Name: Microprocessor automation and relay protection of electrical power systems, 2nd edition
Publisher: ID MPEI
Dyakov A.F., Ovcharenko N.I.
ISBN: 978-5-383-00467-8
Year: 2010
Pages: 336
Format: pdf, djvu
Size: 69.2 MB
Language: Russian
About the book:
In the book Microprocessor automation and relay protection of electrical power systems talks about electrical power systems - methods of operation, principles of operation. It provides structural and multifunctional diagrams of microprocessor devices for emergency automation and relay protection of the most modern Russian developments.
Preface
Introduction
Chapter first. Measuring conversion of operating parameters into information signals of microprocessor automation and relay protection of electrical systems
1.1. Purpose and types of measurement conversion
1.2. Software measuring converters of information parameters of input signals
1.3. Software measuring converters of active and reactive power
1.4. Software filters for symmetrical components
Self-test questions
Chapter two. Microprocessor automatic synchronizers
2.1. Automatic synchronizers for synchronous generators
2.2. Microprocessor automatic synchronizer type AS-M
2.3. Microprocessor automatic synchronizer "Sprint-M" type
Self-test questions
Chapter three. Microprocessor automatic excitation regulators for synchronous generators
3.1. Modern excitation of generators
3.2. General functional diagram automatic excitation control
3.3. Microprocessor automatic regulators of thyristor excitation of synchronous generators
3.4. Software measuring elements of microprocessor regulators
3.5. Features of the microprocessor automatic excitation regulator KOSUR-Ts
3.6. Features of digital control of exciter thyristors
3.7. Functioning algorithm and structural scheme microprocessor excitation regulators
3.8. Adaptive automatic excitation controllers
Self-test questions
Chapter Four. Microprocessor automatic control of excitation of asynchronized generators
4.1. Features of excitation and excitation control of an asynchronized generator
4.2. Algorithm for the functioning of the automatic regulator
4.3. Microprocessor automatic system control of excitation and power of an asynchronized generator
Self-test questions
Chapter five. Automatic regulation of rotation speed and active power of synchronous generators
5.1. Features of automatic frequency and power control
5.2. Microprocessor automatic speed and active power controllers
5.3. Microprocessor automatic system for regulating the frequency and power of turbogenerators
Self-test questions
Chapter six. Automatic voltage and reactive power regulators for synchronous and static compensators
6.l. Features of operating modes of synchronous and static compensators
6.2. Excitation of modern synchronous compensators
6.3. Automatic reactive power controllers for synchronous compensators
6.4. Automatic reactive power controllers for static compensators
6.5. Microprocessor control of brushless excitation of powerful synchronous electric motors
Self-test questions
Chapter seven. Microprocessor relay protection and automation of auxiliary needs of power plants and electrical networks with a voltage of 6-35 kV
7.1. Types of microprocessor devices
7.2. Relay-action software measuring elements
7.3. Microprocessor complexes STC "Mekhanotronika"
7.4. Microprocessor terminals of JSC RADIUS Avtomatika
7.5. Terminals "IC "BRESLER"
7.6. Features of distance protection and automatic reconnection of 35 kV lines
7.7. Automatic frequency shedding and frequency restart
7.8. Accelerated automatic switching on of the reserve
Self-test questions
Chapter eight. Integrated microprocessor relay protection and automation of synchronous generators and transformers
8.1. Types and features
8.2. Microprocessor protection and automation of synchronous generators and transformers
8.3. Features of integrated microprocessor protection
8.4. Features of microprocessor automation integrated with protection
8.5. Microprocessor protection and automation of transformers
8.b. Features of microprocessor protection and automation of transformers STC "Mekhanotronika"
8.7. Microprocessor protection of transformers "IC "Bresler"
8.8. Microprocessor protection and automation of Sirius type transformers
8.9. Features of microprocessor protection and automation of high and ultra-high voltage autotransformers 000 NPP "EKRA"
Self-test questions
Chapter Nine. Microprocessor relay protection of high and extra high voltage power lines
9.1. Types and features. Unified terminals
9.2. Microprocessor filter directional high frequency protection
9.3. Microprocessor-based differential-phase high-frequency protection
9.4. Microprocessor phase differential protection terminals
9.5. Microprocessor-based distance and current-directed zero-sequence protection of power lines
Self-test questions
Chapter ten. Microprocessor-based emergency automation for high and ultra-high voltage power lines
10.1. Types of microprocessor automation
10.2. Microprocessor automatic restart
10.3. Software automatic single-phase restart
10.4. Microprocessor device for monitoring the extinction of the electric arc and successful inclusion disconnected phase on one side
10.5. Action of automatic single-phase restart
10.6. Microprocessor automation of voltage rise limits
10.7. Microprocessor-based automatic devices for identifying fault locations in power lines
10.8. Automatic recorder of electromagnetic transients
Self-test questions
Chapter Eleven. Microprocessor automation for preventing instability
11.1. Features of microprocessor implementation of automatic dosing and storage of emergency control actions
11.2. Microprocessor-based stability control automation panel
11.3. Microprocessor software and hardware complex for automatic dosing and storage of emergency control actions
11.4. Microprocessor emergency control device SMART-PA
11.5. Functioning and development of microprocessor automation for preventing instability
Self-test questions
Chapter twelve. Microprocessor automation for eliminating asynchronous mode
12.1. Purpose and types of automatic devices
12.2. Electrical signs of asynchronous mode
12.3. Options for microprocessor automation for eliminating asynchronous mode
12.4. Microprocessor automation 000 "ABB Automation"
12.5. Microprocessor automation of the Far Eastern State Technical University
12.6. Microprocessor automation JSC "Institute "Energosetproekt"
Self-test questions
Chapter thirteen. Automated control systems for power plants and power systems
13.1. Purpose and principles of implementation automated control power plants
13.2. Microprocessor automated control system for hydroelectric power plants
13.3. Microprocessor automated control system for thermal power plants
13.4. Technical implementation automated system control of the electrical part of thermal power plants
13.5. Digital automatic system for controlling frequency and active power of the electric power system
Self-test questions
Bibliography
Relay operator's handbook. Full version books by the famous author Chernobrovov N.V.
