Rated primary and secondary voltages

The equations of the electrical state of the transformer are written according to the second Kirchhoff's law for the equivalent circuits of the primary and secondary windings of the transformer (Fig. 4.5):

where R 1, X 1, R 2, X 2 are respectively the active and reactive resistances of the dissipation of the primary and secondary windings of the transformer.

Rated data of the transformer

The rated parameters of the transformer include the rated power S n, voltage

and

and currents of the primary and secondary windings .

The rated power of the transformer S n is the value of the total power indicated in the factory passport, to which the transformer can be continuously loaded under the rated conditions of the installation site and the cooling medium at the rated frequency and voltage.

Rated voltage of windings

and

- these are the voltages of the primary and secondary windings at the no-load of the transformer.

Transformer ratio of two-winding transformer is the ratio of the rated voltages of the high and low voltage windings.

.

The rated currents of the transformer are the values ​​of the currents in the windings at which long-term normal operation of the transformer is allowed. The rated current of any transformer winding determines its rated power and rated voltage.

,

.

Efficiency of the transformer.

Power losses in the transformer

Net power of the transformer

,

where φ 2 is the phase angle between U 2 and I 2, which depends on the nature of the load of the transformer.

Power consumed by the transformer from the network

.

.

Power losses add up

,

where

- magnetic losses in the steel core of the transformer spent on magnetization reversal of the core (losses for hysteresis) and eddy currents, the power of these losses depends on the frequency and amplitude of magnetic induction in the magnetic core and the material from which it is made; at a constant effective value of the voltage of the primary winding, the steel losses are constant and do not depend on the load, therefore they are called constant losses; to reduce losses due to magnetization reversal, transformer cores are made of electrical steel, which has a narrow hysteresis loop; to reduce eddy current losses, the transformer cores are made of thin sheets of electrical steel isolated from each other with a varnish film;

- heat losses in copper windings, which depend on currents and are therefore called variable losses; copper losses are proportional to the square of the load factor

.

Transformer efficiency

.

Transformer operating modes

Idle mode . The idle running of the transformer is understood as such a mode of operation in which voltage is applied to the terminals of the primary winding, and the secondary winding is open, current I 2 = 0 (Fig. 4.6). At the input of the transformer, a voltage is set equal to the rated voltage of the primary winding U 1 = U 1n and U 1, I 1x, cosφ 1x, U 2x are measured.

According to this experiment, the transformation ratio k is determined; rated no-load current I 1xn; rated power of no-load losses Р 10, equal to power losses in the steel of the core Р SN at rated voltage.

At idle speed I 2 = 0 and

, so

and U 2x = E 2.

Consequently,

.

In no-load mode, the useful power of the transformer is P 2 = 0, so the power P 1x consumed in the network is completely used to compensate for losses

P 1x = ΔP s + ΔP m 1 = U 1 I 1x cosφ 1x,

where ΔP with - power losses in the steel of the core from hysteresis and eddy currents; ΔP m1 - power loss in copper of the primary winding; φ 1x is the angle of displacement between the voltage and current of the primary winding U 1 and I 1x.

Primary copper loss

,

then the losses in steel can be easily determined as

ΔP s = P 1 x - ΔP m1 = U 1 I 1 x cosφ 1 x -

.

Since the no-load current I 1x is very small, the power

is negligible and negligible. Therefore, in this case, you can take P 1x = ΔP s. Since the voltage of the primary winding is equal to the nominal one, then P 1x = ΔP SN = P 10. The values ​​of I 1x and P 10 judge the quality of the steel of the core and the quality of its assembly.

R hedgehog short circuit ... A distinction is made between a sudden (emergency) short circuit of a transformer that occurs under operating conditions and a laboratory short circuit during testing. A sudden short circuit occurs with a short circuit of the secondary winding (z n = 0, U 2 = 0), when the primary winding is connected Rated voltage U 1n. This is accompanied by a sharp current surge up to the value of I kz = (20-40) I 1n.

When performing a laboratory short-circuit experiment, the secondary winding of the transformer is short-circuited (Figure 4.7).

A voltage U 1k is set at the input of the transformer, at which the currents of the primary and secondary windings become equal to the nominal I 1 = I 1n and I 2 = I 2n. When U 1 = U 1k measure U 1k, I 1k, cosφ 1k.

The rated currents of a single-phase transformer are calculated based on the formula

where S n is the rated power of the transformer according to the passport data.

