Basic requirements for relay protection

In the next article from the section “Relay protection and automation for beginners ” we will introduce you to the basic requirements for relay protection.

Most often, the following four main requirements are distinguished:

• selectivity;
• sensitivity;
• performance;
• reliability.

The concept of relay protection and automation (RPiA)

Rules for technical operation and installation of electrical installations (PUE, PTE) regulate the use of relay types of protection. These devices, or rather complexes of special elements, are often combined with automation, therefore they are abbreviated as RZiA (and this is also an abbreviation without “and” or RZ).

According to PTE standards, power units and lines of electrical installations - power plants, substations, electrical networks - are protected from short circuits (short circuits), abnormal conditions, overloads by relay protection and automation units. Such devices are integrated into structures, are part of them (installed at the design stage), and less often - they are mounted separately. According to the rules, they must be in a constant state of readiness (waiting), with the exception of those withdrawn from operation according to the characteristics of their tasks, design principles, modes of power facilities, and selectivity requirements. Alarm units (warning and reporting the development of breakdowns) must also always be ready for activation.

Where is it used?

Relay protection and automation are installed at power plants, generators, at any electrical installations, on similar large-sized powerful devices, that is, the scope of use is not limited if relay protection is required by the project. The scope of application is specified by the PUE:

Contents of Chapter 3.2 PUE:

What is RZiA used for?

We will explain what relay protection is more specifically, describing its purpose. When using electrical equipment and networks, there are always risks of their damage, incorrect modes, often they cannot be avoided, or such conditions are characteristic of the operation of the power plant. The most critical are overloads and short circuits. Reasons: breakdowns, damage to insulating parts, ruptures, worker errors, for example, disconnection of components under load, incorrect supply of voltage to grounded structures.

A short circuit in the area where it occurred provokes the appearance of an electric arc, the thermal effect of which leads, as a rule, to the irreversible destruction of current-carrying elements, insulating parts, and electrical devices in general (less often, but such cases are very common). In this case, high short circuit currents of thousands of amperes are supplied to the damaged segment. Almost instantaneous heating occurs; in seconds the elements heat up. Thermal processes also damage serviceable areas, causing a problem to develop and a fire to occur. On connected highways and objects, the parameters of electricity are deeply reduced, which causes electric motors to stop, and the functioning of parallel structures and generating devices is critically disrupted.

In the situations described, it is important to immediately stop the development of consequences; usually this is enough to completely prevent accidents. This is achieved by promptly disconnecting the dangerous section of the power plant, network - with automatic devices that function to disconnect contacts and de-energize. This is relay type protection, also known as relay protection or relay protection.

RP tasks

The relay deactivates the switches of the structure with the malfunction, while the electric arc goes out or does not even have time to arise. The flow of short-circuit amperes instantly stops, and at the same time, normal electricity levels are restored on the working part of the power plant or in the network. Damage to short-circuit equipment is minimized and eliminated, and the operating equipment mode is normalized.

Tasks:

• identifies the short circuit point, the location of the breakdown;
• quickly provides automatic shutdown, separates equipment with an unsafe factor, a dangerous segment from working structures and networks;
• detects a problem and reports it, warns about the possibility of an accident;
• creates a delay before deactivation if necessary.

Peculiarities

Other disturbances in the power plant are also possible: overload, short circuits of various kinds, the formation of gas masses in transformers, a decrease in oil volume there, etc. If such problems are not dangerous, they correct themselves, and immediate de-energization may not be required. Usually, if the installation has constant maintenance by specialists, issuing a notification to them is sufficient. In other cases, switching off is enough, but with a pause.

Relays RZ are devices, units with an automatic principle that carry out a change in a characteristic periodic type (“relay action”, intermittently) with an established transformation (modification) of the observed characteristics.

Simply put, when detecting violations of the parameters of the power plant, the relay de-energizes and separates the contacts. Example: when the ampere on the controlled circuit critically increases (its current winding is wound there) the relay disconnects the connections to the prescribed mark.

A relay protection device is an interacting system of relays and auxiliary, automatic, device units that turn off equipment when it is damaged or under abnormal conditions.

Selectivity

To understand the selectivity requirement, consider the figure

In the event of a short circuit at point K, the protection on switch Q5 should operate and give a command to turn it off. In this case, the remaining undamaged part of the electrical installation will remain in operation. This protection action is called selective.

Protections that affect upstream switches (located closer to the power source) should not operate.

For example, if the protections on switch Q3 are triggered, then together with the damaged section, motor M2 will be de-energized, which was normally in operation and there was no damage to the electrical network supplying it. This protection action is called non-selective.

Here is another example of the selectivity principle, although it concerns low-voltage protection.

Let's imagine that in the apartment there are several lines from the electrical panel to the sockets. Each line is protected by its own circuit breaker. There is also an input circuit breaker that powers the entire apartment.