The book examines relay protection of electrical networks, power plant equipment and distribution busbars.
The book is intended as teaching aid for students of energy technical schools and can be used by students of electrical engineering and energy universities, as well as engineers and technicians involved in the operation, installation and design of relay protection of power plants and networks. ...
Chapter first. General concepts about relay protection
Purpose of relay protection
Damage in electrical installations
Abnormal modes
Basic requirements for relay protection
Protection elements, relays and their varieties
Methods for depicting relays and protection circuits in drawings
Methods for turning on a relay
Methods of influencing protection on a circuit breaker
Operating current sources
Chapter two. Relay
General principles of relay implementation
Electromechanical relays
Electromagnetic relays
Electromagnetic current and voltage relays
Electromagnetic intermediate relays
Indicator relays
Time relay
Polarized relays
Induction relays
Induction current and voltage relays
Current induction relay series RT-80 and RT-90
Inductive Power Direction Relays
Magnetoelectric relays
Relays using semiconductors
Rectified current relays that respond to one electrical quantity
Relay comparing the absolute values of two voltages U1 and U2
Relay for direct comparison of the phases of two electrical quantities U1 and U2
Chapter three. Current transformers and their connection diagrams
Current transformer errors
Parameters influencing the decrease in magnetizing current
Accuracy requirements for current transformers and their selection
Pin designation
Image of secondary current vectors
Typical current transformer connection diagrams
Current transformer load
Filters for symmetrical current components
Chapter Four. Overcurrent protection
Operating principle of current protection
Protecting lines with overcurrent protection
Protection circuits
Behavior of maximum protection for double earth faults
Protection current
Protection time delay
Overcurrent protection with starting (blocking) from the minimum voltage relay
Maximum protection on alternating operating current
Maximum protection with direct acting relays
General assessment and scope of overcurrent protection
Chapter five. Current cut-offs
Operating principle of current cut-offs
Cut-off schemes
Instantaneous cut-offs on single-sided feed lines
Non-selective cutoffs
Cut-offs on lines with double-sided feeding
Timed cut-offs
Current three-stage protection
Estimation of current cut-offs
Chapter six. Voltage transformers and their connection diagrams
Basic information
Voltage transformer errors
Voltage transformer connection diagrams
Damage in voltage transformer circuits and monitoring their serviceability
Capacitive voltage dividers
Negative sequence voltage filter
Chapter seven. Directional current protection
The need for directional protection in networks with two-way power supply
Scheme and principle of operation of current directional protection
Power direction relay connection diagrams
Behavior of power relays connected to the current of an undamaged phase
Blocking of maximum directional protection for earth faults
Selecting protection settings
Dead zone
Current directional cut-offs
Brief assessment of directional current protection
Chapter eight. Ground fault protection in networks with high ground fault current
General information
Zero sequence overcurrent protection
Current directional zero sequence protection
Zero Sequence Cutoffs
Staged zero sequence protection
Power supply of the polarizing winding of the zero-sequence power relay from current transformers
Assessment and scope of protection
Chapter nine. Protection against earth faults in networks with low current Kaniya on land
Currents and voltages during a single-phase ground fault
Basic protection requirements
Principles of earth fault protection
Protections that respond to artificially created zero-sequence currents
Protections reacting to residual currents of a compensated network
Protections responding to transient currents
Chapter ten. Line differential protection
Purpose and types of differential protection
The operating principle is longitudinal. differential protection
Unbalance currents in differential protection
General principles of differential line protection
Device for monitoring the health of connecting wires
Longitudinal differential protection of lines type DZL
Longitudinal differential protection
Operating principle and types of transverse differential protection of parallel lines
Current transverse differential protection
Directional transverse differential protection....
Directional transverse residual residual protection
Directional transverse differential protection with separate sets against phase-to-phase and single-phase short circuits.
Ways to increase the sensitivity of trigger organs
transverse differential protection
. Evaluation of directional transverse differential protections
Current balanced protection
Chapter Eleven. Distance protection
Purpose and principle of operation
Characteristics of time delay of distance protection
Elements of distance protection and their interaction....
Characteristics of operation of distance relays and their image on the complex plane
Principles of resistance relays and basic requirements for their designs
Resistance relays on rectified current, performed using semiconductor devices
Electromechanical resistance relays
Accuracy of relay operation. Resistance and current of accurate operation
Remote protection controls
Simplified circuits with a reduced number of remote controls
Reasons that distort the work of remote organs. . .
Starting elements of distance protection
Distance protection schemes
Semiconductor protection circuits
Selecting distance protection settings
Brief conclusions -
Chapter twelve. High frequency protection
Purpose and types of high-frequency protection
Operating principle of directional protection with high frequency blocking
High frequency protection part
Directional protection with high frequency blocking
Types of directional high-frequency protection and their circuits
Differential-phase high-frequency protection
Differential-phase high-frequency protection type DFZ-2
Selecting settings for differential-phase high-frequency protection
Evaluation of high frequency protection
Chapter thirteen. Preventing incorrect swing protection actions
The nature of the change in current, voltage and resistance at the relay terminals during swings
Behavior of protection during swings
Measures to prevent incorrect actions of the protection during swings
Swing protection interlocking device sensing negative sequence current or voltage....