The voltage U 1k is called the short-circuit voltage, it is usually expressed as a percentage of U 1n and is denoted

According to the experience, the short-circuit voltage U 1k, the active and reactive components of the short-circuit voltage U ka and U cr, the rated power of short-circuit losses P kn (power losses in the copper of the windings at I 1 = I 1n and I 2 = I 2n) are determined.

In the event of a short circuit, the useful power of the transformer P 2 = 0. Therefore, the power consumed by it from the network in this mode is completely used to compensate for losses

P 1 x = ΔP s + ΔP m1

Р 1к = ΔР c + ΔР ml = U 1 I 1к cosφ 1к where ΔР ml is the power losses in copper of the primary and secondary windings at rated currents I 1n, I 2n:

Then the losses in steel are determined as

ΔP с = P 1к - ΔP ml = U 1 I 1 x cosφ 1 x -

-

.

Since the voltage

U 1n is very small, then the power losses in steel

in this experiment will be negligible and can be neglected. Therefore, in this case, we can take P 1k = ΔP mn = P kn.

Operation of the transformer under load ... To study the operation of the transformer in this mode, a load is connected to the output terminals of the transformer (Fig. 4.8), consisting of several resistors connected in parallel. A sinusoidal voltage U 1 = U 1n = const is supplied to the primary winding of the transformer at f = f n = const, its load I 2 is changed from I 2 = 0 to I 2 = 1.2 I 2n and U 1, I 1, cosφ 1, I 2, U 2.

H The rated currents of a single-phase transformer are calculated based on the formula

where S n is the rated full power of the transformer according to the passport data.

Measurement data determine net power transformer P 2, power consumed from the network P 1, total power losses ΔР Σ, power losses in steel ΔР с and copper ΔР m and build the characteristics I 1, P 1, U 2, , cosφ 1 = f (P 2).

The power of the transformer is determined

,

,

since the load of the transformer is purely active, cosφ 2 = 1.

Total power losses of the transformer

ΔP Σ = P 1 -P 2.

Power losses in copper of transformer windings

,

where ΔP SN is the nominal power loss in the steel of the core (at U 1 = U 1n).

Efficiency d. transformer:

.

In the nominal operating mode, the constant losses in the steel of the core

denote P 10, variable losses in copper windings

denote P kn, then

.

The characteristics of the transformer during its operation under load are as shown in Fig. 4.9.

External characteristic of the transformer

In the practice of operating a transformer, its external characteristic is often used, which is understood as the dependence of the output voltage on the load, i.e.

or

at constant primary voltage

and network frequency

with a constant nature of the load

.

The secondary voltage U 2 at load differs from the open circuit voltage by the amount of voltage loss, which depends on the size of the load.

The voltage U 2 at any load is calculated by the formula

where U 2x = U 2n - voltage at the terminals of the secondary winding at no-load; - the relative change in the secondary voltage (in percent).

T Since the change in voltage is proportional to the load current , then the external characteristic is practically a straight line (Fig. 4.10).

As you know, to construct a linear relationship, it is enough to define two points on the plane:

For power transformers with a secondary winding current equal to the rated I 2n, the ratio

has an order of 5-10%. Therefore, transformers are designed in such a way that their open circuit voltage

was 5% more than the rated voltage of the receiver.

Page 2 of 4

3. Basic technical data, design features voltage transformers.

Transformers with rated primary voltage up to 110 kV are manufactured in accuracy classes for the main secondary winding 0.2; 0.5; 1.0 and 3.0. The accuracy class of the secondary additional winding of all transformers is 3.0.
The rated winding voltages, power limits and power ratings for each accuracy class of voltage transformers are shown in Table 1.
Transformers with a rated voltage of the secondary additional winding of 100: 3 V are intended for operation in networks with an isolated neutral, and with a rated voltage of 100 V - in networks with an effectively grounded neutral.
The errors of transformers, depending on the accuracy classes, satisfy the standards given in Table 2, provided that:
1. power supply frequency (50 ± 0.5) Hz;
2. primary voltage - from 80 to 120% of the nominal value;
3. load power of windings at rated voltage - from 25 to 100% of rated value;
4. active power factor - inductive load of the secondary winding equal to 0.8.

Table 1 - Specifications voltage transformers.

Table 2 - Errors of voltage transformers.