If a short circuit occurs on one of the lines, the circuit breaker of only this line should turn off, and the input circuit breaker should remain in operation. This will be the selective work of the defenses.

Selectivity or selectivity is the ability of relay protection to detect the location of a fault and turn it off only by the switches closest to it

Alexey Bobkov

Author of the article, design engineer of relay protection systems for stations and substations

Kinds

First, we will separately describe the logical protection for buses, abbreviated as LPS. Principle: compares the state of protection of supply parts and outgoing feeders (cable outlets). Sample algorithm: the protection on one of the last ones was turned off, which means there is a short circuit on it; did not start on them at all - short circuit on bus elements. In the event of a short circuit on the tap, protections (current releases) are activated on it and on the power nodes of the section (CT inputs, segment switches).

Further, upon triggering, the power parts are switched off without a pause. In the event of a short circuit on the bus parts of the distribution circuit, the relay protection on the taps does not start, and when activated on the supply nodes, it is allowed without delay.

Other types of relay protection:

Reliability

It is unlikely that anyone would want to have a system at their disposal that could fail at any moment. And even more so if the integrity of expensive equipment depends on this system. After all, the main task of relay protection is to eliminate abnormal or emergency operating conditions of equipment, which can lead to its damage and failure. And if at the moment of a short circuit, for example, on a line, the relay protection fails, then destruction, losses and other serious consequences cannot be avoided.

Therefore, reliability is the main requirement for relay protection systems.

Reliability is the property of protection that is guaranteed to perform its functions throughout the entire period of operation. The protection must operate correctly and reliably to shut down equipment switches in the event of all types of damage and violations of the normal operating mode and not operate under normal conditions, as well as in the event of such damage and violations of the normal operating mode in which the action of this protection is not provided.

Reliability of relay protection is achieved by using several sets of protection (if one set fails, another will work), implementing protection on different operating principles (current protection, differential, distance, etc.).

Sometimes, to protect an object, protection devices made on completely different element bases can be used simultaneously - microprocessor and microelectronic, microprocessor and electromechanical. Although the trend is that microprocessor protection is replacing all other types of relays.

Automation

Electrical automation, unlike relay protection, not only turns off the equipment, but also turns it on. First of all, these are auto-on: restart (reclose) and power reserve (ABP).

There are also varieties with personnel control of relay protection equipment, these are automatic:

• regulation of the activation of generators, synchronous motors (ASM);
• for circuit breakers (AUV), for redundancy of their failures (breaker failure protection);
• control of CT switch positions (ARNT);
• setting up arc suppression windings (ARC), static capacitors;
• transformer cooling;
• adjustment (synchronization) of generators;
• frequency start of hydrogenerators (HG);
• identification of circuit fault locations (OCF).

Emergency:

• mode: frequency. unloading (AFR)
• activation of deactivated AFR systems (DFA);
• automatic control of frequency and operating power (ARFC);
• automatic voltage unloading (DARN); by current (DART);

Device

Let's consider the device in the process of describing the operation of relay protection and automation:

Requirements for relay protection and automation

The requirements for relay protection are comprehensively specified in the PUE (R. 3 Ch. 3.2), as well as in numerous manuals - there is no point in duplicating them in the article. Let us summarize them so that the reader can navigate what to pay attention to, quickly find and clarify them in the indicated sources.

What principles ensure performance?

Violations in the operation of relay protection and automation equipment due to incorrect selection, installation, or non-compliance with standards:

• false alarms when the control unit and network are working properly;
• unnecessary activations, for example, when the activation of executive nodes is unnecessary;
• damage to the protection structure.

The PUE and related regulations impose requirements that exclude the above (concerning design, installation, setup and start-up, maintenance):

• compliance with classes, reliability levels;
• sensitivity;
• speed of operation;
• selectivity - ensuring protection activation levels are in the correct order. This parameter is closely related to the previous two.

Reliability

Defined by the following characteristics:

• reliability;
• compliance with the number of operation cycles specified during the creation of the relay protection;
• maintainability;
• service life, shelf life. It must be guaranteed by the manufacturer, designer in accordance with the specifications (which must be consistent with GOSTs, PUE) of the product. The product must have a passport and certificate.

Each position has its own assessment, indicated in the technical documentation, in the project approved in accordance with regulatory documents.

There are 3 positions on reliability during maintenance and operation of relay protection by activation: during internal short circuits at work locations, outside their boundaries, when functioning without faults. There are two types of reliability: operational and hardware.

Sensitivity

The requirements for relay protection and relay protection primarily relate to functional settings, since the fixation of threshold values ​​and violation of settings imply that the relay has a certain sensitivity.

It is necessary to correctly determine what the expected degree of violation of the regime, overload is dangerous, and select a correspondingly configured version of the relay protection for it.

There is an equation for sensitivity (its numerical value) when a short circuit occurs. A special characteristic is used - Kch, coefficient.

