CONNECTION DIAGRAMS FOR CURRENT TRANSFORMERS AND RELAY WINDINGS

Power supply of relay protection devices with network current is carried out according to various connection diagrams of current transformers and relay windings.

For each circuit, it is possible to determine the ratio of the current in the relay Iр to the current of the corresponding phase of the secondary winding I2: i.e.

, (1.3)

where Ksh is the circuit coefficient.

Let's look at some of the schemes (Fig. 1.3).

Connection diagram of current transformers and relay windings in a full star. In this circuit (Fig. 1.3a), relays installed in phases (I, II, III) react to all types of short circuits, and relay IV in the neutral wire reacts only to short circuits to ground. The circuit is universal and therefore used in protections operating for all types of short circuits. Relay current Iр=I2. those. Ксх=1.

Connection diagram of the current transformer and relay windings in a partial star . It is used for protection operating during phase-to-phase short circuits Ксх=1 (Fig. 1.3b).

Connection diagram of the current transformer in a triangle, and the relay windings in a star . The circuit is used mainly for differential and distance protection. Ксх= , in three-phase symmetrical modes.

Rice. 1.2

Connection diagram with two current transformers and one relay connected to the current difference. This circuit (Fig. 1.3c) is used for protection against phase-to-phase short circuits, in cases where it provides the necessary sensitivity. Ksh, in symmetrical modes is equal to .

Rice. 1.3

Connection diagram of a current transformer to a zero current filter after the sequence . (Fig. 1.3d), current in the relay appears only during one or two phase faults to ground, thus the circuit is used for protection against ground faults.

Power supply of relay protection devices with network current is carried out according to various connection diagrams of current transformers and relay windings.

For each circuit, it is possible to determine the ratio of the current in the relay Iр to the current of the corresponding phase of the secondary winding I2: i.e.

, (1.3)

where Ksh is the circuit coefficient.

Let's look at some of the schemes (Fig. 1.3).

Connection diagram of the current transformer and relay windings in a partial star . It is used for protection operating during phase-to-phase short circuits Ксх=1 (Fig. 1.3b).

Rice. 1.2

Rice. 1.3

Connection diagrams for voltage transformers in open and open delta

An open delta connection means that the equipment is connected between the sides of two phases. In this case, electric current is conducted from the outside, from the secondary windings of a number proportional to this indicator. The relay and the main load are connected between the secondary network, which allows you to obtain the desired level of resistance.

This circuit allows you to connect three sources at a time. It should be noted that the supply is organized in a linear manner, and the passage of current from the first to the third source and vice versa must be avoided.

The open type of connection is used in rectifier equipment. Using a type connection, a current of triple frequency is achieved, which is impossible when working with a star or open symmetrical. An option is used when three transformers with one phase are connected to a device that increases proportionally the three operating frequencies.

Using the figure under consideration, a zero sequence is obtained, that is, in the normal functional UP will be equal to zero.

The neutral of the primary winding must be grounded, and for the secondary winding, parameters of at least 100 Volts are selected, if grounded. For an isolated one, the coefficient is taken to be 100 to 3 V. The coefficient is three times, therefore, the secondary windings sum up the transformation coefficient also three times. Therefore, for the example described above, it is 6 thousand to one hundred to three. The peak is obtained from the transformer windings of the outer surface, since the supply is carried out through the secondary. Grounding is required.

On the contrary, there is a risk not for the device, but for the personnel who service it. In production, it is strictly prohibited to install protective or switching equipment between devices of this type.

Features of operation of current transformers in relay protection circuits

Voltage transformers in relay protection circuits

Purpose

Transducers are common elements for all relay protection schemes. Their main purpose is to isolate high voltage circuits from secondary protection circuits and convert input values into values convenient for measurements. The most common are electromagnetic current transformers and voltage transformers. Current transformers are designed to produce secondary currents of 5 A

or 1
A
, using voltage transformers, secondary voltages of 100
V
or

100 IN .

As an example, Fig. 8 shows the appearance of low-voltage cable and high-voltage current transformers.

a) b)

Rice. 8 Current transformers:

a) low-voltage cable current transformer; b) current transformer for voltage 220 kV

Features of operation of current transformers in relay circuits

protection

Structurally, the current transformer is a steel ser-

deck with two windings: primary

1 and secondary

2 (Fig.9).

Fig. 9 Current transformer design

When current flows through the primary winding of a current transformer, a magnetic flux is created, which induces a circuit in the secondary winding.

per load resistance, current

2. For a perfect transformation -

current torus, the sum of the magnetizing forces of the windings is zero:

1
w
1 +
I
2
w
2 = 0,

from here

2 = —

1 .

