Types of measuring current transformers. Measuring current transformers - purpose, design, types of designs. Inductive couplings in CT

If it is necessary to control the currents flowing in the electrical network, current and voltage measuring transformers are used. Such devices connected in a special way reduce the measured parameters of the electrical circuit to values ​​​​suitable for their measurement. Thus, the high-current circuit is separated from the low-current circuit. This is necessary so that the measuring or other equipment, which includes the secondary winding of the transformers, does not fail.

Inductive couplings in current transformers (CTs)

According to the basic law of electromagnetic induction, which Faraday substantiated, all voltage transformers (VT) and current transformers (CT) operate on the principle of mutual induction. If you place two windings on one closed magnetic core and connect one of them to an alternating current source, then the changing magnetic flux will cause the appearance of an electromotive force (EMF).

Important! This EMF is called induced. In the second (secondary) winding of the device, as a result of the interaction of magnetic fields, an EMF is also induced, and an electric current will begin to flow.

Features of energy transformation for TT

To understand why current transformers are needed, and how they differ from voltage transformers (VT), you can consider their design. The presence of such devices in electrical circuits is associated with the need to transform: lower or increase voltage or current. Alternating electricity generated by generators at power plants is first subjected to transformation before being transmitted through the power grid.

How the device works

When it became clear what transformation is, it’s time to take a closer look at the operating principle of the current transformer.

Two windings are placed on a closed core (magnetic circuit) assembled from plates. The first coil is connected in series to the load power circuit. The secondary coil is connected to the meters with its terminals. The core is assembled from cold-rolled silicon steel plates.

For your information. Electricity metering is done in exactly this way. Current transformers are included in single-phase and three-phase circuits, which allow readings to be taken for each phase, feeding data to the meter.

When alternating electricity passes through the turns of the first (main) winding, an alternating magnetic flux F1 is formed around it. Flow F1, penetrating all windings of the transformer, induces EMF (E) in them. In this case, E1 and E2 appear. When any load is connected to the secondary winding circuit, electricity will begin to flow through it.

Design Features

What are these transformers made of? What is the difference between a current transformer and a voltage transformer? The answers to these questions can be found in the description of the design features. Current transformers, their purpose and principle of operation, imply the constancy of certain conditions:

• every CT must have more than one winding on its magnetic core;
• windings that are secondary are certainly connected to the load (Rн);
• resistance Rн should not contain deviations from the TT stated in the documents;
• The primary winding is made as a busbar passing through the core or in the form of a coil.

The absence of a load on the secondary winding does not ensure the occurrence of magnetic flux F2 in the core, which has a compensating property. This leads to an increase in the temperature of the core and its melting. Heating occurs because F1 becomes too high.

Resistance deviation Rн affects the measurement error and worsens it. If the resistance in the secondary winding is exceeded, the voltage U2 increases and the CT insulation may fail. A breakdown will occur and the device will fail.

Information. Voltage transformers (VTs) differ from CTs in their method of application and connection diagram. They are connected in parallel and are designed to increase or decrease the voltage, decoupling the power circuit from the control and monitoring circuit. The main operating regulations of the fuel pump are close to the idle mode (idle). This is due to the fact that parallel-connected elements of the control circuit consume low current, and their Rн is large.

Connection diagrams for measuring CTs

Installation of current transformers is carried out according to a certain scheme. It depends on the voltage of the measured network, namely:

• in 3-phase networks with Un up to 1000 V, CTs are built into the circuit of each phase;
• in 3-phase networks with Un 6-10 kV, installation is carried out in two phases (A and C).

In the first option, in electrical installations (EI), where the neutral is solidly grounded, the ends of the secondary windings of the CTs are closed to each other in a “star” circuit.

In the second case, in an electrical device with an isolated neutral, they are connected according to the “partial star” circuit.

Classification of current transformers

The operating principle of the current transformer, as well as the methods of connection and purpose, allow them to be divided according to the following differences:

• purpose;
• type of installation;
• placement method;
• performing the primary winding;
• type of insulation;
• operating voltage;
• number of transformation steps.

In addition, there are other qualities that allow the classification of TT. One of the distinguishing features is the specificity of the design.

According to the design features, CTs differ in:

• single-turn;
• multi-turn;
• optical-electronic.

Each of these types has model types that it is advisable to consider separately.

Coil type CT

These are some of the simplest current transformers. They belong to early CTs, built and promoted on a structure based on a power transformer. Both windings (first and second) are mounted on a frame with insulating properties. Each of them is a coil. This is where the name comes from. In addition to the fact that they are compact and cheap to manufacture, there is a drawback: low discharge voltage due to poor insulation of the coils.

This design allows them to be used only for voltages up to 3 kV. To increase the value of Udispersion, it is necessary to increase the core window and separate the primary winding from the inner surface of the plates. An insulating gasket having a U-shaped form is inserted into the resulting gap.

Pass-through transformer

Distribution devices (distribution devices) with voltages from 6 to 35 kV imply the installation of similar current transformers. This is a multi-turn CT, where the base is a pair of bushings connected to each other in the middle. This assembly allows them to pass through walls and be used in closed switchgears. In this case, there is no need to specifically use the bushing.

