satisfy the conditions of high electrical and mechanical strength, as well as heat resistance.
In addition, their manufacturing technology should be as simple as possible, and losses in the windings should be minimal.

Question 15. What are the transformer windings made of?
Answer. The windings are made of copper or aluminum wire. The current density in the copper windings of oil transformers is in the range of 2...4.5 A/mm 2, and in dry transformers 1.2...3.0 A/mm 2. Upper limits belong to more powerful transformers. In aluminum windings, the current density is 40...45% less. The winding wires can be of a round cross-section with an area of ​​0.02...10 mm 2 or a rectangular cross-section with an area of ​​6...60 mm 2. In many cases, winding coils are wound from several parallel conductors. The winding wires are covered with enamel and cotton or silk insulation. Dry-type transformers use wires with heat-resistant fiberglass insulation.

Question 16. How are transformer windings divided according to the method of arrangement on the rods?
Answer. According to the method of arrangement on the rods, the windings are divided into concentric and alternating. Concentric windings are made in the form of cylinders, the geometric axes of which coincide with the axis of the rods. The low voltage winding is usually located closer to the rod, because this allows you to reduce the insulating gap between the winding and the rod. In alternating windings, the HV and LV coils are alternately positioned along the height of the rod. This design allows increasing the electromagnetic coupling between the windings, but significantly complicates the insulation and winding manufacturing technology, therefore alternating windings are not used in power transformers.

Question 17. How is the transformer windings insulated?
Answer. One of the most important design elements of transformer windings is insulation.
There are main and longitudinal insulation.
The main thing is the insulation of the winding from the rod, tank and other windings. It is made in the form of insulating gaps, electrical insulating frames and washers. At low powers and low voltages, the main insulation function is performed by a frame made of plastic or electrical cardboard, on which the windings are wound, as well as several layers of varnished cloth or cardboard that insulate one winding from the other.
Longitudinal insulation is called insulation between different points of one winding, i.e. between turns, layers and coils. Turn-to-turn insulation is provided by the winding wire's own insulation. For interlayer insulation, several layers of cable paper are used, and the intercoil insulation is carried out either by insulating gaps, or by a frame or insulating washers.
The insulation design becomes more complicated as the voltage of the HV winding increases, and for transformers operating at voltages of 200...500 kV, the cost of insulation reaches 25% of the cost of the transformer.

Literature: Usoltsev Alexander Anatolyevich. Electric cars. Tutorial. 2013

Updated: September 7, 2016 by: admin

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How does a transformer work?

A transformer is a static (i.e., without moving parts) electromagnetic device, single-phase or three-phase, in which the phenomenon of mutual induction is used to convert electrical energy. A transformer converts alternating current of one voltage into alternating current of the same frequency but a different voltage.

The transformer has several electrical windings isolated from one another: single-phase - at least two, three-phase - at least six.

The windings connected to the source of electricity are called primary; the remaining windings, which supply energy to external circuits, are called secondary. The figure below schematically shows the primary and secondary windings of a single-phase transformer; they are equipped with a common closed core assembled from sheet electrical steel.

The ferromagnetic core serves to strengthen the magnetic coupling between the windings, that is, to ensure that most of the magnetic flux of the primary winding meshes with the turns of the secondary winding. In Fig. on the right is the core and six windings of a three-phase transformer. These windings are connected in a star or delta configuration.

To improve cooling and insulation conditions, the transformer is placed in a tank filled with mineral oil (a product of petroleum distillation). This is the so-called oil transformer.

At an alternating current frequency above approximately 20 kHz, the use of a steel core in transformers is impractical due to large losses in steel from hysteresis and eddy currents.

For high frequencies, transformers without ferromagnetic cores are used - air transformers.

If the voltage at the terminals of the primary winding, the primary voltage U1, is less than the secondary voltage U2, then the transformer is called a step-up transformer. If the primary voltage is greater than the secondary one, then it is a step-down voltage (U1>U2). In accordance with the relative value of the rated voltage, it is customary to distinguish between the high voltage (HV) winding and the low voltage (LV) winding.

Let's take a brief look at the operation of a single-phase two-winding transformer with a steel core. Its working process and electrical relationships can be considered characteristic basically of all types of transformers.