The most widespread are single-phase transformers manufactured for operating voltages from 380 V to 500 kV. Three-phase voltage transformers are also widely used, which are available for operating voltages up to 18 kV.
Single-phase and three-phase voltage transformers can have one or two secondary windings.
Phase voltage is applied to the primary winding of the voltage transformer with two secondary windings, connected to the phase-to-earth voltage in normal mode. In the event of an earth fault in a network with an isolated neutral, the phase-to-earth voltage may rise to line-to-line. Therefore, voltage transformers with two secondary windings, intended for use in a network with an isolated neutral and having a rated voltage equal to the phase voltage of the network, are designed for long-term operation under line voltage.
Voltage transformers with two secondary windings, produced for operation in a network with a grounded neutral, must withstand an increase in the primary phase voltage up to 1.5 UF.NOM for 30 s without damage.
The design dimensions and weight of voltage transformers are not determined by power, as in power transformers, but mainly by the volume of insulation of the primary winding and the dimensions of its high voltage terminals. This is due to the fact that at low power of the voltage transformer, which operates, as a rule, in a mode close to no-load, the volume of high-voltage insulation significantly exceeds the volume of copper in the primary winding required in terms of power. To ensure the necessary mechanical strength of the primary winding, it is necessary to overestimate the cross-section of its wire. An increase in the volume of the high voltage winding against the required power, naturally, causes an increase in the size of the magnetic circuit. As a result, the size and weight of a voltage transformer designed for a higher voltage is always larger than a transformer of the same design and power with a lower rated voltage of the primary winding.
To reduce the size and weight of 110 kV and higher voltage transformers, their cascade (step) design is used. In this case, the operating voltage is distributed between the stages and the isolation of each of them is performed at a lower voltage. For the same purpose, at high voltage, voltage transformers of 10-15 kV are used, which are switched on through a capacitive voltage divider.
Voltage transformers with rated voltage from 380 V to 6 kV are made with dry insulation (windings are made with PEL wire and impregnated with asphalt varnish). 10-500 kV voltage transformers have oil insulation (the magnetic circuit is immersed in transformer oil). There are also versions of voltage transformers for 2-6 kV with oil insulation and for 6-24 kV with dry (cast) insulation.
To reduce the effect of atmospheric overvoltages on the turns of the upper (input) layers of the primary winding, they are protected in all voltage transformers of 3 kV and above by electrostatic screens connected to the line inputs. The screen is made in the form of a metal strip covering the winding with a small gap between its edges (to avoid the formation of a short-circuited turn).
Single phase voltage transformers. Isolation of the primary winding and its both terminals is performed at full operating voltage only for transformers with one secondary winding, which can be switched to phase-to-phase voltage. Voltage transformers with two secondary windings, connected to a phase-to-earth voltage, have only one primary terminal, designed for the full operating voltage; the other end is led out through the low voltage input. The section of the primary winding close to the grounded terminal is usually made with reduced insulation relative to the ground and the secondary winding.

Three-phase voltage transformers. Figure 1 shows a diagram of a voltage transformer (with one secondary winding on each phase) with a three-rod magnetic circuit. The primary windings (terminals A, B, C) are connected to a star, due to which a phase voltage is applied to each of them. The secondary windings are also connected to a star, and their beginnings are brought out to terminals a, b, c, and the neutral to terminals 0.
In fig. 2 shows a diagram of a three-phase voltage transformer with two secondary windings on each phase. The main secondary windings are connected to a star and have terminals a, b, c, 0. Additional windings of all three phases are connected in series (as in Fig. 5), and the 3Uo circuit is connected to the terminals АД, хД. The zero point of the primary windings must be grounded to ensure the operation of the earth fault signaling relay switched on at voltage 3U0.
Transformers with two secondary windings are made on five-rod magnetic circuits (Fig. 2). The outer rods, free from windings, are designed to close the magnetic flux of asymmetry, proportional to the voltage 3U0 and arising from single-phase ground faults, when the primary winding of one of the phases is short-circuited and, as a result, there is no magnetic flux in its rod, and the magnetic flux in the other two rods increases in time.
When used instead of a five-rod three-rod magnetic circuit, the magnetic flux of asymmetry could be closed only through the air and through the transformer casing, that is, along a path with high magnetic resistance, which would lead to a significant increase in the magnetizing currents of undamaged phases and dangerous overheating of their primary windings. Therefore, in order to avoid damage to transformers with three-rod magnetic circuits, grounding of the zero point of their primary windings is not allowed. Their primary and secondary windings are made for phase voltage; zero of the primary winding is not displayed.

Cascade voltage transformers... The principle of implementation is illustrated by the diagram of a transformer consisting of two stages (I and II), shown in Fig. 3. Each stage is a transformer with a rated voltage equal to half the operating voltage, which is applied to the terminals A and X of the HV winding. The transformer of each of the cascades is placed in a porcelain casing filled with transformer oil, and the casing of the first cascade is installed directly on the casing of the second, as a result of which the high voltage input A is double insulated with respect to the ground.