The winding, which serves as the primary winding, is laid through the void located inside. The number of turns is taken from the calculation of the required “ampere-turns” for the corresponding accuracy class. Bushings are placed under the flange, which is grounded. The magnetic cores of the secondary windings, covered with a casing, are fixed in their middle.

Attention! The location of the winding terminal for the primary winding is on the upper plane, relative to the grounded flange.

Rod device

This type of device is designed to work with U = 10-20 kV and In = 600 and 1500 A. This CT refers to feed-through single-turn transformers with porcelain insulation. It has a current-carrying rod piercing a porcelain insulator and serves as the primary winding.

Tire device

The following design is intended for installation in complete transformer substations (CTS). They implement the transfer of measurement information to control and measuring instruments (instruments). Signals from similar CTs are also transmitted to protection and control circuits.

Advantages and disadvantages

Each of the listed devices has its own pros and cons. It is preferable to consider them separately: single-turn and multi-turn models.

The advantages of single-turn CTs include:

• simplicity of the device;
• low cost;
• small dimensions;
• resistance to short circuit currents (short circuit).

Here we can add that by changing the cross-section of the current conductor (rod), we achieve a change in thermal stability.

The disadvantage of such models is their low accuracy at low measured currents.

As for multi-turn models, a clear positive point is the presence of a certain number of turns in the primary winding. This made it possible to significantly increase the measurement accuracy class. Negative characteristics include:

• design complexity;
• rise in price;
• susceptibility of the primary winding to interturn overvoltages.

At the same time, this also includes low resistance to short-circuit currents.

Current transformer parameters

Knowing, by definition, that these parts serve for measurements and protective functions, one can guess that their main characteristics will be: KI and accuracy class.

Transformation coefficient KI

Transformer units only perform scaling of electricity parameters, they do not produce it themselves. To determine the amount of scaling, the transformation ratio is used.

The relationship between the amount of current (I) or voltage (U) applied to the input and removed at the output is called the transformation ratio (CT).

In the case of current conversion, we are talking about:

• КI – CT transformation coefficient;
• I1 – input current;
• I2 – output current.

For CTs, a proportional relationship is observed between the primary and secondary currents. This follows from the expressions:

• I1 =I2 / KI;
• I2 = I1 * KI.

Clarification. The nominal Ctr of the CT is displayed as a fractional expression. The numerator is the rated value of the current flowing in the primary coil, the denominator is the rated current value in the secondary electrical winding. It is always greater than one.

Thus, the nominal value of the measured current reflects KI nom. The specified passport data of the part (KI = 65/5) indicates that when 65 A is passed through the primary coil, a current of 5 A will flow in the secondary coil.

When using CTs, the current in the secondary circuit is reduced, which makes it possible to ensure safe operation. The secondary circuit includes not only measuring equipment that records the current value, but also protection or automatic switching systems. In this case KI< 1.

For voltage values, the coefficient formula is different:

The scaling changes (sign) depend on the value of K. When K>1, the transformer increases the supplied electrical quantity, when the value of K<1 он её понижает.

If the inductive coupling between the two windings of the transformer remains unchanged, then the conversion coefficient can be changed by changing the ratio of the number of turns of winding wire in coils W1 and W2. Referring to his formula:

it can be equated to the following form:

• KU – transformation coefficient;
• W2 – number of turns of coil No. 2;
• W1 – number of turns of coil No. 1.

The diameter of the wound wire depends on the amount of current planned to pass through the winding.

Accuracy class

This is the main characteristic of the CT, affecting the metrology of the process. The accuracy class depends on two errors:

• current error (%);
• angular error (min).

The first option, when the actual KId. differs from the nominal KIn coefficient.

The error formula looks like:

f = (I2d – I2n)/ I2n * 100%,

• f – current error;
• I2d – secondary real (real) current;
• I2н – secondary rated current.

The angular error is the angle between the current vectors: primary and secondary. Moreover, the secondary current vector is rotated by 1800.

Attention! These errors are motivated by the influence of magnetizing currents. Accuracy classes are selected from the 0.2 line; 0.2S; 0.5; 0.5S and other values ​​according to GOST 7746-2015.

Current transformer designations

The alphanumeric marking of domestically produced products is deciphered as follows:

• 1 letter T – transformer;
• 2nd letter – model type;
• 3rd letter – isolation.

After the letters, separated by a dash, the following are listed:

• insulation class (kV);
• design by climate zone (letter abbreviation);
• installation category (in numbers);
• transformation coefficient (fraction).

More accurate recognition of CT markings can be found in reference literature or the device passport.

Purpose and application

Based on their operating principle, current transformers are used for use and inclusion in technical and commercial electricity metering units. They are designed for a certain voltage class. When determining the purpose of current transformers, pay attention to Ktr and measurement accuracy class.

Possible faults

Errors in the installation and connection of current transformers, as well as incorrectly selected equipment, cause CT malfunction.

Important! Troubleshooting should begin if the secondary current of the CT is not combined with the primary. Too low a current that does not correspond to the declared ratio indicates damage to the device.