Swing locking device that responds to the rate of change of current, voltage or resistance
Chapter fourteen. Protection of extra high voltage lines and protection of branches with branches
Protection of extra high voltage lines
Branch line protection
Chapter fifteen. Generator protection
Damage and abnormal operating conditions of generators, basic requirements for the protection of generators
Protection against phase-to-phase short circuits in the stator winding
Protection against short circuits between turns of one phase
Protection against short circuit of the stator winding to the housing (to ground)
Overcurrent protection during external short circuits and overloads
Protection of hydro generators from voltage increases
Rotor protection
Complete generator protection circuit
Protection of synchronous compensators
Chapter sixteen. Protection of transformers and autotransformers
Damage and abnormal operating conditions of transformers and autotransformers, types of protection and requirements for them
Overcurrent protection for external short circuits
Overload protection
Current cut-off
Differential protection
Unbalance currents in differential protection of autotransformers
Magnetizing currents of power transformers when energized
Differential protection schemes
Brief assessment of transformer differential protection
. Gas protection of transformers
Current protection against short circuits to the transformer body (casing)
Features of protection of transformers without circuit breakers on the high voltage side
Protection of booster control transformers
Chapter seventeen. Protection of generator-transformer and generator-transformer-line blocks
Block protection features
Generator-transformer unit protection
Features of protection of generator-transformer-line units
Chapter Eighteen. Motor protection
General requirements to the protection of electric motors
Main types of protection used on electric motors
Some properties of asynchronous electric motors
Protection of electric motors against short circuits between phases
Protection of electric motors against single phase ground faults
Motor overload protection
Motor undervoltage protection
Protection of electric motors with voltages below 1000 V
Calculation of self-starting currents of electric motors and residual voltage at their terminals
Protection of synchronous electric motors
Chapter nineteen. Busbar protection
Types of tire protection and requirements for them
Tire differential protection
Measures to improve the reliability of differential busbar protection
Actuation current of differential protection of buses with relays connected via BIT
Types of bus differential protection schemes
Evaluation of busbar differential protection and its applications
Incomplete differential busbar protection
Busbar protection with current cut-off
Distance tire protection
Busbar protection 110-500 kV with current transformers having increased error
Chapter twenty. Redundancy of relay protection and switches
Necessity and methods of reservation
Principles of implementation of a circuit breaker failure backup device (CBF)
Redundancy Device Evaluation
Application. Overcurrent protection on semiconductor devices
Literature
N. V. Chernobrovov
RELAY PROTECTION
FIFTH EDITION, REVISED
Approved by the USSR Ministry of Energy and Electrification
as a teaching aid for energy students
and energy construction technical schools
“ENERGY” MOSCOW 1974
UDC 621.316..925 (075)
Chernobrovov N.V.
Ch-49 Relay protection. Textbook for technical schools. Ed. 5th, revised and additional M., “Energy”, 1974. 680 p. With ill.
The book examines relay protection of electrical networks, power plant equipment and distribution busbars. The fourth edition of the book was published in
The book is intended as a textbook for students of power engineering colleges and can be used by students of electrical engineering and power engineering universities, as well as by engineers and technicians involved in the operation, installation and design of relay protection of power plants and networks.
30311-601 051(01)-74
Publishing house "Energy", 1974
PREFACE TO THE FIFTH EDITION
Relay protection automatically eliminates damage and abnormal conditions in the electrical part of power systems and is the most important automation ensuring their reliable and stable operation.
IN In modern energy systems, the importance of relay protection is especially increasing due to the rapid growth in the power of energy systems, their unification into single electrically connected systems within several regions, the entire country, and even several states.
Characteristic of modern energy systems is the development of high and ultra-high voltage networks, with the help of which energy systems are interconnected and large flows of electrical energy are transferred from powerful power plants to large consumption centers.
IN In the Soviet Union, on the basis of 500 kV networks, the Unified Energy System of the country (UES) is being created, powerful and long transmission lines are being built 500-750 kV, and in the near future it is planned to create even more powerful transmissions of 1150 kV alternating and 1500 kV direct current, the largest thermal, hydraulic and nuclear power plants are being built, the capacity of energy units is increasing. Accordingly, the power of electrical substations increases, the configuration of electrical networks becomes more complex and their load increases.
Growing loads, increasing the length of power transmission lines, and tightening requirements for the stability of power systems complicate the operating conditions of relay protection and increase the requirements for its speed, sensitivity and reliability. In this regard, there is a continuous process of development and improvement of relay protection technology, aimed at creating more and more advanced protection that meets the requirements of modern energy.
New protections are being created and put into operation for long-distance power transmission of ultra-high voltage, for large generators, transformers and power units. Distance protections with complex characteristics are being developed that make it possible to obtain an optimal solution to a very complex problem - reliable detuning of protection against load and swings while maintaining sufficient sensitivity during short circuits. Ways are being sought to improve blocking against swings and damage in voltage circuits. Methods for reserving failures of protections and switches are being improved. The trend towards abandoning electromechanical relays and switching to static, non-contact systems is becoming increasingly clear.
In this regard, it is widely used in relay protection devices for semiconductor devices (diodes, transistors, thyristors). Relay designs based on magnetic elements are being developed. Attempts are being made to use contact relays that are more reliable than conventional electromechanical designs. Such relays include sealed magnetically controlled contacts (reed switches), which are anchor-free relays (used in computer technology). They are characterized by high speed, reliability and small size. The possibility of using a digital computer to perform relay protection functions is being considered.
It is becoming increasingly necessary to use a digital computer to calculate protection settings, since such calculations in modern power systems are very labor-intensive and time-consuming.
Due to the increase in currents short circuit, caused by an increase in the generating capacity of power systems, issues of accuracy of transformation of the primary currents supplying the measuring elements of relay protection become relevant. To solve this problem, studies are being conducted on the behavior of current transformers, possibilities for increasing their accuracy are being studied, practical methods for calculating the errors of current transformers are being developed, and new, more accurate methods for transforming primary currents are being sought.