The core of the first stage is connected to the end of the HV winding, which makes it possible to isolate it by half the operating voltage with attenuation in the layers closest to the end.
The low voltage secondary winding with terminals a, x is located on the grounded core of the lower second stage.
To distribute the secondary load given by the LV winding between the transformers of the lower and upper stages, each of them has connecting windings P, connected to each other. For the first stage, the P winding is secondary, and for the second - an additional primary. Due to the presence of connecting windings, the load is divided in half between the stages. Half of the load is transformed into the LV winding from the HV winding, and the other half from the P winding.
Voltage transformers with two secondary windings are intended not only for supplying measuring instruments and relays, but also for operation in an earth fault signaling device in a network with an isolated neutral or protection against earth faults in a network with a grounded neutral.

A diagram of a voltage transformer with two secondary windings is shown in Fig. 4. The terminals of the second (additional) winding, used for signaling or protection in case of earth faults, are marked as hell and xd. In fig. 5 shows a diagram of the inclusion of three such voltage transformers in a three-phase network. The primary and main secondary windings are star-connected. The neutral of the primary winding is grounded. Three phases and zero can be applied to measuring instruments and relays from the main secondary windings. Additional secondary windings are open delta connected. From them, the sum of the phase voltage vectors of all three phases is fed to the signaling or protection devices. During normal operation of the network in which the voltage transformer is connected, this amount is zero. This can be seen from the vector diagrams in Fig. 6, where UА, UВ and UС are vectors of phase voltages applied to the primary windings.

Under real conditions, there is usually a negligible unbalance voltage at the output of an open triangle, not exceeding 2-3% of the nominal voltage. This imbalance is created by the always existing insignificant asymmetry of the secondary phase voltages and a small deviation of the shape of their curve from the sinusoid. The voltage that ensures the operation of the relays connected to the open delta circuit occurs only in case of ground faults on the side of the primary winding of the voltage transformer. In this case, the vector sum of the phase voltages is not zero and, according to the method of symmetrical components, is triple zero sequence voltage 3U0. The open delta output circuits applied to the alarm or protection relays are also designated 3Uo (fig. 5).
The ZU0 voltage has the highest value with a single-phase earth fault. It should be borne in mind that the maximum voltage value 3U0 in a network with an isolated neutral is much higher than in a network with a grounded neutral.
If the voltage on the additional secondary windings in normal three-phase mode is equal to the rated voltage of these windings, then in the event of a single-phase earth fault, the maximum value of 3Uo in a network with a grounded neutral will be equal to this rated voltage, and in a network with an isolated neutral, it will be 3 times higher.

Rated primary voltage a transformer is called such a voltage that must be supplied to its primary winding in order to obtain the secondary voltage indicated in the transformer passport at the terminals of the open secondary winding.

Rated secondary voltage is the voltage that is set at the terminals of the secondary winding during the idle run of the transformer, when the rated voltage is applied to the terminals of the primary winding, and the secondary winding is open.

The voltage on the secondary winding in the load mode differs from the voltage on the same winding in the no-load mode, since the load current creates a voltage drop across the active and inductive resistances of the winding.

This change in the secondary voltage depends not only on the value of the current and the resistance of the winding, but also on the power factor of the load. If the transformer is loaded with purely active power (Fig. 11.1, a), then the voltage, in comparison with other options, varies within smaller limits. In vector diagram É 2 - emf in the secondary winding of the transformer. The secondary voltage vector is equal to the geometric difference:

Ú 2 = É 2 - i 2 Z Tp , (11.2)

where i 2 - current vector in the secondary winding; Z Tp - impedance of the secondary winding of the transformer, Z Tp = √ R tp 2 + X tr 2 ; R tp them tr - respectively, the active and inductive resistance of the secondary winding.

With an inductive load and at the same current value, the voltage decreases to a greater extent (Fig. 11.1.6). This is due to the fact that the vector i 2 NS tr, lagging behind the current by 90 °, in this case is more abruptly turned towards the vector É 2 than in the previous one.

With a capacitive load, an increase in the load current causes an increase in the voltage across the transformer winding (Figure 11.1, c). Here the vector i 2 NS Tr equal to a similar vector in the first two cases and. also lagging behind the current by 90 °, due to the capacitive nature of this current, it turns out to be rotated along the vector É 2 and increases Ú 2 compared with É 2 .

During operation, it becomes necessary to regulate the voltage. For this, the number of working turns of the high voltage winding is changed by changing the transformation ratio within the range from ± 5 to ± 7.5% of the nominal value.