Evidence of a transformer malfunction is:

• crackling and increased noise during operation;
• the appearance of sparks from the winding on the housing or on the terminals;
• smoke or smell of burning insulation;
• excessive heating of device parts.

A faulty device may produce distorted measurement results, which will cause false activation of protective equipment and incorrect metering of electricity. Periodically, at substations, element-by-element (phase-by-phase) verification is carried out with the measurement of currents under load. The calculated values ​​obtained from the measurement data must coincide with the measured values ​​at the CT output. The permissible error is no more than 10%.

Design requirements

When choosing a design, they start from what the transformer is needed for. Why install a busbar or feed-through CT if the voltage with which it will have to work lies in the range from 1 to 3 kV?

The requirements include the following items:

• the selected device must be suitable for the operating conditions and installation location;
• for outdoor use, the transformer terminals must contain protective covers;
• winding terminals must be marked;
• availability of gripping points for lifting heavy vehicles (more than 50 kg);
• grounding sign at the point where the grounding conductor is connected.

All winding contact terminals are made in accordance with the requirements of GOST 10434-82 (for indoor installation) and GOST 21242-75 (for outdoor installation).

Selecting a current transformer for metering devices

The purpose of a commercial measuring transformer is to keep track of electricity. When choosing such models, pay attention to the following:

• Unom TT – 0.66 kV;
• accuracy class - 0.5 S for the market version, for technical control - 1.0;
• I1н – rated primary current.

The transformation ratio depends on the rated primary current.

Not a single electrical substation can operate without current transformers. These devices work to know and take into account the current load. They provide protection for power circuits and provide timely signals about all changes in current strength in the primary circuit. A properly selected CT will serve without any complaints for a long time.

Video

Powerful electrical installations can operate with a voltage of several hundred kilovolts, while the current in them can reach more than ten kiloamperes. Naturally, it is not possible to use conventional instruments to measure quantities of this order. Even if they could be created, they would be quite bulky and expensive.

In addition, when directly connected to a high-voltage alternating current network, the risk of electric shock when servicing the devices increases. The use of measuring current transformers (hereinafter referred to as ICTs) made it possible to get rid of these problems, thanks to which it was possible to expand the capabilities of measuring devices and provide galvanic isolation.

Purpose and device of ITT

The functions of this type of transformer are to reduce the primary current to an acceptable level, which makes it possible to connect unified measuring devices (for example, ammeters or electronic electricity meters), protective systems, etc. In addition, the current transformer provides galvanic isolation between high and low voltage, thereby ensuring the safety of operating personnel. This brief description allows you to understand why these devices are needed. A simplified design of the ITT is presented below.

Design of measuring current transformer

Designations:

1. Primary winding with a certain number of turns (W 1).
2. A closed core made from electrical steel.
3. Secondary winding (W 2 - number of turns).

As can be seen from the figure, coil 1 with terminals L1 and L2 is connected in series to the circuit where the current I 1 is measured. Devices that allow you to set the current value I 2 , relay protection, automation system, etc. are connected to coil 2.

The main area of ​​application of CT is metering electricity consumption and organizing protection systems for various electrical installations.

In a measuring current transformer, it is necessary to have insulation between the coils, the turns of wire in them and the magnetic circuit. In addition, according to PUE standards and safety requirements, it is necessary to ground the secondary circuits, which provides protection in the event of a short circuit between the coils.

You can get more detailed information about the principle of operation of CTs and their classification on our website.

List of main parameters

The technical characteristics of the current transformer are described by the following parameters:

• The rated voltage, as a rule, is indicated in kilovolts in the passport for the device. This value can be from 0.66 to 1150 kV. You can get complete information about the voltage scale in the reference literature.
• The rated current of the primary coil (I 1) is also indicated in the passport. Depending on the design, this parameter can be in the range from 1.0 to 40000.0 A.
• The current on the secondary coil (I 2), its value can be 1.0 A (for ITT with I 1 no more than 4000.0 A) or 5.0 A. Devices with I 2 equal to 2.0 A or 2.50 A.
• Transformation ratio (CT), it shows the ratio of the current between the primary and secondary coils, which can be represented as the formula: CT = I1/I2. The coefficient determined by this formula is usually called real. But for calculations, the nominal CT is also used, in this case the formula will look like: I NOM1 /I NOM2, that is, in this case we operate not with actual, but with nominal values ​​of the current on the first and second coils.

Below, as an example, is the datasheet of the TT-B model.

List of main parameters of the measuring current transformer TT-V

Types of instrument transformer designs

Depending on the design, these devices are divided into the following types:

Designations:

• A – Secondary terminal block.
• B – Protective housing.
• C – Contacts of the primary winding.
• D – Winding (loop or figure eight).

1. Rod, they are also called single-turn. Depending on the design they can be:

Designations:

• A – built-in TT.
• B – insulator of the power input of the substation transformer.
• C – location of the CT installation (shown in section) on the insulator. That is, in this case, the high-voltage input plays the role of the primary winding.

This design option greatly simplifies installation/dismantling.