The ratio of the number of turns of the windings is called the turn ratio of the current transformer:

nв

=
w
2
w
1.

The ratio of the primary and secondary rated currents is called the rated current transformer ratio.

nTT

= I

1
nom
.

2
nom
Due to losses in the core steel, the values of the turn and rated transformation ratios of current transformers are different. For

To consider the reasons causing this difference, let us turn to the equivalent circuit of the current transformer (Fig. 10).

Primary current

1 passes resistance

1 and further branches along

two parallel branches. The load receives secondary current

2 ,

the magnetization branch closes the current

I us

=
I
1 -
I
2, called

magnetizing current.
The introduction of a magnetization branch into the equivalent circuit of a current transformer makes it possible to take into account errors in the actual transformation process. Fig. 10 Current transformer equivalent circuit

Thus, the ratio of the primary and secondary currents has the form:

uur

2 =

1 —
I us
,

nTT

that is, the real current transformer has errors.

The following types of errors are distinguished.

Current

the error determines the difference between the measured current module and its actual value:

=
I
1 -
I
2 ×100%.

1 I

Phase

the error determines the angle of shift of the secondary current relative to the primary.

From the equivalent circuit it follows that the magnitude of the error depends on

magnetization branch resistance values

zus

and from its correlation

with load resistance

zn .

Magnetization resistance op-

determined by the design of the current transformer, the characteristics of the steel

core and the ratio of the primary current. Primary current increase

leads to saturation of steel and a decrease in resistance

zus

, What

leads to an increase in error. If you increase the load while the primary current remains unchanged, then the error also increases.

For example, Table 1 shows the classification of current transformers. The permissible errors given in the table correspond to loads of the secondary winding not exceeding the rated value, and with a secondary current not exceeding 120% of the rated value

Current transformers designed to power relay protection circuits operate during short circuits or equipment overloads, when the primary currents significantly exceed the rated ones. Such operating conditions are associated with increased errors. And although the cores of current transformers for relay protection devices are made of high-quality electrical steel, which saturates at high current ratios, a prerequisite for the possibility of using a current transformer is its testing for permissible error.

According to regulatory requirements, the error of current transformers in protection mode should not exceed 10%. The following procedure for selecting current transformers is recommended:

1. The operating current of the protected object is determined

I slave

2. Based on the found current value and rated voltage, a current transformer is selected.

3. The maximum possible value of the fault current is determined

protected object

max.

4. The short circuit current multiplicity is calculated as the ratio

=
I k
max .

I slave

5. Based on the equipment supplier’s technical documentation or reference materials and the found primary current ratio

The permissible load of the current torus is determined.

zndop

for the selected transform -

6. The actual load of the current transformers is calculated and compared with the permissible one.

znfact

7. If

zndop

³
In fact
, it is believed that the current transformer satisfies

meets the accuracy requirements and can be used for this protection scheme.

zndop

<
zn fact
, then it is necessary to take measures to reduce the

loads. Such measures include the following:

— selection of a current transformer with an increased transformation ratio;

— increasing the cross-section of the control cable;

— use of a group of transformers connected in series instead of one current transformer.

The actual load of current transformers can be calculated using the expression:

znfact

=
z r
+
zpr
+
zcab
+
ztrans
,

Where

z r

– relay resistance;

zpr

– resistance of devices
;
zkab

–

control cable resistance;

zper

– transient resistance

contacts. To simplify calculations, the addition of total and active resistances can be done arithmetically. In a three-phase network, it is necessary to additionally take into account the connection diagram of the current transformers and the type of short circuit.

Current transformers, unlike power transformers, operate under conditions close to the short circuit mode of the secondary terminals. When the secondary winding opens, the entire primary current goes into the magnetization branch, and the current transformer goes into deep saturation mode (Fig. 11).

The saturation mode is accompanied by heating of the magnetic circuit and the occurrence of dangerous overvoltages at the secondary terminals, which is unacceptable due to the insulation conditions of the secondary circuits.

Taking into account the above, operation of a current transformer with an open secondary winding is unacceptable, and operation with a shorted one is often

this is the case of normal operation. According to electrical safety conditions, the secondary windings of current transformers are grounded.

Fig. 11 Curves of the change in time of current I, ampere turns, induction B and emf. E for a current transformer with an open secondary winding.

Voltage transformers in relay protection circuits
A voltage transformer
is a core made of electrical steel plates with primary and secondary windings placed on it (Fig. 12)

Fig. 12 Voltage transformer design

Primary winding

1
, which has a large number of turns (several thousand
), is connected in parallel to the power network, to the secondary winding w

measuring instruments, protection and alarm circuits are connected.