Kch = Iкз min/Iсз

Calculation: the ratio of the smallest short-circuit current of the working area to the activation current value. The relay functions normally when Iсз < Iкз min. The most optimal sensitivity (coefficient) is 1.5–2.

Performance

The speed of de-energization has 2 components:

• triggering of protective algorithms with a command to the node below;
• activation of the switch drive.

Time response adjustable in the range min.-max. values ​​depending on the capabilities of the relay protection device and the elements used. The response delay is created by the introduction of special relays with the ability to configure; this option is used for the most distant protections. Relay protection placed closer to the location of the problem, to the protected area, is configured for a shorter activation time interval or is used without it.

Selectivity

The second name for this characteristic is selectivity. This option allows you to determine the location of the fault in circuits of any complexity.

The generator generates and supplies electricity to consumers in segments 1–3 (each with its own protection). When there is a short circuit on the consumer's device at the 3rd interval, the current flows through all nodes of the relay protection, starting from the energy source. In such conditions, it is advisable to turn off the circuit of the segment with a malfunction, for example, an electric motor, leaving the remaining serviceable consumers involved. For this purpose, it is possible to make RP settings for each circuit. Typically, such features are laid down at the design stage.

Protection 5 of the 3rd segment should detect fault currents earlier and activate more quickly, disconnecting damaged segments from the circuits. Therefore, the values ​​of the current-time settings at each interval are reduced from the generator to the consumer. Principle: the further away from the breakdown location, the less sensitivity. In this way, redundancy is simultaneously implemented, taking into account the possibility of effective protection in the event of malfunctions of any devices, including lower-level protection systems. The described scheme means that if protection 5 of segment 3 breaks down in an accident, protection devices 3 or 4 of gap 2 should be activated. And these sections, in turn, are protected by the protective units of segment 1.

Sensitivity

Relay protection must be sensitive to the types of damage from which it protects. “Sensitive” in this case means that the protection must perform the action it requires (issue a command to open the circuit breaker, sound a sound signal) immediately when damage occurs in the area it protects. At the same time, the protection should not be overly sensitive, that is, it should not operate in normal mode during short-term overloads.

A measure of the sensitivity of relay protection is the sensitivity coefficient Kch, defined as the ratio of the minimum value of the controlled value during a short circuit at the end of the protected section to the protection setting.

The values ​​of the sensitivity coefficient for various protection zones are standardized in the PUE.

The minimum sensitivity coefficient for short circuit in the main zone is Kch = 1.5, in the redundancy zone - Kch = 1.2, for high-speed differential protection Kch = 2.

Sensitivity is the property of protection to operate reliably during a short circuit at the end of the protected section in the minimum operating mode of the system.

Management nuances

The relay protection is performed as a separate unit and is an independent circuit, despite the fact that it is often integrated into the design of the power plant itself. Such nodes are included in general complexes that make up the emergency control system of the power system, where all nodes are interconnected and implement assigned tasks together.

Below is a diagram (simplified) of the functions and actions of the automation:

Scheme

There are extremely many varieties, combinations, and places of relay protection on networks and in electrical installations. There are also standardized options, original templates - circuit diagrams. But regardless of the complexity, any drawing can be understood only by learning to read it. This skill is necessary to work with relay protection and automation equipment.

In terms of importance and complexity, the “principles” of relay protection and automation kits are second in the design of the entire electrical equipment system. In all cases, when developing or testing finished circuits, at least minimal skills in electrical engineering will be required. Even specialists sometimes find it difficult to understand the relay protection circuit at the elementary input of 10 kV transformers, not to mention in general for a 110/10 kV substation.

Let's look at a technique that makes drawings easier to understand. The method described below is standard and common; it does not harm the quality of the analysis.

Breaking down the diagram into parts

The whole scheme is extremely difficult to understand, so it is conditionally divided into separate sections and analyzed each separately.

Let's consider relay protection and automation with terminals on microprocessors, divide the drawing into 10 positions:

• explanatory;
• circuits: measurements (current, voltage);
• switch mechanism;
• involved current (operational), including power supply to the terminal;
• alarms;
• weekends, including TC and reserve;
• ACS;
• auxiliary (heating, light, sockets, etc.);

Not every set of RP contains all 10 positions, but the absence of any must be justified; if this cannot be done, then there is an error in the diagram.

This method is a kind of checklist, an analysis system. The results obtained can be recorded in a list with checkboxes opposite the items and transferred to the contractor before design.

Test example: the absence of a drive circuit in a 10 kV VT (position 2 in the “circuits” section of the list) is justified by the fact that the latter’s cell does not have a switch, and this is logical. If there is no answer to the question posed, for example, why there is no parameterization data for the 10 kV input into the relay protection, then there is an error, especially for terminals with flexible logic.

Typical circuit errors:

• in current circuits - incorrect polarity when connecting the CT to the terminal;
• drive circuit - platoon (ready for activation). There may be incorrect closing or opening. It is necessary to check by comparing it with the terminal algorithm;
• operational current circuits - control keys, control mode (MU/DU);
• Arc protection circuits, generation designs are particularly susceptible to such complex faults.