Explanation of markings

The designation of domestic models is interpreted as follows:

• The first letter in the model name indicates the type of transformer, in our case it will be the letter “T”, indicating that it belongs to a TT.
• The second letter indicates a design feature, for example, the letter “Ш” indicates that this device is a bus device. If the letter “O” is indicated, then this is a reference CT.
• The third letter encrypts the isolation performance.
• The numbers indicate the voltage class (in kV).
• Letter to indicate climatic version according to GOST 15150 69
• CT, indicating the rated current of the primary and secondary windings.

Let us give an example of deciphering the markings of a current transformer.

As you can see, the figure shows the marking TLSh 10UZ 5000/5A, this indicates that we have a current transformer (the first letter T) with cast insulation (L) and a busbar structure (Sh). This device can be used in a network with voltage up to 10 kV. As for the design, the letter “U” indicates that the device was created for use in a temperate climate zone. KT 1000/5 A, indicates the value of the rated current on the first and second windings.

Connection diagrams

The windings of three-phase CTs can be connected in delta or star (see Fig. 8). The first option is used in cases where it is necessary to obtain a higher current strength in the circuit of the second winding or it is necessary to phase shift the current in the secondary coil relative to the primary one. The second connection method is used if it is necessary to monitor the current strength in each phase.

Figure 8. Connection diagram of a three-winding CT with star and delta

If there is an isolated neutral, a circuit can be used to measure the current difference between the two phases (see A in Fig. 9) or a partial star connection (B).

Figure 9. Connection diagram of a CT for a difference of two phases (A) and a partial star (B)

When it is necessary to power the ground fault protection, a circuit is used that allows the currents of all phases to be summed (see A in Fig. 10.). If a current relay is connected to the output of such a circuit, it will not respond to a short circuit between phases, but will definitely work if a ground fault occurs.

Fig. 10. Connections: A – for the sum of currents of all phases, B and C – serial and parallel connection of two-winding CTs

In conclusion, we give two more examples of connecting the secondary windings of a CT to take readings from one phase:

The secondary coils are connected in series (B in Fig. 10), making it possible to measure the total power.

The secondary windings are connected in parallel, which makes it possible to reduce the CT, since the current in these coils is summed, while in the line this indicator remains unchanged.

Choice

When choosing a current transformer, first of all, it is necessary to take into account the rated voltage of the device is not lower than in the network where it will be installed. For example, for a three-phase network with a voltage of 380 V, you can use a CT with a voltage class of 0.66 kV; accordingly, for installations over 1000 V, such devices cannot be installed.

In addition, I NOM CT must be equal to or exceed the maximum current of the installation where the device will be operated.

Let us briefly outline other rules that allow you not to make a mistake when choosing a TT:

• The cross-section of the cable that will connect the CT to the secondary load circuit should not lead to losses in excess of the permissible norm (for example, for accuracy class 0.5, losses should not exceed 0.25%).
• For custody transfer metering systems, devices with a high accuracy class and a low error threshold must be used.
• It is allowed to install current transformers with an increased CT, provided that at maximum load the current will be up to 40% of the rated one.

You can view the norms and rules by which measuring current transformers (including high-voltage ones) are calculated in the PUE (clause 1.5.1.). An example of the calculation is shown in the picture below.

Example of current transformer calculation

As for choosing a manufacturer, we recommend using branded products whose advantages have been proven over time, for example ABB, Schneider Electric b, etc. In this case, you can be sure that the technical data indicated in the passport and the test methodology complied with the standards.

Service

It is necessary to note that, subject to the operating mode and conditions, correctly selected ratings and regular maintenance, the CT will serve for 30 years or more. To do this you need:

• Pay attention to various types of faults; note that most of them can be detected by visual inspection.
• Monitor the load in the primary circuits and prevent overload above the established norm.
• It is necessary to monitor the condition of the primary circuit contacts (if any) and there should be no external signs of damage.
• Equally important is monitoring the condition of the external insulation; in almost half of the cases, its resistance is impaired due to the accumulation of dirt or moisture, which short-circuits the contacts to the ground.
• For oil transformers, check the oil level, its cleanliness, presence of leaks, etc. Maintenance of such installations is practically not very different from other power plants, for example, NDE capacitive transformers; the difference lies in small technical details.
• CT verification must be carried out in accordance with current standards (GOST 8.217 2003).
• If a malfunction is detected, the device is replaced. The damaged CT is sent for repair, which is carried out by specialized services.

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 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 the phenomenon discovered by Faraday.

Purpose of transformers

Different types of transformers are used in almost all power supply circuits for electrical devices and when transmitting electricity over long distances.

Power plants produce relatively low voltage current - 220 , 380 , 660 B. Transformers, increasing the voltage to values ​​of the order thousand kilovolts, make it possible to significantly reduce losses when transmitting electricity over long distances, and at the same time reduce the cross-sectional area of ​​power transmission line wires.

Immediately before it reaches the consumer (for example, a regular household outlet), the current passes through a step-down transformer. This is how we get what we are used to 220 Volt.

The most common type of transformers 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.