Voltage conversion U

U value

determined according to

sewing the turns of the primary and secondary windings:

1 =
w
1
U
2
w
2

The ratio of the number of turns of the windings is called the transformation ratio of the voltage transformer:

ntn

Voltage transformers are available in single-phase and three-phase versions. Depending on the required information, single-phase transformers can be connected in various circuits (Fig. 13).

Fig. 13 Connection diagrams for single-phase voltage transformers

To obtain one phase-to-phase voltage, the circuit shown in Fig. 13 , b

;
to obtain two or three phase-to-phase voltages, a partial star circuit is used (Fig. 13, cSS
).

In Fig. 13, a

The connection of three voltage transformers in a star circuit is shown. This circuit is used to obtain information about phase or phase-to-phase voltages.

To obtain zero-sequence voltage, along with phase and phase-to-phase voltages, voltage transformers with two secondary windings are used. One of the secondary windings is connected in a star, the other in an open triangle (Fig. 14).

The secondary windings of voltage transformers must be grounded to ensure personnel safety when high voltage enters the secondary circuits. When connecting the secondary winding in a star, the zero point is grounded; in other cases, one of the phase wires is grounded.

Fig. 14 Connection diagram of transformer windings with two secondary windings

To protect against short circuits, fuses or circuit breakers are installed in all ungrounded secondary circuits of voltage transformers.

Voltage transformers have two errors:

1.Voltage error

, which is understood as the deviation of the actual value of the transformation ratio from its nominal value.

2. Angle error

Depending on the errors, voltage transformers are divided into accuracy classes. Table 2 shows the classification of transformers depending on the accuracy class.

Depending on the load, the same voltage transformer can operate in different accuracy classes.

Therefore, the passport data indicates two power values:

— nominal, at which the transformer operates in a guaranteed accuracy class;

— limit, at which the heating of the windings does not exceed permissible limits.

In addition to the main errors, the measurement accuracy is affected by the voltage drop in the control cable. The amount of losses is standardized, so for relay protection circuits it should not exceed 3%.

CONCLUSIONS

1. Current and voltage transformers are designed to convert primary information about current and voltage into values that are convenient for measurements and safe for operating personnel.

2. Normal operating modes for current transformers are short circuit mode, and for voltage transformers - no-load mode.

3. Current transformers intended for powering relay protection circuits operate under conditions of large multiples of the primary current, which leads to increased errors

3. Basic algorithms for the functioning of protections with relative selectivity

Classification of protections

Overcurrent protection

Voltage transformers in relay protection circuits

Purpose

or 1
A
, using voltage transformers, secondary voltages of 100
V
or

100 IN .

As an example, Fig. 8 shows the appearance of low-voltage cable and high-voltage current transformers.

a) b)

Rice. 8 Current transformers:

a) low-voltage cable current transformer; b) current transformer for voltage 220 kV

Features of operation of current transformers in relay circuits

protection

Structurally, the current transformer is a steel ser-

deck with two windings: primary

1 and secondary

2 (Fig.9).

Fig. 9 Current transformer design

When current flows through the primary winding of a current transformer, a magnetic flux is created, which induces a circuit in the secondary winding.

per load resistance, current

2. For a perfect transformation -

current torus, the sum of the magnetizing forces of the windings is zero:

1
w
1 +
I
2
w
2 = 0,

from here

2 = —

1 .

The ratio of the number of turns of the windings is called the turn ratio of the current transformer:

nв

=
w
2
w
1.

The ratio of the primary and secondary rated currents is called the rated current transformer ratio.

nTT

= I

1
nom
.

2
nom
Due to losses in the core steel, the values of the turn and rated transformation ratios of current transformers are different. For

To consider the reasons causing this difference, let us turn to the equivalent circuit of the current transformer (Fig. 10).

Primary current

1 passes resistance

1 and further branches along

two parallel branches. The load receives secondary current

2 ,

the magnetization branch closes the current

I us

=
I
1 -
I
2, called

Thus, the ratio of the primary and secondary currents has the form:

uur

2 =

1 —
I us
,

nTT

that is, the real current transformer has errors.

The following types of errors are distinguished.

Current

the error determines the difference between the measured current module and its actual value:

=
I
1 -
I
2 ×100%.

1 I

Phase

the error determines the angle of shift of the secondary current relative to the primary.

From the equivalent circuit it follows that the magnitude of the error depends on

magnetization branch resistance values

zus

and from its correlation

with load resistance

zn .

Magnetization resistance op-

determined by the design of the current transformer, the characteristics of the steel

core and the ratio of the primary current. Primary current increase

leads to saturation of steel and a decrease in resistance

zus

, What

leads to an increase in error. If you increase the load while the primary current remains unchanged, then the error also increases.