In preparation for the reissue of the book, the author sought to reflect new developments in domestic technology in the areas of its development listed above. The book includes new protections and technical solutions that have already found application in practice or have a real prospect of application. Taking this into account, changes and additions have been made to the third chapter, devoted to current transformers, to chapter fifteen, which sets out the principles of generator protection, and to chapter seventeen, concerning the protection of units. Changes and clarifications have been made to the remaining chapters, mainly aimed at improving the presentation.
The author expresses gratitude to the book reviewer T. N. Dorodnova for a number of useful comments. The author requests that all wishes and comments be sent to the address: 113114, Moscow, Shlyuzovaya embankment, 10, Publishing House "Energia".
CHAPTER FIRST
GENERAL CONCEPTS ABOUT RELAY PROTECTION
1-1.PURPOSE OF RELAY PROTECTION
In energy systems, damage and abnormal operating conditions of electrical equipment of power plants and substations, their switchgear, power lines and electrical installations of electrical energy consumers may occur.
Damage in most cases is accompanied by a significant increase in current and a deep decrease in voltage in the elements of the power system.
Increased current releases a large number of heat, causing destruction at the location of the fault and dangerous heating of undamaged lines and equipment through which this current passes.
A decrease in voltage disrupts the normal operation of electricity consumers and the stability of parallel operation of generators and the power system as a whole.
Abnormal conditions usually lead to deviations of voltage, current and frequency values from permissible values. When the frequency and voltage are reduced, there is a risk of disruption normal operation consumers and the stability of the power system, and increased voltage and current 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 of the power system.
To ensure the normal operation of the energy system and electricity consumers, it is necessary to identify and separate the location of damage from the undamaged network as quickly as possible, thereby restoring normal operating conditions and stopping destruction at the location of the damage.
The dangerous consequences of abnormal modes can also be prevented if a deviation from the normal mode is detected in a timely manner and measures are taken to eliminate it (for example, reduce the current when it increases, lower the voltage when it increases, etc.).
In this regard, there is a need to create and use automatic devices that perform these operations and protect the system and its elements from the dangerous consequences of damage and abnormal conditions.
Initially, fuses were used as such protection. However, as the power and voltage of electrical installations grew and their switching circuits became more complex, this method of protection became insufficient, which is why protective devices were created using special automatic machines - relays, called relay protection.
Relay protection is the main type of electrical automation, without which normal and reliable operation of modern energy systems is impossible.
It continuously monitors the state and operating mode of all elements of the power system and responds to the occurrence of damage and abnormal conditions.
When damage occurs, the protection identifies and disconnects the damaged area from the system by acting on special power switches designed to interrupt fault currents.
When abnormal conditions occur, the protection identifies them and, depending on the nature of the violation, performs the operations necessary to restore normal conditions, or sends a signal to the duty personnel.
In modern electrical systems, relay protection is closely related to electrical automation, designed for rapid automatic recovery normal regime and nutrition of consumers.
The main devices of such automation include: automatic reclosers (AR), automatic switches for backup power supplies and equipment (AVR) and automatic frequency shedding (AFS).
Let us consider in more detail the main types of damage and abnormal conditions that occur in electrical installations and their consequences.
1-2. DAMAGE IN ELECTRICAL INSTALLATIONS
Most faults in electrical systems result in short circuits between phases or to ground (Figure 1-1). In the windings of electrical machines and transformers, in addition to short circuits, there are short circuits between the turns of one phase.
The main causes of damage are:
1) violation of the insulation of live parts caused by its aging, unsatisfactory condition, overvoltage, mechanical damage;
2) damage to wires and power line supports caused by their unsatisfactory condition, ice, hurricane winds, dancing wires and other reasons;
3) personnel errors during operations (turning off disconnectors under load, turning them on to an erroneously left grounding, etc.).
All damage is a consequence of design flaws or imperfections of the equipment, poor-quality manufacturing, installation defects, design errors, unsatisfactory or improper care of the equipment, abnormal operating modes of the equipment, operation of the equipment in conditions where
rye it is not calculated. Therefore, damage cannot be considered inevitable, but at the same time, the possibility of its occurrence cannot be ignored.
Short circuits(k.z.) are the most dangerous and severe type of damage. With short circuit e. d.s. E of the power source (generator) is short-circuited through the relatively low resistance of generators, transformers and lines (see Fig. 1-
1, a - d and f).
Therefore, in a short-circuited circuit. d.s. a large current Ic arises, called short-circuit current.
Short circuits are divided into three-phase, two-phase and single-phase depending on the number of closed phases; for short circuits with and without earth; short circuits at one or two network points (Fig. 1-1).
With short circuit due to an increase in current, the voltage drop in the system elements increases, which leads to a decrease in voltage at all points of the network, since the voltage in
any point M (Fig. 1-2, a) UM - E-Ik zm, where E - e. d.s. power source, and zM is the resistance from the power source to point M.
The greatest reduction in voltage occurs at the short circuit. (point K) and in the immediate vicinity of it (Fig. 1-2, a). At network points remote from the damage site,
tension decreases to a lesser extent.
Occurring as a result of short circuit. An increase in current and a decrease in voltage lead to a number of dangerous consequences:
a) Short-circuit current Ik, according to the Joule-Lenz law, releases heat Q = kIk 2 rt in the active resistance r of the circuit through which it passes during time t.
At the site of damage, this heat and the flame of the electric arc produce large destruction, the size of which is greater, the greater the current Ik and time t.
Passing through undamaged equipment and power lines, short-circuit current. Ik heats them above the permissible limit, which can cause damage to insulation and live parts.
b) Voltage reduction during short circuit. disrupts the work of consumers.
The main consumer of electricity is asynchronous electric motors.
Therefore, with a deep decrease in voltage, the rotational torque of electric motors may be less than the resistance moment of the mechanisms, which leads to their stopping.
The normal operation of lighting installations, which make up the second significant part of electricity consumers, is also disrupted when the voltage decreases.