Transformer operating principle

Transformers are single-phase and multi-phase, with one, two or more windings. Let's consider the circuit and principle of operation of a transformer using the example of a simple single-phase transformer.

What does a transformer consist of? In the simplest case, from one metal core and two windings . The windings are not electrically connected to one another and are insulated wires.

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

When a transformer is connected to an alternating current source, an alternating current of magnitude flows in the turns of its primary winding. I1 . This creates a magnetic flux F , which penetrates 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 no-load mode. Accordingly, if the secondary winding is connected to any consumer, current flows through it I2 , arising under the influence of EMF.

The magnitude of the EMF arising 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 corresponding windings.

By selecting the number of turns on the windings, you can increase or decrease the voltage at the current consumer from the secondary winding.

Ideal transformer

An ideal transformer is a transformer in which there are no energy losses. 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 where heat loss 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 transformer operation. For small power transformers this does not pose a problem, and all the heat goes into the environment - natural air cooling is used. Such transformers are called dry.

In more powerful transformers, air cooling is not enough, and oil cooling is used. In this case, the transformer is placed in a tank with mineral oil, through which heat is transferred to the walls of the tank 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 they might seem at first glance - after all, we briefly examined the principle of operation of a transformer. An electrical engineering test with problems on calculating a transformer can suddenly become a real problem. always ready to help in solving any problems with your studies! Contact Zaochnik and learn easily!

A measuring current transformer is a special narrow direction device that is designed to measure alternating current and control it. Most often used in relay protection systems (automation) and measuring instruments. Its use is necessary when direct connection of the measuring device to an electrical network with alternating voltage is impossible or unsafe for the personnel servicing it. And also for organizing galvanic isolation of primary power circuits from measuring circuits. The calculation and selection of the measuring current transformer is carried out in such a way that changes in the signal shape are reduced to zero, and the impact on the controlled power circuit is minimal.

Purpose of instrument transformers

The main function of this measuring device is to display current changes as proportionally as possible. Current transformers guarantee complete measurement safety by separating the measuring circuits from the primary circuits with dangerously high voltages, which most often amount to thousands of volts. The requirements for their accuracy class are very high, since the operation of expensive, powerful equipment depends on it.

Operating principle and design

Instrument transformers are produced with two or more groups of secondary windings. The first is used to turn on relay protection and alarm devices. And the other, with a higher accuracy class, for connecting precision measurement and accounting devices. They are placed on a specially manufactured ferromagnetic core, which is made from sheets of special electrical steel of fairly thin thickness. The primary winding is directly connected in series to the network being measured, and the coils of various measuring instruments, most often ammeters and electricity meters, are connected to the secondary winding.

In current transformers, as in most other such electromagnetic devices, the magnitude of the primary current is greater than the secondary. The primary winding is made of wire of different sections or buses, depending on the rated current value. In current transformers of 500 A and above, the primary winding is most often made of 1 single turn. It can be in the form of a straight busbar made of copper or aluminum, which passes through a special core window. The accuracy of measurements of any measuring transformer is characterized by the error in the value of the transformation ratio. In order not to confuse the ends, markings must be applied to them.
Emergency unsafe operation is associated with a break in the secondary circuit of the CT when the primary is connected to the circuit, this leads to very strong magnetization of the core and even if the secondary winding breaks. Therefore, when switched on without load, the secondary windings are short-circuited.
According to the accuracy class, all measuring CTs are divided into several levels. Particularly accurate, they are called laboratory and have accuracy classes of no more than 0.01–0.05;

Connection diagrams

The connection diagrams presented below enable personnel to monitor the currents in each phase.

For the safety of personnel, low-voltage measuring equipment and instruments, one terminal of the secondary winding, as well as the housing, is grounded.

Classification and selection

By design and design, current transformers used in measuring circuits are divided into:

When choosing a current transformer, the main thing to know is that when the rated current flows through the primary winding, its secondary winding, which is closed to the measuring device, will necessarily be 5 A. That is, if you need to measure current circuits where its calculated operating value will be approximately 200 A This means that when installing a 200/5 measuring transformer, the device will constantly show the upper measurement limits, this is inconvenient. It is necessary that the operating limits be approximately in the middle of the scale, so in this particular case you need to choose a 400/5 current transformer. This means that at 200 A of the rated current of the equipment on the secondary winding there will be 2.5 A and the device will display this value with a margin in the direction of increase or decrease. That is, even with changes in the controlled circuit, it will be visible to what extent this electrical equipment has left the normal operating mode.

Here are the main values ​​that you should pay attention to when choosing measuring current transformers:

1. Rated and maximum voltage in the primary winding;
2. Rated value of primary current;
3. AC frequency;
4. The accuracy class is different for measurement and protection circuits.

Maintenance

The operation of instrument transformers is not a very complex and time-consuming process. The actions of the personnel consist mainly of monitoring the serviceability of its secondary circuits, the presence of protective grounding and the readings of control devices, as well as meters. Inspection is most often carried out visually; due to the danger of human injury from high voltage, entry beyond the fences where transformers are installed is strictly prohibited. However, this applies to a greater extent to systems with voltages above 1000 Volts. For low-voltage circuits, visual inspection for heating of connections, as well as corrosion of contact terminals, is an integral job of electrical personnel. The most commonly used device for measuring current in 0.4 kV circuits is a current clamp. Since when calculating and developing starting equipment, stationary transformers are very rarely used for measurement.