Example of diagram breakdown and reading

To explain, we took a popular example from the Internet; there are also video materials for it on the Internet. We will show the circuit in parts (connected in a horizontal plane), since it is quite large.

Drawing for a relay protection with electromechanical relays, a VVTEL Tavridaelektrik switch (without a terminal, this is not a digital design on microprocessors) of a 10 kV line at a 110/10 kV substation:

Second part of the diagram:

Third part of the diagram:

The diagram is available online; to open it you will need special drawing programs. Next, let's select and show the parts.

Explanatory diagram:

Measuring circuits (electromechanics, MTZ, MTO, power circuits from the alternating opercurrent of the switch drive, converter counters):

Drive circuits (power supply and control unit, electromagnet circuits):

Operating current circuits (automatic circuit breaker, executive relays, power supply):

Arc protection refers to opercurrent circuits:

Emergency and warning light circuits, central (below):

Output circuits (in this case this is what is included in the remote alarm):

Auxiliary circuits:

List of elements:

In this example, there are no logic tables, parameterization data, ACS circuits, or parallel automation protection, since this relay protection does not have a terminal and is not based on microprocessors.

GENERAL INFORMATION ABOUT RELAY PROTECTION

LECTURE COURSE

By relay protection

GENERAL INFORMATION ABOUT RELAY PROTECTION

PURPOSE OF RELAY PROTECTION

When operating any electrical power system, one has to take into account the possibility of damage and abnormal operating conditions occurring in it.

The danger
of damage
and
abnormal operation
of power lines and electrical equipment is as follows:

Damage

in most cases, they are accompanied by a significant increase in current and a deep decrease in voltage in the elements of the power system.

Increased current

generates a large amount of heat, causing destruction at the site of damage and dangerous heating of undamaged equipment through which this current passes.

Undervoltage

disrupts the normal operation of electricity consumers and the stability of parallel operation of generators and the power system as a whole.

Abnormal operating conditions

usually lead to a deviation of voltage, current and frequency values ​​from permissible values, which creates a risk of disruption to the normal operation of consumers and the stability of the power system, and also threatens damage to equipment and power lines.

Thus, damage disrupts the operation of the power system and electricity consumers, and abnormal conditions create the possibility of damage or disruption to the operation of the power system.

To prevent dangerous consequences of damage and abnormal conditions, a set of special automatic devices is used, called relay protection .

Its name is Relay protection

received from the word
“relay”,
which is an automatically operating device that comes into action (triggered) at a certain value of the input value acting on it.
Relay protection
is the main type of electrical automation, without which normal and reliable operation of a modern power system is impossible. It continuously monitors the condition and operating mode of all elements of the power system.

The main purpose of relay protection is:

— if damage occurs, identify and disconnect the damaged area by acting on special power switches designed to interrupt fault currents;

— identify abnormal conditions and, depending on the nature of the violation, perform the operations necessary to restore normal conditions, or send a signal to the duty personnel.

Relay protection is closely related to other electrical automation devices - automatic reclosing devices ( AR ), automatic transfer switch ( ATS ), automatic frequency unloading ( AFD ) and other system automation devices designed for quick automatic restoration of normal operation of electric power systems.

Three-phase short circuit

Three-phase short circuit (K(3)) is a symmetrical mode in which the currents and voltages in all phases are equal in magnitude both at the short-circuit location and at another point in the network:

; ;

Vector diagram of currents and voltages at K(3 )

is shown in Figure 1.1 a.

The short-circuit current passing in each phase lags behind the emf that creates it. at the same angle, determined by the ratio of the active and reactance of the short circuit circuit:

;

All phase and phase-to-phase voltages at the location of the 3-phase short circuit. are equal to zero:

; ;

At points remote from the short circuit location. over a short distance, phase and phase-to-phase voltages are insignificant in magnitude, therefore a 3-phase short circuit. represents the greatest danger and is the design mode when determining the maximum short-circuit current.

Two-phase short circuit

With a two-phase short circuit . (K(2)) currents and voltages of different phases are not the same.

In damaged phases at the site of a two-phase short circuit. currents of equal magnitude but opposite in direction pass through, and in the damaged phase the short-circuit current absent. For example, for the case of a 2-phase short circuit. between phases B

and
C
the following relations are valid:

— phase-to-phase voltage between closed phases;

- phase voltages of closed phases.

The same as with a three-phase short circuit. phase currents in damaged phases lag behind the emf creating them. by an angle determined by the ratio of active and reactive resistances of the circuit.

Vector diagram of currents and voltages for a two-phase short circuit. shown in Fig. 1.1 c. As you move away from the location of the 2-phase short circuit. phase voltages and phase-to-phase voltages will increase.