Computing and control machines, which have been widely introduced recently, are especially sensitive to voltage drops.
c) The second, most severe consequence of a decrease in voltage is a violation of the stability of parallel operation of generators. This can lead to the collapse of the system and loss of power to all its consumers.
The reasons for this decay can be explained using the example of the system shown in Fig. 1-2, b. In normal mode, the mechanical torque of the turbines is balanced by the counteracting torque created by the electrical load of the generators, as a result of which the rotation speed of all turbogenerators is constant and equal to synchronous. If a short circuit occurs at point K at the busbars of power plant A, the voltage on them will become equal to zero, as a result of this the electrical load, and therefore the counteracting torque of the generators, will also become equal to zero. At the same time, the same amount of steam (or water) enters the turbine and its torque remains unchanged. As a result, the rotation speed of the turbogenerator will begin to increase rapidly, since the turbine speed regulator acts slowly and will not be able to prevent the acceleration of rotation of the turbogenerators of station A.
The generators at station B are in different conditions. They are far from point K, so the voltage on their buses may be close to normal. Due to the fact that the generators of power station A are unloaded, the entire load of the system will fall on the generators of station B, which may overload and reduce the rotation speed. Thus, as a result of short circuit. the rotation speed of the generators of power plants A and B becomes different, which leads to disruption of their synchronous operation.
With a long short circuit. there may also be a violation of the stability of asynchronous electrical
motors. When the voltage drops, the rotation speed of asynchronous electric motors decreases.
If the slip exceeds a critical value, the engine will go into an area of unstable operation, it will roll over and completely brake.
With increasing slip, the reactive power consumed by asynchronous motors increases, which can lead to a short circuit after switching off. to a shortage of reactive power and, as a consequence, to an avalanche-like decrease in voltage in the entire system and the cessation of its operation.
Accidents with a violation of the stability of the system are the most severe in terms of the amount of damage caused to the power supply.
Considered consequences of short circuit. confirm the conclusion made above that they are a severe and dangerous type of damage that requires rapid shutdown (see § 1-4).
Ground fault of one phase in a network with an isolated neutral or grounded
connected through the high resistance of the arc extinguishing coil (AGC). In Fig. 1-1, d it can be seen that a ground fault does not cause a short circuit, since e. d.s. Ea of the damaged phase A is not shunted by the connection to the ground that appears at point K. The resulting current 1A at the point of damage is closed through the capacitance C of the wires relative to the ground and therefore, as a rule, has a small value, for example, several tens of amperes. Linear voltages with this type of damage remain unchanged (see Chapter 9).
Due to this, in terms of its consequences, a single-phase ground fault in networks with an isolated neutral or grounded through a DGK is significantly different from a short circuit. It does not affect the operation of consumers and does not disrupt the synchronous operation of generators. However, this type of damage creates an abnormal mode, causing overvoltages, which is dangerous from the point of view of the possibility of breaking the insulation relative to the ground of two undamaged phases and the transition of a single-phase ground fault to a phase-to-phase short circuit. (Fig. 1, f).
1-3. ABNORMAL MODES
Abnormal modes include those associated with deviations from the permissible values of current, voltage and frequency that are dangerous for equipment or stable operation of the power system.
Let's consider the most typical abnormal modes.
a) Equipment overload caused by an increase in current above the rated value. The rated current is the maximum current allowed for a given circuit.
mining for an unlimited time.
If the current passing through the equipment exceeds the rated value, then due to the additional heat generated by it, the temperature of live parts and insulation after some time exceeds the permissible value, which leads to accelerated wear of the insulation and its damage. The time allowed for the passage of increased currents depends on their magnitude. The nature of this dependence is shown in Fig. 1-3 and is determined by the design of the equipment and the type of insulating materials. For warning
damage to the equipment when it is overloaded, it is necessary to take measures to unload or turn off the equipment.
b) Oscillations in systems occur when generators (or power plants) A and B operating in parallel are out of synchronism (Fig. 1-2, b). When swinging, a periodic change (“swing”) of current and voltage occurs at each point of the system. The current in all network elements connecting generators A and B that are out of synchronism ranges from zero to maximum value, many times higher than normal weight
disguise The voltage drops from normal to a certain minimum value, which has a different value at each point in the network. At point C, called the electrical swing center, it drops to zero, at other points of the network the voltage drops, but remains above zero, increasing from the swing center C to power sources A and B. The nature of the change in swing current and voltage is similar to a short circuit . An increase in current causes heating of the equipment, and a decrease in voltage disrupts the operation of all consumers of the system. Swinging is a very dangerous abnormal mode that affects the operation of the entire energy system.
c) Increasing the voltage beyond permissible value usually occurs on hydrogenerators when their load is suddenly switched off. The unloaded hydrogenerator increases the rotation speed, which causes an increase in e. d.s. stator to values dangerous for its insulation. Protection in such cases should reduce the excitation current of the generator or turn it off.
An increase in voltage that is dangerous for the insulation of equipment can also occur during one-sided switching off or switching on. long lines power transmission with high capacitance.
In addition to the noted abnormal modes, there are others, the elimination of which is possible using relay protection.
1-4. BASIC REQUIREMENTS FOR RELAY PROTECTION
/. REQUIREMENTS FOR PROTECTION FROM K. 3.
a) Selectivity
Selectivity or selectivity of protection is the ability of protection to turn off during a short circuit. only the damaged section of the network.
In Fig. 1-4 show examples of selective fault tripping. So, with short circuit at point K 1, the protection must disconnect the damaged line with switch B in, i.e., the switch closest to the location of the damage. In this case, all consumers, except those fed from the damaged line, remain in operation.
In the case of short circuit at point K2, with selective action of protection, the damaged line I should be switched off, line II remains in operation. During such a shutdown, all network consumers retain power. This example shows that if a substation is connected to the network by several lines, then selective shutdown of the short circuit. on one of the lines allows you to maintain the connection of this substation with the network, thereby ensuring uninterruptible power supply consumers.