In any case, you need to pay attention and take measures to eliminate detected defects such as:

1. Detection of cracks in insulators and porcelain dielectric elements;
2. Poor condition of reinforced seams;
3. Crackling and discharges inside the device;
4. Lack of grounding of the frame or secondary winding.

When carrying out maintenance of instrument transformers, on the switchboards where the devices are installed, you need to look not only at the readings of the devices, but also at the contact connections of the wires that are connected to them. By the way, their cross-section should not be less than 2.5 mm² for copper wires, and 4 mm² for aluminum.

Testing instrument transformers

Testing of instrument transformers comes down to measuring the insulation resistance and transformation ratio, which is determined according to the following diagram.

In this case, a current of at least 20% of the rated one is supplied to the primary winding from a special load transformer or autotransformer. As is known, the transformation ratio will be equal to the ratio of the current in the primary winding to the current in the secondary. After which this value is compared with the nominal value. If the transformer has several secondary windings, then it is necessary to check each one. And we also must not forget about having the correct labeling.

The choice of current transformer required, as well as their test characteristics, are determined in laboratory conditions by special highly qualified electrical personnel, where the corresponding document is issued based on its results.

A current transformer (CT) is a transformer in which, under normal operating conditions, the secondary current is practically proportional to the primary current and, when switched on correctly, is shifted relative to it by an angle close to zero.

The primary winding of the current transformer is connected in series to the circuit (in the current conductor section), and the secondary winding is closed to a certain load (measuring instruments and relays), ensuring the passage of a current proportional to the current in the primary winding.

In high voltage current transformers, the primary winding is isolated from the secondary winding (from ground) at full operating voltage. One end of the secondary winding is usually grounded. Therefore, it has a potential close to the potential of the earth.

Current transformers, according to their purpose, are divided into current transformers for measurements and current transformers for protection. In some cases, these functions are combined in one current transformer.

Current transformers for measurements are designed to transmit measurement information to measuring instruments. They are installed in high voltage circuits or in circuits with high current, that is, in circuits in which direct connection of measuring instruments is not possible. Ammeters, current windings of wattmeters, meters and similar devices are connected to the secondary winding of the CT for measurements. Thus, the current transformer for measurements provides:
1) conversion of alternating current of any value into alternating current of an acceptable value for direct measurement using standard measuring instruments;
2) isolation of measuring instruments to which maintenance personnel have access from the high voltage circuit.

Current transformers for protection are designed to transmit measurement information to protection and control devices. Accordingly, the current transformer for protection provides:
1) conversion of alternating current of any value into alternating current acceptable in value for powering relay protection devices;
2) isolating relays that are accessible to maintenance personnel from the high voltage circuit.

The use of current transformers in high voltage installations is necessary even in cases where current reduction for measuring instruments or relays is not required.

Classification of current transformers

All current transformers - both for measurements and for protection - can be classified according to the following main characteristics.

By type of installation: current transformers for outdoor operation (location category 1 according to GOST 15150-69); for work in enclosed spaces (according to GOST 15150-69); for installation into internal cavities of electrical equipment (category in accordance with Table 1); for special installations (in mines, on ships, electric locomotives, etc.).

Table 1

By installation method: feed-through current transformers intended for use as input and installed in openings of walls, ceilings or in metal structures; supporting ones, intended for installation on a supporting plane; built-in, that is, intended for installation in the internal cavities of electrical equipment.

By the number of transformation coefficients: with one transformation ratio; with several transformation ratios obtained by changing the number of turns of the primary or secondary winding, or both windings, or by using several secondary windings with different numbers of turns corresponding to different values ​​of the rated current.

According to the number of transformation steps: single stage; cascade (multi-stage), that is, with several stages of current transformation.

To complete the primary winding: single-turn; multi-turn.

Single turn current transformers

Single-turn current transformers (Figure 1) have two varieties: without their own primary winding; with its own primary winding. Single-turn CTs that do not have their own primary winding are built-in, busbar or detachable.

Built-in current transformer 1 (Figure 1) is a magnetic circuit with a secondary winding wound on it and does not have its own primary winding. Its role is played by the current-carrying rod of the bushing. This current transformer has no insulating elements between the primary and secondary windings. Their role is performed by the insulation of the bushing.

In a bus current transformer 1 The role of the primary winding is performed by one or several busbars of the switchgear, passed during installation through the internal cavity of the bushing. The latter isolates the primary winding from the secondary.

Figure 1. Current transformer circuit.
–––––– CT’s own primary winding; – – – – current-carrying rod of the bushing (bus)

Split current transformer 2 also does not have its own primary winding. Its magnetic core consists of two parts, held together by bolts. It can open and close around the current carrying conductor, which is the primary winding of this CT. The insulation between the primary and secondary windings is applied to the magnetic circuit with the secondary winding.