From the point of view of the influence on the stability of parallel operation of generators and on the operation of electric motors, a 2-phase short circuit. poses less danger than a 3-phase short circuit.

Single-phase short circuit

Single-phase short circuit can only occur in networks with a grounded neutral (networks 110 kV and above).

Vector diagram of currents and voltages at the location of a 1-phase short circuit. phase C

is shown in Figure 1.1 d.

In place of a 1-phase short circuit. phase C

phase voltage of the damaged phase and phase currents. undamaged phases will be equal to zero:

; ;

Short-circuit current flows only in phase C

:

;

Voltages of undamaged phases a A

and
B
will exceed the emf. corresponding phases due to the fact that emf is induced in undamaged phases. mutual induction under the action of short-circuit current occurring in the damaged phase.

In some cases, single-phase short circuit current may be greater than the current of a three-phase short circuit, however, this mode poses less of a danger to the normal operation of the power system than 3 and 2 phase short circuits, since at the point of damage only the phase voltage of the damaged phase is reduced to zero.

Requirements for relay protection.

Relay protection is carried out in the form of autonomous devices installed on the elements of the power system. Relay protection devices respond to short circuits. and abnormal modes and act on disconnecting the switches of the protected elements.

Relay protection must operate in case of damage in the protected area (in case of internal damage) and should not operate in case of damage outside the protected area (in case of external damage), as well as in the absence of damage.

Protections are divided into main and backup .

Basic

is called protection designed to work with all or part of the types of short circuits. within the entire protected object, the response time is shorter than that of other installed protections.

Reserve

is called protection provided for operation instead of the main protection of a given object in the event of its failure or withdrawal from operation, as well as instead of protection of adjacent elements in the event of their failure or failure of switches of adjacent elements.

Basic requirements for short circuit protection:

Performance.

Quick shutdown of damaged equipment or a section of an electrical installation reduces the extent of damage, preserves the normal operation of consumers of the undamaged part of the installation, and prevents disruption of the parallel operation of generators, power plants and the power system as a whole. The last condition is the main one.

Allowable short circuit tripping time according to the condition of maintaining stability, it depends on a number of factors, the most important of which is the amount of residual voltage on the buses of power plants and node substations of the power system. The lower the residual voltage, the worse the stability conditions, the faster you need to turn off the short circuit. PUE recommend determining the residual voltage on the buses of power plants and central substations during three-phase short circuits. at the network point of interest to us. If the residual voltage is less than 60% of the rated voltage, then high-speed protection should be used to maintain stability.

Total fault shutdown time toff

consists of the protection operating time
tз
and the operating time of the switch
tв
, breaking the short-circuit current.
toff
=
tз
+
tв
.

Modern high-speed relay protection devices have an action time of 0.02-0.1 s .

Sensitivity.

The protection must have such sensitivity within the zone established for it that its action is ensured at the very beginning of the occurrence of damage, thereby reducing the extent of equipment damage at the short circuit site.

Thus, sensitivity is a protection property that ensures that damage to electrical equipment is detected at the very beginning of its occurrence.

The sensitivity of the protection should also ensure that it operates in the event of damage in adjacent sections of the network. So, for example, if in case of damage in current K1

(Figure 6) for some reason switch
B1

B4
next to the power source must act and turn off this switch.
This protection action is called long-range reservation of an adjacent or next section.
The sensitivity of the protection must be such that it operates during a short circuit. at the end of the zone established for it in the minimum operating mode of the system and during short circuits through an electric arc

.

The sensitivity of protection can be assessed by the sensitivity coefficient Kch

. For protections that respond to short-circuit current.

, Where

Iк.min

– minimum short-circuit current,
Iс.з
– protection operation current.

Reliability.

The reliability requirement is that the protection must operate correctly and reliably within the zone established for it and must not operate incorrectly in modes in which its operation was not intended.

.

Unreliable protection itself becomes a source of accidents.

During operation, the following types of failures in the functioning of relay protection devices are possible:

− trip failures

when required;

− excessive triggering

in case of damage in the protected area with the requirement of non-operation;

− false positives

if there is no damage in the protected area.

The requirement of reliability is ensured by the perfection of the principles of protection and the design of the equipment, ease of implementation, as well as the level of operation.

Requirements for relay protection against abnormal conditions:

Protection against abnormal conditions must also be selective

, sufficient
sensitivity
and
reliability
.
But,
as a rule, protection against abnormal conditions
does not require
high performance .

Abnormal regimes are often short-term in nature and self-destruct. For example, during short-term overloads when starting an asynchronous electric motor, a quick shutdown is not only unnecessary, but can also cause damage to consumers. Therefore, the action to disable protection from abnormal conditions should be carried out with a time delay and only when there is a danger to the protected equipment.

In cases where the elimination of an abnormal mode can be carried out by the personnel on duty of the electrical installation, protection against abnormal modes can be carried out in response to a warning signal.