The tap-off diagram for simple switching windings is shown in Figure 11.2. In the passport of such a transformer, the minimum, rated and maximum voltage values ​​are indicated. If, for example, the rated secondary voltage of the transformer is 10 000 V, then the maximum voltage is 1.05 U n= 10 500 V, and the minimum is 0.95 U n= 9500 V.

The number of turns of the high voltage winding is changed using a special switch, the contacts of which are located inside the transformer, and the handle is brought to its cover.

Usually, for transformers that are installed near a 35/10 kV step-down substation or 0.4 / 10 kV step-up substation, the transformation ratio is taken to be 1.05 k, that is, the switch is set to the + 5% position. If the consumer substation is removed from the district, a significant voltage loss occurs in the power line, so the switch is set to the -5% position. The transformer at the midpoint of the transmission line is set to the nominal transformation ratio (Figure 11.3).

For automatic stabilization of the secondary voltage under load, voltage stabilizers of the STS type are used for 10, 16, 25, 40, 63, 100 kV A. According to the stabilization method, they can be stabilized for three phase voltages of 220 V or with stabilization [for three line voltages. They provide voltage stability within ± 1.5% of the nominal voltage when the supply voltage changes from +10 to -15% of the nominal. The stabilizer control circuit is made on semiconductor elements.

To regulate the voltage, the transformers are equipped with PRB or OLTC devices. PRB means: switching of windings without excitation, that is, with the transformer off. The taps from the windings are made in such a way that the voltage can be adjusted in the range from -5 to + 5% every 2.5%. On-load tap-changer means: voltage regulation under load (automatic). In this case, the voltage is changed in the range from -7.5 to + 7.5%, in six steps or every 2.5%. Such devices can be used to equip transformers with a capacity of 63 kVA and above.

RELAY PROTECTION

LECTURE No. 2

TIME - 2 hours

PURPOSE OF THE LESSON: To get acquainted with the types of measuring transducers. To study the working conditions and circuits for switching on current transformers.

EDUCATIONAL QUESTIONS:

INTRODUCTION - 5 min

TYPES OF MEASURING CONVERTERS - 30 min.

WORKING CONDITIONS AND CURRENT TRANSFORMER CIRCUITS - 50 min.

CONCLUSION - 5 min.

LITERATURE:

Figurnov E.P. Relay protection: Textbook. In 2 hours, Part 1. 3rd ed. , revised and add. - M.: GOU "Training and Methodological Center for Education in Railway Transport", 2009. - P. 45 ... 58.

GOST 18685-73. CURRENT AND VOLTAGE TRANSFORMERS.

TYPES OF MEASURING CONVERTERS - 10 min.

Measuring transducers of current and voltage are used in relay protection as sensors of information about the modes of operation of the protected object. In AC installations, current transformers and voltage transformers are widely used as such converters.

GOST 18685-73. A current (voltage) transformer is a transformer in which, under normal conditions of use, the secondary current (secondary voltage) is practically proportional (proportional) to the primary current (primary voltage) and, when correctly switched on, is shifted (shifted) relative to it in phase by an angle close to zero.

Current transformer- a transformer, the primary winding of which is connected to a current source.

Measuring current transformer- a transformer designed to convert current to a value convenient for measurement.

The primary winding of the current transformer is connected in series to the circuit with the measured alternating current, and the measuring instruments are connected to the secondary winding. The current flowing through the secondary winding of a current transformer is proportional to the current flowing in its primary winding.

Current transformers are widely used for measuring electric current and in relay protection devices for electrical power systems.

Current transformers provide measurement safety by isolating the measuring circuits from the high voltage primary circuit, often hundreds of kilovolts.

High accuracy requirements are imposed on current transformers. As a rule, a current transformer is performed with two or more groups of secondary windings: one is used to connect protection devices, the other, more accurate, to connect metering and measuring devices

Reference. The secondary windings of the current transformer (at least one for each magnetic circuit) must be loaded. The load resistance is strictly regulated by the requirements for the accuracy of the transformation ratio. A slight deviation of the resistance of the secondary circuit from the rating (indicated on the plate) modulo total Z or cos φ (usually cos = 0.8) leads to a change in the conversion error. This leads to a deterioration in the measuring qualities of the transformer. A significant increase in load resistance creates a high voltage in the secondary winding, sufficient for breakdown isolation of the transformer, which leads to the failure of the transformer, and also poses a threat to the life of the operating personnel. In addition, due to increasing losses in the core, the magnetic circuit of the transformer begins to overheat, which can also lead to damage (or at least wear) of the insulation and its further breakdown.