Single-turn CTs, which have their own primary winding, are made with a rod primary winding or with a U-shaped one.
Current transformer 3 has a primary winding in the form of a rod of circular or rectangular cross-section, fixed in a bushing.

Transformer 4 has a U-shaped primary winding, designed in such a way that almost all of the internal insulation of the CT is superimposed on it.

Multi-turn current transformers

Multi-turn current transformers (Figure 1) are manufactured with a coil primary winding placed on a magnetic core; with loop primary winding 5 , consisting of several turns; with link primary winding 6 , made in such a way that the internal insulation of the current transformer is structurally distributed between the primary and secondary windings, and the relative position of the windings resembles chain links; with an eye-shaped primary winding designed in such a way that the internal insulation of the current transformer is applied mainly only to the eye-shaped primary winding.

By the type of insulation between the primary and secondary windings CTs are made with solid (porcelain, cast insulation, extruded insulation, and so on); with viscous (pouring compounds); with combined (paper-oil, capacitor type) or gaseous (air, SF6) insulation.

Based on the principle of current conversion CTs are divided into electromagnetic and optical-electronic.

Main parameters and characteristics of current transformers

The main parameters and characteristics of the current transformer in accordance with GOST 7746-2001 are:

1. Rated voltage – the effective value of the line voltage at which the CT is intended to operate, indicated in the current transformer data sheet. For domestic transformers, the rated voltage scale, kV, is adopted:

0,66; 6; 10; 15; 20; 24; 27; 35; 110; 150; 220; 330; 500; 750; 1150.

2. Rated primary current I 1n – current indicated in the CT rating table, passing through the primary winding, at which continuous operation of the CT is provided. For domestic CTs, the following scale of rated primary currents, A, is adopted:

1; 5; 10; 15; 20; 30; 40; 50; 75; 80; 100; 150; 200; 300; 400; 500; 600; 750; 800; 1000; 1200; 1500; 2000; 3000; 4000; 5000; 6000; 8000; 10000; 12000; 14000; 16000; 18000; 20000; 25000; 28000; 30000; 35000; 40000.

In current transformers intended for completing turbo and hydrogen generators, rated current values ​​over 10,000 A are recommended.

Current transformers designed for rated primary current 15; thirty; 75; 150; 300; 600; 750; 1200; 1500; 3000 and 6000 A, must allow an unlimited time for the passage of the largest operating primary current, equal to 16, respectively; 32; 80; 160; 320; 630; 800; 1250; 1600; 3200 and 6300 A. In other cases, the highest primary current is equal to the rated primary current.

3. Rated secondary current I 2n – current indicated in the CT rating table passing through the secondary winding. The rated secondary current is assumed to be 1, 2 or 5 A.

2n corresponds to the impedance of its external secondary circuit, expressed in ohms, indicating the power factor. A secondary load can also be characterized by the total power in volt-amperes it draws at a given power factor and secondary current rating.

A secondary load with a power factor cosφ 2 = 0.8, at which the established accuracy class of the CT or the maximum multiple of the primary current relative to its rated value is guaranteed, is called the rated secondary load of the CT z 2n.no.

For domestic current transformers the following values ​​of rated secondary load are established: S 2n.nom, expressed in volt-amperes, with a power factor cosφ 2 = 0.8:

3; 5; 10; 15; 20; 25; 30; 50; 60; 75; 100.

Corresponding secondary load ratings z 2n.nom (in ohms) are determined by the expression:

z 2n.nom = S 2n.nom / I 2 2nom.

5. The CT transformation ratio is equal to the ratio of the primary current to the secondary current.

Two terms are used in current transformer calculations: actual transformation ratio n and rated transformation ratio n n. Under actual transformation ratio n refers to the ratio of the actual primary current to the actual secondary current. Under rated power factor n n refers to the ratio of the rated primary current to the rated secondary current.

6. CT resistance to mechanical and thermal influences is characterized by electrodynamic resistance current and thermal resistance current.

Electrodynamic withstand current I d is equal to the greatest amplitude of the short circuit current for the entire duration of its flow, which the CT can withstand without damage preventing its further proper operation. Current I d characterizes the ability of a CT to withstand the mechanical (electrodynamic) effects of short circuit current. Electrodynamic resistance can also be characterized by the multiplicity K d, which is the ratio of the electrodynamic resistance current to the amplitude of the rated primary current. Electrodynamic resistance requirements do not apply to busbar, built-in and detachable CTs.

Thermal current I tt is equal to the highest effective value of the short circuit current over the period t t, which the CT can withstand during this period of time without heating the current-carrying parts to temperatures exceeding those permissible for short-circuit currents, and without damage that prevents its further operation.

Thermal resistance characterizes the ability of a CT to withstand the thermal effects of short circuit current. To judge the thermal resistance of a CT, it is necessary to know not only the values ​​of the current passing through the transformer, but also the time of its passage or, in other words, to know the total amount of heat generated, which is proportional to the product of the square of the current I tt and the time of its passage t t. This time, in turn, depends on the parameters of the network in which the TT is installed, and varies from one to several seconds.