Relay

The main element of any relay protection circuit is a relay . Under the term relay

It is customary to understand
an automatically operating device designed to produce an abrupt change in the state of the controlled circuit at given values ​​of a quantity characterizing a certain deviation of the mode of the controlled object.
Relay protection and automation includes a complex of relays for various purposes, which operate together in a given sequence (according to a given program). Relays close or open various electrical circuits or otherwise abruptly change their state (for example, abruptly change their resistance), or mechanically influence power devices (switches, etc.).

Electrical and mechanical relays are used in relay protection devices

and
thermal
. Electrical relays

react to electrical quantities - current, voltage, power, frequency, resistance, the angle between current and voltage, or two currents, or two voltages.

Mechanical relay

react to non-electrical quantities - pressure, flow rate of liquid or gas, rotation speed, etc.

Thermal relays

react to the amount of heat generated or temperature changes.

Electric relays are most widely used in relay protection and automation.

Operating current

Indirect action relays act on turning on and off switches through special turning on and off electromagnets by supplying them with a current called operative current .

Operating current is also used to power auxiliary relays in relay protection and automation circuits (intermediate, time relays, indicating), as well as for the operation of light and sound alarms

Thus , the operating current

is called the current that supplies remote control circuits for switches, operational circuits for relay protection, automation and various types of alarms.

Operating current sources must provide a high degree of reliability, be constantly ready for action and provide the required voltage or current in the windings of the electromagnets for turning on and off switching devices

(switches and disconnectors).

To control switches and power relay protection devices in electrical installations, two types of operating current are used: direct and alternating .

Constant operating current

The main sources of constant operating current are rechargeable batteries (AB) with chargers. The standard values ​​for rated voltages of direct operating current are 24, 48, 110 and 220 V.

A distribution network is created to power relay protection and automation devices, control switches, emergency and warning alarms, as well as other devices that require an independent DC source (Figure 12). To charge the battery, rectifier or electric machine charging units are used.

The DC operating current distribution network is divided into separate sections so that a fault in one of them does not disrupt the operation of others.

Figure 12 – Example of a schematic diagram of a DC distribution network.

All consumers of operational current are divided into categories according to the degree of their responsibility. The most responsible consumers are the operational circuits

current relay protection, automation and trip coils of switches, powered from control buses

.
The second very important section is the circuits of switching coils, powered from individual bus
due to the large currents consumed by the switching coils of oil switches.
The third, less responsible consumer of operational current is the alarm system, powered by the ShS
.

Typically, critical circuits are powered by two batteries operating in different sections of DC switchboards.

DC distribution networks make extensive use of sectioning and redundancy.

On each line extending from the DC busbars, automatic switches (or fuses) are installed to protect the network in the event of a short circuit. on outgoing lines.

Short-circuit current determined by the formula:

, Where

e

– e.m.f. one battery cell, V;

Re – internal resistance of one battery cell, Ohm;

n – number of elements in the discharge circuit, pcs.;

– circuit resistance from the battery buses to the short-circuit point. both ways, Om.

ℓ – distance along the cable route from the battery buses to the short-circuit location, m;

γ – specific conductivity, equal to approximately 57 for copper and 34 for aluminum; m/Ohm×mm2.

S - cross-section of cable cores, mm2.

Violation of the insulation relative to the ground of the DC network can lead to ground faults and the formation of bypass circuits and false shutdown of equipment, therefore DC switchboards are equipped with insulation monitoring devices that continuously monitor the insulation state of the DC network relative to the ground.

The circuit of the simplest insulation monitoring device is shown in Figure 13 and consists of two voltmeters connected between each pole and ground.

Figure 13 – Scheme for monitoring the insulation of DC circuits using two voltmeters.

Under normal conditions, when the insulation resistance of each pole relative to ground is R(+)

and
R(-)
are the same, the voltage of each pole relative to ground is equal to half the voltage between the poles, i.e.
U(+) =U(-) = 0.5U
.

If one of the poles, for example ( + ), is shorted to ground, i.e. R(+) = 0

, then, accordingly
, U(+)
will also become equal to zero, and the voltage
U(-)
will increase to the full voltage between the poles, i.e.
U(+) = 0 and U(-) =
U.
Consequently, when the insulation resistance on one of the poles decreases, the voltage of this pole relative to the ground, which is equal to 0.5 U
, decreases, and the voltage of the other pole relative to the ground increases by the same amount.

Using the K(+)

and
K(-)

K(+)
and
K(-)
buttons are opened alternately and the readings of the voltmeters
U(-)
and
U(+)
. The network insulation resistance relative to the ground is determined by the formulas:

; ,

where Rв is the internal resistance of voltmeters;

Other insulation monitoring devices can be used in operation, including those that automatically act on a warning signal when the network insulation decreases to a certain value.

Rechargeable batteries are the most independent and reliable sources of operational current and therefore they are widely used in power plants and substations to power operational circuits of relay protection, automation and circuit breaker control.