The voltage U1 applied to the terminals of the primary winding creates an alternating current i1 in this winding. The current excites an alternating magnetic flux F in the transformer core. Due to the periodic change of this flux, an EMF is induced in both windings of the transformer.

e1= - w1 (?ф: ?t) and e2= - w2 (?ф:?t), where

w1 and w2 - the number of turns of both windings.

Thus, the ratio of EDEs induced in the windings is equal to the ratio of the number of turns of these windings:

e1: e2 = w1: w2

This is the transformation ratio of the transformer.

The efficiency of the transformer is relatively very high, on average about 98%, which makes it possible, at rated load, to consider the primary power received by the transformer and the secondary power supplied to them to be approximately equal, i.e. p1? p2 or u1i1? u2i2, on the basis of which

i1:i2? u2: u1? w 2: w 1

This ratio of instantaneous values ​​of currents and voltages is valid for both amplitudes and effective values:

L1: l2? w 2: w 1?u2: u1,

i.e., the ratio of currents in the windings of a transformer (at a load close to the rated load) can be considered the inverse of the ratio of voltages and the number of turns of the corresponding windings. The smaller the load, the more the no-load current influences, and the given approximate current ratio is violated.

When a transformer operates, the role of the EMF in its primary and secondary windings is completely different. The EMF induced by it in the primary winding arises as the circuit’s opposition to the change in current i1 in it. The phase of this EMF is almost opposite to the voltage.

As in a circuit containing inductance, the current in the primary winding of a transformer

i1=(u1 + e1) : r1,

where g 1 is the active resistance of the primary winding.

From here we obtain the equation for the instantaneous value of the primary voltage:

u1 = -e1 + i1r1 = w t(?ф: ?t) + i1r1,

which can be read as the condition of electrical equilibrium: the voltage u1 applied to the terminals of the primary winding is always balanced by the emf and the voltage drop in the active resistance of the winding (the second term is relatively very small).

Other conditions occur in the secondary circuit. Here, the current i2 is created by the emf e1, which plays the role of the emf of the current source, and with an active load r/n in the secondary circuit this current

i2= l2: (r2 +r/n),

where r2 is the active resistance of the secondary winding.

To a first approximation, the effect of the secondary current i2 on the primary circuit of the transformer can be described as follows.

Current i2, passing through the secondary winding, tends to create a magnetic flux in the transformer core, determined by the magnetizing force (MF) i2w2. According to Lenz's principle, this flow should be in the opposite direction to the direction of the main flow. Otherwise, we can say that the secondary current tends to weaken the magnetic flux inducing it. However, such a decrease in the main magnetic flux F t would disrupt the electrical equilibrium:

u 1 = (-е 1) + i1r1,

since e1 is proportional to the magnetic flux.

A predominance of the primary voltage U1 is created, therefore, simultaneously with the appearance of the secondary current, the primary current increases, moreover, so much as to compensate for the demagnetizing effect of the secondary current and, thus, maintain electrical equilibrium. Consequently, any change in the secondary current should cause a corresponding change in the primary current, while the current of the secondary winding, due to the relatively small value of the component i1r1, has almost no effect on the amplitude and nature of changes over time in the main magnetic flux of the transformer. Therefore, the amplitude of this flow Ft can be considered almost constant. This constancy of Ft is typical for the transformer mode, in which the voltage U1 applied to the terminals of the primary winding is maintained constant.

The simplest is a device consisting of a steel core and two windings (Fig. 1). When an alternating voltage is supplied to the primary winding, an emf of the same frequency is induced in the secondary winding. If you connect some electrical receiver to the secondary winding, then an electric current arises in it and a voltage is established at the secondary terminals of the transformer, which is somewhat less than the EMF and depends to some relatively small extent on the load. The ratio of the primary to secondary voltage (transformation ratio) is approximately equal to the ratio of the number of turns of the primary and secondary windings.

Rice. 1. The principle of the design of a single-phase two-winding transformer. 1 primary winding, 2 secondary winding, 3 core. U1 primary voltage, U2 secondary voltage, I1 primary current, I2 secondary current, F magnetic flux

The simplest symbols of transformers are shown in Fig. 2; For clarity, different windings of the transformer can be represented in different colors, as in the figure.

Rice. 2. Symbol of a transformer in detailed (multilinear) diagrams (a) and in electrical network diagrams (b)

Transformers can be single- or multi-phase, and there can be more than one secondary winding. Electrical networks typically use three-phase transformers with one or two secondary windings. If the primary and secondary voltages are relatively close to each other, then single-winding autotransformers can be used, the circuit diagrams of which are presented in Fig. 3.