Thermal resistance can be characterized by a factor of K t of thermal resistance current, which is the ratio of the thermal resistance current to the effective value of the rated primary current.

In accordance with GOST 7746-2001, the following thermal resistance currents are established for domestic current transformers:
a) two seconds I 2t (or its multiple K 2t in relation to the rated primary current) for current transformers with rated voltages of 330 kV and above;
b) three seconds I 3t (or its multiple K 3t in relation to the rated primary current) for current transformers for rated voltages up to 220 kV inclusive.

Time t t of thermal stability current flow may be less than the specified values ​​and must be set in the technical specifications for a specific type of CT.

The ratio between the currents of electrodynamic and thermal resistance must be observed

I d ≥ 1.8 × √2 × I T

The temperature of the current-carrying parts of the CT during the passage of thermal resistance current should not exceed: 200°C for live parts made of aluminum; 250°C for live parts made of copper and its alloys, and 300°C for live parts made of copper and its alloys not in contact with organic insulation or oil. When determining the indicated temperature values, one should proceed from its initial values ​​corresponding to long-term operation of the current transformer at the rated current.

The values ​​of electrodynamic and thermal resistance currents are not standardized by the state standard. However, they must comply with the electrodynamic and thermal resistance of other high-voltage devices installed in the same circuit with the current transformer. Table 2 shows practical data on the dynamic and thermal resistance of domestic current transformers.

Table 2

Data on the electrodynamic and thermal resistance of some types of domestic current transformers

Current transformer	Rated primary current, A	Multiplicity
		electrodynamic TO d	Thermal TO T
Single-turn feed-through: normal performance Enhanced performance	up to 600 1000 1500 up to 600 1000	160 – 170 100 – 110 60 – 70 150 – 170 100 – 110	80 80 80 120 – 140 120 – 140
Tire	2000 – 6000	250 – 300	–
Feedthrough multi-turn: normal performance enhanced performance	5 – 300 5 – 300	45 – 250 90 – 500	70 – 80 100 – 250
Support outdoor installation: with link winding with ring winding	up to 2000 up to 2000	60 – 150 80 – 100	60 – 150 30 – 45

7. The mechanical load is determined by the wind pressure at a speed of 40 m/s on the surface of the current transformer and the tension of the supply wires (in the horizontal direction in the plane of the primary winding terminals), which must be no less than:
500 N (50 kgf) – for transformers with rated voltage up to 35 kV inclusive;
1000 N (100 kgf) – for transformers with a rated voltage of 110 – 220 kV;
1500 N (150 kgf) – for transformers with a rated voltage of 330 kV and above.

These are the main technical parameters and characteristics of current transformers. When designing a CT, in addition to these parameters, the following design requirements must be taken into account:

1. Contact terminals of the terminals of the primary winding of current transformers must be made taking into account the requirements of GOST 10434-82, and for outdoor current transformers - taking into account, in addition, the requirements of GOST 21242-75. Contact terminals of the secondary windings must be made taking into account the requirements of GOST 10434-82. The contact clamps of the secondary windings of built-in current transformers can be located on the structural elements of the device in which the current transformer is built. In outdoor current transformers, the output terminals of the secondary winding must be in special boxes that reliably protect them from precipitation.

The designation of the lead ends of the primary and secondary windings in accordance with GOST 7746-2001 must be made in accordance with Table 3. The linear leads of the primary winding are indicated by symbols L 1 and L 2, which must be applied so that when the current in the primary winding is directed from L 1 and N 1 respectively to TO i and L 2 secondary current passed through the external circuit (devices) from AND 1 to AND 2 .

Table 3

Designations of the lead ends of the primary and secondary windings

2. An oil-filled current transformer must have an oil conservator (compensator) and an oil level indicator. The capacity of the oil conservator must ensure the constant presence of oil in it in all modes of operation of the current transformer - from the off state to the normalized current load - and with fluctuations in ambient temperature established for a given climatic region.

In current transformers with rated voltages of 330 kV and more, oil protection from moisture must be provided, for example, by means of bellows. It is advisable to provide the same protection in current transformers for lower voltages.

3. The dimensions of the oil level indicator must be such that maintenance personnel can observe the oil level in the current transformer from a safe distance.

4. Current transformers weighing more than 50 kg must have lifting devices. If such devices cannot be made, then the manufacturer must indicate in the instructions where to grip the current transformers when lifting.

5. Current transformers, in which the voltage amplitude on the open secondary winding at a rated current in the primary winding exceeds 350 V, must have the inscription: “Attention! Danger! There is high voltage on the open winding.”

6. Current transformers, except built-in ones, must have a contact pad for connecting a grounding conductor and a grounding clamp in accordance with the requirements of GOST 21130-75 and GOST 12.2.007.3-75. A grounding sign in accordance with GOST 21130-75 must be installed near the grounding clamp. The specified requirements do not apply to CTs with a body made of cast resin or plastic that do not have metal parts that must be grounded, as well as to CTs that are not subject to grounding in accordance with GOST 12.2.007.0-75.