Thus, selective shutdown of faults is the main condition for ensuring reliable power supply to consumers. Non-selective action of protection leads to the development of accidents. As will be shown below, non-selective shutdowns may be allowed, but only in cases where this is dictated by necessity and does not affect the power supply of consumers.
b) Speed of action
Switching off the short circuit should be carried out as quickly as possible to limit the extent of equipment destruction, increase the efficiency of automatic reconnection of lines and busbars, reduce the duration of voltage reduction for consumers and maintain the stability of the parallel operation of generators, power plants and the power system as a whole. The last of the listed conditions is the main one.
Allowable short-circuit disconnection time (1-2, b) according to the condition of maintaining stability depends on a number of factors. The most important of them is the amount of residual voltage on the buses of power plants and hub substations connecting power plants with the power system. The lower the residual voltage, the more likely the instability is and, therefore, the faster the short circuit needs to be turned off. The most severe in terms of stability conditions are three-phase short circuits. and two-phase short circuits to the ground online with a deaf person
ground neutral (Fig. 1-2, a and d), since with these damages the greatest decreases in all phase-to-phase voltages occur.
IN Modern power systems require a very short short-circuit disconnection time to maintain stability. For example, on power lines 300-500 kV it is necessary to disconnect the fault within 0.1-0.12 s after its occurrence, and in 110-220 kV networks - within 0.15-0.3 s. In 6 and 10 kV distribution networks, separated from power sources by high resistance, short circuit. can be turned off over a period of approximately 1.5-3 s, since they do not cause a dangerous drop in voltage on the generators and therefore do not affect the stability of the system. An accurate assessment of the permissible outage time is made using special stability calculations carried out for this purpose.
IN as an approximate criterion (measure) of the need to use high-speed protection Rules for the construction of electrical installations (PUE) [L. 1] 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 receives -
is less than 60% of the nominal value, then to maintain stability, fast shutdown should be used damage, i.e. apply quick-acting protection.
The total fault shutdown time t open is the sum of the protection operating time
you t 3 and the operating time of the switch t in, breaking the short-circuit current, i.e. t off =t a + t in. Thus, to speed up the shutdown, it is necessary to speed up the action of both the protection and the shutdown.
tel. The most common switches operate with a time of 0.15-0.06 s. In order to ensure the above requirement for disconnection with such switches,
short circuit, for example, with t = 0.2 s, the protection should operate with a time of 0.05-0.12 s, and if it is necessary to turn off with t = 0.12 s and the switch operates with a time of 0.08 s protection operation should not exceed 0.04 s.
Protections operating for up to 0.1-0.2 s are considered fast-acting. Modern high-speed protection can operate with a time of 0.02-0.04 s.
The requirement for speed is in some cases a determining condition that ensures the stability of parallel operation of power plants and power systems.
Creating selective high-speed protection is an important and difficult task in relay protection technology. These protections are quite complex and expensive, so they should be used only in cases where simpler time-delay protections do not provide the required speed of action.
For the sake of simplicity, it is allowed to use simple high-speed protections that do not provide the necessary selectivity. In this case, to correct non-selectivity, automatic reclosure is used, which quickly turns back on the non-selectively disconnected section of the system.
c) Sensitivity
(Document)
n1.doc
N. V. ChernobrovovR E L E Y N A Y
PROTECTION
FIFTH EDITION,
RECYCLED
Approved by the Ministry
Energy and electrification of the USSR
As a teaching aid
For energy students
And energy construction technical schools
“ENERGY” MOSCOW 1974
6P2.11
UDC 621.316..925 (075)
Chernobrovov N.V.
Ch-49 Relay protection. Textbook for technical schools.
Ed. 5th, revised and additional M., “Energy”, 1974. 680 p. With ill.
The book examines relay protection of electrical networks, power plant equipment and distribution busbars. The fourth edition of the book was published in 1971.
The book is intended as a textbook for students of power engineering colleges and can be used by students of electrical engineering and power engineering universities, as well as by engineers and technicians involved in the operation, installation and design of relay protection of power plants and networks.
30311-601
051(01)-74
75-74 6P2.11
Publishing house "Energy", 1974.
PREFACE TO THE FIFTH EDITION
Relay protection automatically eliminates damage and abnormal conditions in the electrical part of power systems and is the most important automation ensuring their reliable and stable operation.
In modern energy systems, the importance of relay protection is especially increasing due to the rapid growth in the power of energy systems, their unification into single electrically connected systems within several regions, the entire country, and even several states.
Characteristic of modern energy systems is the development of high and ultra-high voltage networks, with the help of which energy systems are interconnected and large flows of electrical energy are transferred from powerful power plants to large consumption centers.
In the Soviet Union, on the basis of 500 kV networks, the Unified Energy System of the country (UES) is being created, powerful and extended transmissions of 500-750 kV are being built, and in the near future it is planned to create even more powerful transmissions of 1150 kV alternating current and 1500 kV direct current, the largest thermal, hydraulic and nuclear power plants, the power of energy units is increasing. Accordingly, the power of electrical substations increases, the configuration of electrical networks becomes more complex and their load increases.
Growing loads, increasing the length of power transmission lines, and tightening requirements for the stability of power systems complicate the operating conditions of relay protection and increase the requirements for its speed, sensitivity and reliability. In this regard, there is a continuous process of development and improvement of relay protection technology, aimed at creating more and more advanced protection that meets the requirements of modern energy.
New protections are being created and put into operation for long-distance power transmission of ultra-high voltage, for large generators, transformers and power units. Distance protections with complex characteristics are being developed that make it possible to obtain an optimal solution to a very complex problem - reliable detuning of protection against load and swings while maintaining sufficient sensitivity during short circuits. Ways are being sought to improve blocking against swings and damage in voltage circuits. Methods for reserving failures of protections and switches are being improved. The trend towards abandoning electromechanical relays and switching to static, non-contact systems is becoming increasingly clear.