According to regulatory requirements, the error of current transformers in protection mode should not exceed 10%. The following procedure for selecting current transformers is recommended:

1. The operating current of the protected object is determined

I slave

2. Based on the found current value and rated voltage, a current transformer is selected.

3. The maximum possible value of the fault current is determined

protected object

max.

4. The short circuit current multiplicity is calculated as the ratio

=
I k
max .

I slave

5. Based on the equipment supplier’s technical documentation or reference materials and the found primary current ratio

The permissible load of the current torus is determined.

zndop

for the selected transform -

6. The actual load of the current transformers is calculated and compared with the permissible one.

znfact

7. If

zndop

³
In fact
, it is believed that the current transformer satisfies

meets the accuracy requirements and can be used for this protection scheme.

zndop

<
zn fact
, then it is necessary to take measures to reduce the

loads. Such measures include the following:

— selection of a current transformer with an increased transformation ratio;

— increasing the cross-section of the control cable;

— use of a group of transformers connected in series instead of one current transformer.

The actual load of current transformers can be calculated using the expression:

znfact

=
z r
+
zpr
+
zcab
+
ztrans
,

Where

z r

– relay resistance;

zpr

– resistance of devices
;
zkab

–

control cable resistance;

zper

– transient resistance

Taking into account the above, operation of a current transformer with an open secondary winding is unacceptable, and operation with a shorted one is often

this is the case of normal operation. According to electrical safety conditions, the secondary windings of current transformers are grounded.

Fig. 11 Curves of the change in time of current I, ampere turns, induction B and emf. E for a current transformer with an open secondary winding.

Voltage transformers in relay protection circuits
A voltage transformer
is a core made of electrical steel plates with primary and secondary windings placed on it (Fig. 12)

Fig. 12 Voltage transformer design

Primary winding

1
, which has a large number of turns (several thousand
), is connected in parallel to the power network, to the secondary winding w

measuring instruments, protection and alarm circuits are connected.

Voltage conversion U

U value

determined according to

sewing the turns of the primary and secondary windings:

1 =
w
1
U
2
w
2

The ratio of the number of turns of the windings is called the transformation ratio of the voltage transformer:

ntn

Voltage transformers are available in single-phase and three-phase versions. Depending on the required information, single-phase transformers can be connected in various circuits (Fig. 13).

Fig. 13 Connection diagrams for single-phase voltage transformers

To obtain one phase-to-phase voltage, the circuit shown in Fig. 13 , b

;
to obtain two or three phase-to-phase voltages, a partial star circuit is used (Fig. 13, cSS
).

In Fig. 13, a

The connection of three voltage transformers in a star circuit is shown. This circuit is used to obtain information about phase or phase-to-phase voltages.

Fig. 14 Connection diagram of transformer windings with two secondary windings

To protect against short circuits, fuses or circuit breakers are installed in all ungrounded secondary circuits of voltage transformers.

Voltage transformers have two errors:

1.Voltage error

, which is understood as the deviation of the actual value of the transformation ratio from its nominal value.

2. Angle error

Depending on the errors, voltage transformers are divided into accuracy classes. Table 2 shows the classification of transformers depending on the accuracy class.

Depending on the load, the same voltage transformer can operate in different accuracy classes.

Therefore, the passport data indicates two power values:

— nominal, at which the transformer operates in a guaranteed accuracy class;

— limit, at which the heating of the windings does not exceed permissible limits.

CONCLUSIONS

1. Current and voltage transformers are designed to convert primary information about current and voltage into values that are convenient for measurements and safe for operating personnel.

2. Normal operating modes for current transformers are short circuit mode, and for voltage transformers - no-load mode.

3. Current transformers intended for powering relay protection circuits operate under conditions of large multiples of the primary current, which leads to increased errors

3. Basic algorithms for the functioning of protections with relative selectivity

Classification of protections

Overcurrent protection

Connection of transformer windings in a triangle

The triangle connection is so called because of its external resemblance to a triangle (seen in the figure).

When connected into a triangle, the following relations apply:

linear currents are √3 times greater than phase currents
line voltages are equal to phase voltages

The three secondary windings, when connected in a delta, are connected in series, thereby forming a closed circuit. There is no current in this circuit, since the EMF of the phases are shifted by 120 degrees and their sum at each moment of time is zero. Also, the current is zero if the following conditions are met: the EMF has a sinusoidal shape, the windings have the same number of turns.

Star and triangle in the question of third harmonics of transformers

In transformers, the triangle circuit is used, among other things, to obtain third harmonic currents, which are necessary to create a sinusoidal EMF of the secondary windings. In other words, to eliminate the third harmonic component in the magnetic flux.

To introduce third harmonics when connecting to a star, the neutral of the star is connected to the neutral of the generator, and the third harmonics begin to run along this path.