The transformation ratio of measuring current transformers is their main characteristic. The rated (ideal) coefficient is indicated on the transformer nameplate as the ratio of the rated current of the primary (primary) windings to the rated current of the secondary (secondary) windings, for example, 100/5 A or 10-15-50-100 / 5 A (for primary windings with several sections of turns). In this case, the real transformation ratio is slightly different from the nominal value. This difference is characterized by the magnitude of the conversion error, which consists of two components - in-phase and quadrature. The first characterizes the deviation in magnitude, the second is the deviation in phase of the secondary current of the real from the nominal. These values ​​are regulated by GOST and serve as the basis for assigning accuracy classes to current transformers during design and manufacture. Since in magnetic systems there are losses associated with magnetization and heating of the magnetic circuit, the secondary current turns out to be less than the rated one (i.e., the error is negative) for all current transformers. In this regard, to improve the characteristics and introduce a positive offset in the conversion error, a turn correction is applied. This means that the transformation ratio of such corrected transformers does not correspond to the usual formula for the ratio of turns of the primary and secondary windings.

Voltage transformer- a transformer powered by a voltage source. Typical applications are high to low voltage conversion and isolation in measuring circuits. The use of a voltage transformer allows the protection logic and measurement circuits to be isolated from the high voltage circuit.

reference ... Types of voltage transformers:

Grounded voltage transformer - single phase voltage transformer, one end of the primary winding of which must be tight grounded, or a three-phase voltage transformer, the neutral of the primary winding of which must be firmly grounded.

Ungrounded voltage transformer - a voltage transformer in which all parts of the primary winding, including the terminals, are isolated from earth to a level corresponding to the voltage class.

A cascade voltage transformer is a voltage transformer, the primary winding of which is divided into several series-connected sections, the transmission of power from which to the secondary windings is carried out using connecting and equalizing windings.

Capacitive voltage transformer - voltage transformer,containingcapacitive divider.

A two-winding transformer is a voltage transformer having one secondary voltage winding.

Three-winding voltage transformer - a voltage transformer with two secondary windings: the main and additional

The rated secondary current for the current transformer (CT) is 1 or 5 A.

The rated secondary voltage of the voltage transformer is 100 V.

Relay protection devices are usually connected to the same current and voltage transformers to which the measuring instruments are connected.

A decrease in voltage during a short circuit does not lead to an increase in VT errors. An increase in short-circuit current causes an increase in CT errors. To exclude incorrect actions of the relay, TT are checked for permissible error during short circuit.

Along with TT and VT as signal sensors for relay protection are used transreactors.

Transreactor - current transformer with a magnetic circuit having an air gap.

The current passing through the primary circuit determines the voltage at the terminals of the magnetizing branch of the equivalent circuit. Thus, at the terminals of the secondary load, a voltage is generated that is proportional to the current in the primary circuit. Transreactors are installed in protection devices when it is required to electrically add current and voltage vectors or when it is required to have a small cross-section of connecting wires in the secondary circuit of protection and automation devices.

Transreactors operate in a mode close to idling. The angle between the current in the primary winding and the EMF of the secondary winding is close to 90 0.

Current transformers and transreactors for installations with a voltage of 500 kV and above are quite complex and have significant errors. At such high voltages, magnetic-type current converters are used. "Magnetic" current transformers use inductive coupling between primary and secondary circuits.

The main feature of such a current transformer is the presence of a U-shaped ferromagnetic core, on which the secondary windings are located. The core is installed under the wire of the phase in which the current is to be controlled. The magnetic flux created in the core by the current of the controlled phase creates an EMF in the secondary winding, which is proportional to the current and is used as signals for protection circuits. Such converters are simple and economical, but they have several disadvantages:

Low power of the secondary circuit;

Influence on the EMF of the secondary circuit of currents in other phases.

To eliminate these disadvantages, amplifiers and various protection against interference can be used.

Other types of primary current (voltage) to secondary signal converters are devices that convert primary signals to radio signals or light signals. The receiver receives these signals and converts them back into current or voltage used for the input circuits of the relay.

2. OPERATING CONDITIONS AND CURRENT TRANSFORMER CIRCUITS

Operating conditions of current transformers

A schematic diagram of a current transformer and its equivalent circuit are shown in Fig. 4.

Rice. 4.a) circuit diagram current transformer; b) the equivalent circuit of the current transformer.

The primary winding with the number of turns W 1 is connected in series in the controlled current circuit I 1. The load resistance Zн is connected to the secondary winding with the number of turns W 2. The load resistance is the sum of the resistances of the series-connected relay windings, the resistances of the measuring devices and the connecting wires.