However, rechargeable batteries are expensive and require a special room and a charger; and they must be serviced by specially trained qualified personnel. In addition, the implementation of a DC distribution network requires a large amount of control cable.

In Russia, power supply of operational circuits from direct operating current sources has become widespread at power plants and substations with voltages of 110 kV and higher.

AC operative current

To power operating circuits with alternating current, mains current or . In this case, the sources of alternating operating current are: current transformers, voltage transformers and auxiliary transformers.

Current transformers are a reliable source of power supply for short-circuit protection operational circuits. With short circuit The current and voltage at the terminals of the current transformer increase and, consequently, the power of the current transformers increases, which ensures reliable power supply to the operating circuits.

The power supply circuit for operational protection circuits with alternating operational current directly from current transformers is shown in Figure 14 a). In normal mode, the circuit breaker trip coil 2

is bypassed by the contacts of relay
1
and there is no current in it.
With short circuit relay 1
is triggered, its contacts open, and current from the current transformers flows into trip coil
2
, activating it.

However, current transformers do not provide the necessary power in case of damage and abnormal conditions that are not accompanied by an increase in current. They cannot be used to power relay protection devices against ground faults in networks with an isolated neutral, protection against turn faults of electrical machines and for protection against abnormal conditions of electrical installations, such as increased or decreased voltage and decreased frequency. , voltage transformers or auxiliary transformers should be used as sources of operational current .

The operating current supply circuit from the voltage transformer and from the auxiliary transformer is shown in Figure 14 b), c). Scheme b) is used to power operational protection circuits, and to power circuit breaker control circuits, scheme c) is usually used, where rectified current is used to power control circuits.

Figure 14 – Power supply circuit for operational protection circuits with alternating operational current

a) directly from current transformers; b) from voltage transformers; c) from the auxiliary transformer

However, voltage transformers and auxiliary transformers

unsuitable for powering operational circuits of short-circuit protection. because at short circuit The network voltage drops sharply, and they can be used for such protections as, for example, overload protection, ground fault protection, overvoltage protection, etc.

In addition to directly using the power of current and voltage transformers, it is possible to use the energy stored in pre-charged capacitors . The capacitor is usually charged in normal mode from the mains voltage. When the voltage in the electrical installation disappears, the energy stored by the capacitor is stored and can be used to power the protections that must operate when the voltage disappears.

A circuit powered by a charged capacitor is shown in Figure 15. Capacitor 1

powered by a voltage transformer through a rectifier
2
. In normal mode, the capacitor is charged. When the protection operates, it closes to the trip coil, feeding it with discharge current.

Figure 15 – Circuit for powering operational protection circuits with alternating current using the energy of a charged capacitor

In Russia, power supply of operational circuits from alternating current sources has become widespread in electrical networks with a voltage of 6-35 kV.

Current transformers

A current transformer is a transformer in which, under the right conditions of use, the secondary current is practically proportional to the primary current and, when turned on correctly, is shifted in phase relative to it by an angle close to zero.

Selection of current transformers

The selection of current transformers for relay protection is carried out according to the following algorithm:

1. The operating current of the protected object I slave

.

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

3. The maximum possible value of the damage current of the protected object I k.max is determined.

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

,

where I1.nom

– rated primary current of the CT.

5. Knowing the multiplicity of K

, the permissible load ZH is determined from the 10% error curve
.
additional for the selected current transformer.

6. Taking into account the CT connection diagram, the actual load of the current transformers ZН.fact is calculated.

and is compared with the permissible
ZH.
add. 7. If ZН.fact ≤ ZН. extra

The current transformer is considered to satisfy the accuracy requirements and can be used for this protection circuit.
If ZН.fact > ZН.
additionally , it is necessary to take
measures to reduce the load
. Such measures include the following:

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

— increasing the cross-section of the control cable;

- using a group of transformers connected in series instead of one current transformer.

Normal operating mode for TT

is the short circuit mode, in which
the CT
have the smallest values.

Operation of a current transformer with an open secondary winding is unacceptable, since in this case there is no demagnetizing flux in the CT

, which leads to its saturation, a sharp increase in the magnetizing current and, as a consequence, unacceptable heating of the transformer and destruction of the insulation.
Opening up the secondary winding of a CT
in the presence of current in the primary leads to overvoltage in the secondary circuits and insulation breakdown.

Voltage transformers

A voltage transformer ( VT ) is similar in principle of operation and design to a conventional power transformer and consists of: a steel core (magnetic core) assembled from thin plates of transformer steel, and 2 windings - primary and secondary, isolated from each other and from the core.

The device and connection diagram of the voltage transformer are shown in Fig. 2-1.

The primary winding W1 , which has a very large number of turns, is connected directly to the high-voltage network, and are connected in parallel W2 .

Under the influence of the network voltage, a current passes through the primary winding, creating a flux F , which, crossing the turns of the secondary winding, induces an emf in it. E , which when the secondary winding is open (idling transformer) is equal to the voltage at its terminals U2xx .