Rice. 3. Schematic diagrams step-down (a) and step-up (b) autotransformers

The most important ratings of a transformer are its rated primary and secondary voltages, rated primary and secondary currents, and rated secondary apparent power (rated power). Transformers can be manufactured for both very low power (for example, for microelectronic circuits) and very high power (for example, for high-power power systems), covering a power range from 0.1 mVA to 1000 MVA.

Energy losses in the transformer - due to active resistance windings, copper losses and steel losses caused by eddy currents and hysteresis in the core are usually so small that transformer efficiency is typically above 99%. Despite this, the heat generation in powerful transformers can be so strong that it is necessary to resort to effective ways heat sink. Most often, the active part of the transformer is placed in a tank filled with mineral (transformer) oil, which, if necessary, is supplied with forced air or water cooling. With a power of up to 10 MVA (sometimes higher), dry transformers can also be used, the windings of which are usually filled with epoxy resin. The main advantages of dry-type transformers are higher fire safety and the elimination of leakage of transformer oil, so they can be installed without obstacles in any part of buildings, including on any floor. To measure variable current or voltage (especially in the case of high currents and high voltages), instrument transformers are often used.

The design of a voltage transformer is no different in principle from power transformers, but it operates in a mode close to idle; The transformation coefficient in this case is quite constant. The rated secondary voltage of such transformers is usually 100 V. The secondary winding of the current transformer is ideally short-circuited and the secondary current is then proportional to the primary. The rated secondary current is usually 5 A, but can sometimes be less (eg 1 A). Examples of current transformer symbols are shown in Fig. 4.

Rice. 4. Symbol of a current transformer in expanded diagrams (a) and in single-line diagrams (b)

The first can be considered the induction ring manufactured by Michael Faraday, consisting of an annular steel core and two windings, with the help of which he discovered the phenomenon of electromagnetic induction on August 29, 1831 (Fig. 5). During the fast transient process that occurs when the primary winding connected to a direct current source is turned on or off, a pulse emf is induced in the secondary winding. Such a device may therefore be called a pulse or transient transformer.

Rice. 5. The principle of the transient transformer by Michael Faraday. i1 primary current, i2 secondary current, t time

Based on Faraday's discovery, a physics teacher at Margnooth College near Dublin (Dublin, Ireland), Nicholas Callan (1799–1864), built an induction coil (spark inductor) in 1836, consisting of a chopper and a transformer; This device made it possible to convert direct current into high voltage alternating current and cause long spark discharges. Induction coils began to improve rapidly and were widely used in the study of electrical discharges in the 19th century. These may also include the ignition coils of modern cars. The first alternating current transformer was patented in 1876 by Russian electrical engineer Pavel Yablochkov, who lived in Paris, and used it in the power circuits of his arc lamps. The core of Yablochkov's transformer was a straight bundle of steel wires, as a result of which the magnetic circuit was not closed, like Faraday's, but open, and such a transformer was not used in other installations. In 1885, electrical engineers of the Budapest plant Ganz & Co. Max Deri (172 1854–1938), Otto Titus Blathy (1860–1939) and Karoly Zipernovsky (1853– 1942) manufactured a transformer with a toroidal wire core and at the same time developed an alternating current power distribution system based on the use of these transformers. A transformer with even better properties, the core of which was assembled from E- and I-shaped steel sheets, was created in the same year by the American electrical engineer William Stanley (1858–1916), after which the rapid development of alternating current systems began both in Europe and and in America. The first three-phase transformer was built in 1889 by Mikhail Dolivo-Dobrovolsky.

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 the 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 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. The main function of such transformers is to convert any current value to values ​​that are most convenient for 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.

Measuring current transformers are designed for electrical circuits with high voltage, when there is no possibility of direct connection of 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 the external network is supplied to the power primary winding with a certain number of turns and overcomes its total resistance. 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, the electromotive force is activated by the magnetic flux. 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 latter option is used when connecting measuring instruments to current circuits of 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 external and internal 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 ensure normal operation of the 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. The absolute value of the primary and secondary current differs by exactly 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 due to violations of the electrical resistance of the insulation of the windings, a decrease in their conductivity under the influence of elevated temperatures. Accidental events have a negative impact mechanical influences or poor quality 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.