In this regard, it is widely used in relay protection devices for semiconductor devices (diodes, transistors, thyristors). Relay designs based on magnetic elements are being developed. Attempts are being made to use contact relays that are more reliable than conventional electromechanical designs. Such relays include sealed magnetically controlled contacts (reed switches), which are anchor-free relays (used in computer technology). They are characterized by high speed, reliability and small size. The possibility of using a digital computer to perform relay protection functions is being considered.
It is becoming increasingly necessary to use a digital computer to calculate protection settings, since such calculations in modern power systems are very labor-intensive and time-consuming.
In connection with the increase in short-circuit currents caused by an increase in the generating capacity of power systems, issues of accuracy of transformation of the primary currents supplying the measuring elements of relay protection become relevant. To solve this problem, studies are being conducted on the behavior of current transformers, possibilities for increasing their accuracy are being studied, practical methods for calculating the errors of current transformers are being developed, and new, more accurate methods for transforming primary currents are being sought.
In preparation for the reissue of the book, the author sought to reflect new developments in domestic technology in the areas of its development listed above. The book includes new protections and technical solutions that have already found application in practice or have a real prospect of application. Taking this into account, changes and additions have been made to the third chapter, devoted to current transformers, to chapter fifteen, which sets out the principles of generator protection, and to chapter seventeen, concerning the protection of units. Changes and clarifications have been made to the remaining chapters, mainly aimed at improving the presentation.
The author expresses gratitude to the book reviewer T. N. Dorodnova for a number of useful comments. The author requests that all wishes and comments be sent to the address: 113114, Moscow, Shlyuzovaya embankment, 10, Publishing House "Energia".
PURPOSE OF RELAY PROTECTION
In energy systems, damage and abnormal operating conditions of electrical equipment of power plants and substations, their switchgear, power lines and electrical installations of electrical energy consumers may occur.
Damage in most cases is accompanied by a significant increase in current and a deep decrease in voltage in the elements of the power system.
The increased current generates a large amount of heat, causing destruction at the fault site and dangerous heating of undamaged lines and equipment through which this current passes.
A decrease in voltage disrupts the normal operation of electricity consumers and the stability of parallel operation of generators and the power system as a whole.
Abnormal conditions usually lead to deviations of voltage, current and frequency values from permissible values. When the frequency and voltage decrease, there is a danger of disruption to the normal operation of consumers and the stability of the power system, and an increase in voltage and current threatens to damage 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 of the power system.
To ensure the normal operation of the energy system and electricity consumers, it is necessary to identify and separate the location of damage from the undamaged network as quickly as possible, thereby restoring normal operating conditions and stopping destruction at the location of the damage.
The dangerous consequences of abnormal modes can also be prevented if a deviation from the normal mode is detected in a timely manner and measures are taken to eliminate it (for example, reduce the current when it increases, lower the voltage when it increases, etc.).
In this regard, there is a need to create and use automatic devices that perform these operations and protect the system and its elements from the dangerous consequences of damage and abnormal conditions.
Initially, fuses were used as such protection. However, as the power and voltage of electrical installations grew and their switching circuits became more complex, this method of protection became insufficient, which is why protective devices were created using special automatic machines - relays, called relay protection.
Relay protection is the main type of electrical automation, without which normal and reliable operation of modern energy systems is impossible. It continuously monitors the state and operating mode of all elements of the power system and responds to the occurrence of damage and abnormal conditions.
When damage occurs, the protection identifies and disconnects the damaged area from the system by acting on special power switches designed to interrupt fault currents.
When abnormal conditions occur, the protection identifies them and, depending on the nature of the violation, performs the operations necessary to restore normal conditions, or sends a signal to the duty personnel.
In modern electrical systems, relay protection is closely related to electrical automation, designed to quickly automatically restore normal operation and supply power to consumers.
The main devices of such automation include: automatic reclosers (AR), automatic switches for backup power supplies and equipment (AVR) and automatic frequency shedding (AFS).
Let us consider in more detail the main types of damage and abnormal conditions that occur in electrical installations and their consequences.
1-2. DAMAGE IN ELECTRICAL INSTALLATIONS
Most faults in electrical systems result in short circuits between phases or to ground (Figure 1-1). In the windings of electrical machines and transformers, in addition to short circuits, there are short circuits between the turns of one phase.
The main causes of damage are:
1) violation of the insulation of live parts caused by its aging, unsatisfactory condition, overvoltage, mechanical damage;
2) damage to wires and supports of power lines caused by their unsatisfactory condition, ice, hurricane winds, dancing wires and other reasons;
3) personnel errors during operations (turning off disconnectors under load, turning them on to an erroneously left grounding, etc.).
All damage is a consequence of design flaws or imperfections of the equipment, poor quality of its manufacture, installation defects, design errors, unsatisfactory or improper care of the equipment, abnormal operating modes of the equipment, operation of the equipment in conditions for which it was not designed. Therefore, damage cannot be considered inevitable, but at the same time, the possibility of its occurrence cannot be ignored.
Short circuits(k.z.) are the most dangerous and severe type of damage. With short circuit e. d.s. E the power source (generator) is short-circuited through the relatively low resistance of generators, transformers and lines (see Fig. 1-1, a - G And e).
Therefore, in a short-circuited circuit. d.s. a large current occurs I To, called short circuit current.
Short circuits are divided into three-phase, two-phase and single-phase depending on the number of closed phases; for short circuits with and without earth; short circuits at one or two network points (Fig. 1-1).
With short circuit due to an increase in current, the voltage drop in the system elements increases, which leads to a decrease in voltage at all points of the network, since the voltage at any point M (Fig. 1-2, a) U M - E-I k z m, where E - e. d.s. power source, and z M is the resistance from the power source to point M.