According to the law of total current:

I 1 W 1 - I 2 W 2 = Iμ W 1 = F μ ,

where Iμ is the magnetizing current; Fμ is the resulting magnetomotive force.

We divide the left and right sides of the total current equation by W 2 and get:

I 1 W 1 / W 2 - I 2 W 2 / W 2 = Iμ W 1 / W 2 or I 1 W 1 / W 2 - I 2 = Iμ W 1 / W 2.

Let us introduce the notation:

I 1 W 1 / W 2 = I 1 " ; Iμ W 1 / W 2 = I μ " .

Taking into account the notation, the equation for the total current will take the form:

I 1 " = I 2 + I μ " .

This equation corresponds to the equivalent circuit shown in Fig. 4 b). Primary resistance Z 1 " and the resistance of the magnetization branch Z μ " also given to the secondary winding.

Based on the equivalent circuit, we build a vector diagram (Fig. 5).

Rice. 5. Vector diagram of the equivalent circuit

Magnetizing current I μ " creates the resulting magnetic flux F... The magnetic flux vector lags behind the current vector by an angle γ, which is explained by losses in steel. Magnetic flux induces EMF E 2. Current I 2 lags behind E 2 by the angle ψ, determined by the ratio of the active and reactive components of the resistance Z 2 and Z n.

It can be seen from the diagram that the secondary current differs from the reduced primary current in absolute value by the current error ΔI.

ΔI = I 1 " - I 2

Current I 2 lags behind the current I 1 " by an angle δ, called the angular error.

Relative current transformer error,%:

f i = (ΔI / I 1 " ) 100 = (I 1 - I 2 K t) 100 / I 1 " ,

where Кт = W 2 / W 1 - turns transformation ratio.

The total error ε of the current transformer is determined by the formula:

ε = ( Iμ / I 1) 100%.

Ideal is a transformer that has no magnetizing current, i.e. Iμ = 0. Under this condition I 2 = I 1 / Kt.

As can be seen from the analysis of the equivalent circuit, the lower the value of the load resistance Z n, the greater the value of the current I 2 and the less the current Iμ. Therefore, the most favorable for the operation of the current transformer is the mode of the minimum resistance of the load circuit, i.e. short circuit mode. And, conversely, with an increase in the resistance of the load circuit, the magnitude of the magnetizing current Iμ increases and the error ε of the current transformer rises sharply. When opening the magnetizing circuit Iμ = I 1. Error ε = ( Iμ / I 1) 100% = 100%. An open secondary winding generates voltages of several thousand volts, which is dangerous for people and equipment in the measuring circuit.

The accuracy of converting the primary current into a proportional secondary current depends not only on the load resistance, but also on the value of the primary current. With an increase in the current I 1 to the value of the saturation current I we saturation of the transformer core occurs. This leads to a violation of proportionality between the values ​​of I 1 and I 2 (Fig. 6).

Rice. 6. Currents diagram

The value of ΔI determines the current measurement error at the current in the primary winding I s. The growth of the current error leads to the fact that the relay protection receives inaccurate information about the mode of operation of the protected object. This causes either unnecessary tripping or failure of the protection operation. For normal work protection, the current error should not exceed 10%, and the angular error should not exceed 7 0. These conditions are met if the total error ε does not exceed 10%.

To assess the limiting value of the primary current, the concept of the limiting current multiplicity is used K 10.

K 10 = I 1 max / I 1nom,

where I 1 max is the maximum value of the primary winding current at which the total error of the current transformer ε at a given load Z n, does not exceed 10%.

Along with the K 10 value, the concept of the maximum rated current ratio By 10th. This is the limiting current ratio at rated load. Z n nom with cos φ = 0.8.

Typical dependence of the limiting current ratio K 10nom on the value of the load resistance Z n is shown in Fig. 7.

Rice. 7. Typical dependence K 10 = f ( Z n)

Factories manufacturers of current transformers in the passport data indicate the values Z n nom and K 10nom, and also give curves of the form K 10 = f ( Z n).

There are 3 basic requirements for current transformers:

To prevent unnecessary operation of relay protection at short circuit outside the protected area, the total current error of the current transformer should not exceed 10%.

To prevent protection failures in case of short-circuit at the beginning of the protected zone, the current error of the current transformer should not exceed the value permissible for this type of relay - under the conditions of increased vibration of the contacts and under the conditions of the magnitude of the angular error.

The voltage at the terminals of the secondary winding of the current transformer during a short circuit in the protected area should not exceed the values ​​permissible for relay protection devices under the conditions of the insulation strength of the secondary circuits.