Voltage U2xx is less than the primary voltage U1 as many times as the number of turns of the secondary winding W2 is less than the number of turns of the primary winding W1 :

(2-1)

The ratio of the number of turns of the windings is called the transformation ratio and is denoted nн:

Therefore, we can write:

(2-2)

If a load in the form of devices and relays is connected to the secondary winding, then the voltage at its terminals U2 will be less than the emf. by the magnitude of the voltage drop in the resistance of the secondary winding. However, this voltage drop is small and can be neglected, then:

U1=U2nn; (2-3)

In passports for voltage transformers, their transformation ratios are indicated as a fraction, the numerator of which is the rated primary voltage, and the denominator is the rated secondary voltage.

For the correct connection of the VT to each other and the correct connection to them of power direction relays, wattmeters and meters, the winding terminals are marked in a certain way: the beginning of the primary winding is A , the end is X ; the beginning of the main secondary winding is a , the end is x ; start of additional winding a

d
, end – x
d
.
Voltage transformers are made in single-phase or three-phase versions.

When single-phase VTs on to phase voltages, the beginnings of their primary windings are connected to the phases, and the ends are collected at the zero point.

When a VT for phase-to-phase voltages, the beginnings of the primary windings are connected to the initial phases in the order of their alternation (for example, when 2 single-phase VTs to phase-to-phase voltages AB and BC with phase alternation A-B-C, the first VT is connected by the beginning of the primary winding to phase A , the end - to phase B , and the second VT - the beginning to phase B and the end to phase C) .

When marking the terminals of the secondary windings of voltage transformers, the beginning
a is taken to be the terminal from which the current comes out, while in the primary winding the current passes from the beginning A to the end X (Fig. 2-2).

Thus, the rule for marking the windings of voltage transformers is as follows:

If on the primary side the current enters the beginning of A, then the beginning of the secondary winding a will be its terminal from which the current leaves at that moment .

When marking and turning on the windings according to this rule, the direction of the current in the load (device or relay) when connected through a VT will remain the same as when connected directly to the network.

Table 2-1

Permissible VT errors

The same voltage transformer can operate with different accuracy classes when the load connected to its secondary winding changes. Therefore, in passports and reference books for voltage transformers, two power values ​​are indicated: the rated power at which the transformer can operate in a guaranteed accuracy class and the maximum power at which it can operate with permissible heating of the windings.

In addition to the main errors (in magnitude and angle), the operation of relay protection and measurement accuracy are also affected by additional errors associated with the voltage drop in the voltage circuits from the voltage transformer to the installation site of the protection or measurement panels. Thus, for relay protection voltage circuits, the normalized voltage drop should not exceed 3% , for panel electrical measuring instruments no more than 1.5% , and for electricity meters - no more than 0.5% .

CURRENT LINE PROTECTION

The main types of damage to power lines are interphase and single-phase short circuits. Line protection must detect the occurrence of a fault and generate a command to disconnect the switches of the damaged line from the power sources.

To protect lines from short circuits. Protections that respond to an increase in current above a predetermined value (set) have become widespread. Such protections are called maximum current protection (MCP) .

Overcurrent protection can be implemented using various technical means:

− fuses with fuse links;

− electromagnetic and thermal releases of automatic switches (automatic machines);

− maximum current relays in combination with time relays, intermediate and indicating relays.

MTZ is widely used in radial electrical networks with voltages up to 35 kV .

In voltage networks up to 1 kV, current protection is usually carried out using fuses or circuit breakers, and in networks above 1 kV using relays.

Automatic circuit breakers (automatic circuit breakers) are devices consisting of a switch with a powerful contact system for disconnecting short-circuit currents. and current protection relays that act to turn off the machine when damage or overload occurs.

2. Automatic machines have a number of significant advantages compared to fuses:

— readiness to quickly turn on immediately after disconnecting the protected line;

— the ability to disconnect all three phases of the protected connection, while a blown fuse in one phase can lead to dangerous operation on 2 phases for electric motors.

The presence of better protective (time-current) characteristics of circuit breakers, compared to fuses, predetermined their widespread use as the main current protection in electrical networks up to 1 kV.

Protection current

The minimum current at which the protection is triggered is called the tripping current of the maximum current protection.

The overcurrent protection current is selected to be greater than the maximum operating current of the protected line (maximum load current), taking into account the need to return the protection after disconnecting the short circuit. protection of the previous section of the network .

To solve this problem, the following conditions must be met:

1. The protection trip current must be greater than the maximum operating load current:

Is.z. > Iwork.max (3-7)

2. After disconnecting the external short circuit. the starting protection elements must return to their original state:

(3-8)

3. When choosing the operating current, it is necessary to take into account the increase in current when starting motors:

(3-9)

Usually the value Ks.zap . is in the range from 1.2 to