To check the transformer for the condition of 10% error, it is necessary to calculate the value of I 1 max at a short circuit at the end of the protected zone and determine the value of the resistance Z n of the secondary circuit of the transformer.

Depending on the connection diagram and the type of short circuit (single-phase, two-phase, three-phase), the value of Z n is determined from tables from reference books.

With a single-phase short circuit for the circuit Fig. 8 the value of the resistance of the secondary circuit of the transformer is determined by the formula:

Z n = r pr + z rf + z p0 + r lane,

where r pr is the resistance of the connecting wires; z рф - resistance of all relays in this phase; z p0 is the resistance of the relay in the neutral wire; r lane - contact resistance (0.1 Ohm).

Relay resistance is determined by the formula:

where S is the power consumption of the relay; I is the relay current at a given power consumption.

Resistance Z n is calculated for the most loaded phase and for the type of short circuit at which this resistance is greatest.

Then calculate the value of the limiting current ratio K 10.

Certain values ​​of Z n and K 10 are compared with passport data. If the conditions Z n ≤ Z n, nom and K 10 ≤ K 10, nom are met, then the error of the current transformer will not exceed 10%. If these conditions are not met, then it is necessary to either reduce the value of Z n or choose a transformer with a different value of I 1n.

Rice. 8. Secondary load connection diagram

In the event of a short circuit at the place of protection installation, the primary currents of the current transformers can be many more current I sat, at which saturation of the core occurs (Fig. 6). If the current in the primary winding is greater than the current I s, then the current error will be greater than 10%. In this case, there may be cases of relay failure. With a current error of more than 50%, contact vibration can occur in electromagnetic relays. For overcurrent protections and overcurrent cutoffs with the RT-40 relay for semiconductor relays and for distance protections with inductive relays, the maximum current error should not exceed 50%. Relay on integrated circuits type PCT11 can operate with current errors of 80 ... 90%.

The error of the current transformer at a short circuit at the beginning of the protected zone is determined as follows. Depending on the switching circuit and the type of short circuit, the load value Z n is calculated. Curves K 10 = f ( Z n) determine the limiting current ratio K 10. Calculate the value of the current I 1 max, at a given short circuit, and determine the corresponding value of the current multiplicity K max = I 1 max / I 1nom. Based on the type of relay used, the maximum permissible current error f% is set. According to the reference books for this type of relay, the value of the coefficient A is found (Fig. 9) and the condition is checked:

A K 10 ≤ K max.

If this condition is met, then the current error will not exceed the maximum permissible value for this case. If the condition is not met, then it is necessary to either reduce the value of Z n or choose a transformer with a different value of I 1nom or choose another type of transformer.

Rice. 9. Dependence of the current error on the coefficient A

Another check is carried out according to the value of the maximum value of the voltage in the secondary circuit of the current transformer U 2 m, which occurs at the maximum value of the primary current I 1 max in the event of a short circuit at the place of installation of the protection:

U 2m = k y I 1max Z n / a t,

where = 1.8 is the shock coefficient of the short-circuit current.

The condition U 2 m ≤ U isp must be met,

where U isp = 1000 V is the test voltage for the insulation of current circuits.

Current transformer connection diagrams

The most common wiring diagram of the current transformer and relay windings (KA) into a star (Fig. 10). Also, connection schemes are used in an incomplete star, in a star and a triangle, in the geometric difference of the currents of the two phases.

Rice. 10. Connection diagram of the current transformer and relay windings

This scheme is used for protection against all types of single-phase and multi-phase short circuits. In the neutral wire n, the current flows only with a short circuit to the ground, if the circuit has a grounded neutral, as well as in the event of an open circuit in the secondary circuit of one of the phases. All secondary windings of current transformers, according to electrical safety conditions, must be grounded. The circuit coefficient (the ratio of the current in the relay to the secondary current of the transformer of the same phase) k cx = 1.

With different types of short-circuit (three-phase, two-phase, single-phase), currents of different magnitude flow through the relays installed in different phases. Protection operation conditions are determined by that of the secondary currents, which is the largest for a given short circuit. For a relay, this highest current is called the relay current I p and is calculated.

Each switching circuit has its own formulas for determining the relay current Iр. In the case of the considered star connection scheme, the formulas are used:

With three-phase short-circuit and symmetrical modes: I p = I (3) k / K t;

In case of two-phase short-circuit at the place of protection installation: I p = I (2) k / K t = I (3) k / (2K t);

With two-phase short circuit behind the current transformer: I p = 2I (2) to / (K t) = I (3) to / K t.

In the latter case, k cx = 2 /.