The greatest reduction in voltage occurs at the short circuit. (point K) and in the immediate vicinity of it (Fig. 1-2, a). At network points remote from the fault site, the voltage decreases to a lesser extent.
Occurring as a result of short circuit. An increase in current and a decrease in voltage lead to a number of dangerous consequences:
A) Short-circuit current I k, according to the Joule-Lenz law, releases heat Q = kI k 2 rt in the active resistance r of the circuit through which it passes during time t.
At the site of damage, this heat and the flame of the electric arc produce great destruction, the size of which is greater, the greater the current Ik and the time t.
Passing through undamaged equipment and power lines, short-circuit current. I k heats them above the permissible limit, which can cause damage to insulation and live parts.
B) Voltage reduction during short circuit. disrupts the work of consumers.
The main consumer of electricity is asynchronous electric motors. The rotational torque of motors MD is proportional to the square of the voltage U at their terminals: M d = kU 2.
Therefore, with a deep decrease in voltage, the rotational torque of electric motors may be less than the resistance moment of the mechanisms, which leads to their stopping.
The normal operation of lighting installations, which make up the second significant part of electricity consumers, is also disrupted when the voltage decreases.
Computing and control machines, which have been widely introduced recently, are especially sensitive to voltage drops.
C) The second, most severe consequence of a decrease in voltage is a violation of the stability of parallel operation of generators. This can lead to the collapse of the system and loss of power to all its consumers.
The reasons for this decay can be explained using the example of the system shown in Fig. 1-2, b. In normal mode, the mechanical torque of the turbines is balanced by the counteracting torque created by the electrical load of the generators, as a result of which the rotation speed of all turbogenerators is constant and equal to synchronous. If a short circuit occurs at point K near the power plant buses A the voltage across them will become zero, as a result of which the electrical load, and therefore the counteracting torque of the generators, will also become zero. At the same time, the same amount of steam (or water) enters the turbine and its torque remains unchanged. As a result, the rotation speed of the turbogenerator will begin to increase rapidly, since the turbine speed regulator acts slowly and will not be able to prevent the acceleration of rotation of the station’s turbogenerators A.
The station generators are in different conditions IN. They are far from point K, so the voltage on their buses can be close to normal. Due to the fact that power plant generators A unloaded, the entire load of the system will fall on the generators of station B, which may overload and reduce the rotation speed. Thus, as a result of short circuit. rotation speed of power plant generators A And IN becomes different, which leads to disruption of their synchronous operation.
With a long short circuit. The stability of the operation of asynchronous electric motors may also be disrupted. When the voltage drops, the rotation speed of asynchronous electric motors decreases.
If the slip exceeds a critical value, the engine will go into an area of unstable operation, it will roll over and completely brake.
With increasing slip, the reactive power consumed by asynchronous motors increases, which can lead to a short circuit after switching off. to a shortage of reactive power and, as a consequence, to an avalanche-like decrease in voltage in the entire system and the cessation of its operation.
Accidents with a violation of the stability of the system are the most severe in terms of the amount of damage caused to the power supply.
Considered consequences of short circuit. confirm the conclusion made above that they are a severe and dangerous type of damage that requires rapid shutdown (see § 1-4).
Ground fault of one phase in a network with isolated neutral or an arc extinguishing coil (AEC) grounded through a high resistance. In Fig. 1-1, d it can be seen that a ground fault does not cause a short circuit, since e. d.s. Ea of the damaged phase A is not shunted by the connection to the ground that appears at point K. The resulting current of 1 A at the point of damage is closed through the capacitance C of the wires relative to the ground and therefore, as a rule, has a small value, for example, several tens of amperes. Linear voltages with this type of damage remain unchanged (see Chapter 9).
Due to this, in terms of its consequences, a single-phase ground fault in networks with an isolated neutral or grounded through a DGK is significantly different from a short circuit. It does not affect the operation of consumers and does not disrupt the synchronous operation of generators. However, this type of damage creates an abnormal mode, causing overvoltages, which is dangerous from the point of view of the possibility of breaking the insulation relative to the ground of two undamaged phases and the transition of a single-phase ground fault to a phase-to-phase short circuit. (Fig. 1, f).
1-3. ABNORMAL MODES
Abnormal modes include those associated with deviations from the permissible values of current, voltage and frequency that are dangerous for equipment or stable operation of the power system.
Let's consider the most typical abnormal modes.
A) Equipment overload caused by an increase in current above the rated value. Rated is the maximum current allowed for of this equipment for an unlimited time.
If the current passing through the equipment exceeds the rated value, then due to the additional heat generated by it, the temperature of live parts and insulation after some time exceeds the permissible value, which leads to accelerated wear of the insulation and its damage. The time allowed for the passage of increased currents depends 
from their size. The nature of this dependence is shown in Fig. 1-3 and is determined by the design of the equipment and the type of insulating materials. To prevent damage to equipment when it is overloaded, measures must be taken to unload or turn off the equipment.
B) Oscillations in systems occur when generators (or power plants) running in parallel are out of synchronism. A And IN(Fig. 1-2, b). When swinging, a periodic change (“swing”) of current and voltage occurs at each point of the system. Current in all network elements connecting generators out of synchronization A And IN, fluctuates from zero to a maximum value many times higher than the normal value. The voltage drops from normal to a certain minimum value, which has a different value at each point in the network. At the point WITH, called the electrical swing center, it drops to zero, at other points of the network the voltage drops, but remains above zero, increasing from the swing center WITH to power supplies A And IN. In terms of the nature of the change in current and voltage, the swing is similar to a short circuit. An increase in current causes heating of the equipment, and a decrease in voltage disrupts the operation of all consumers of the system. Swinging is a very dangerous abnormal mode that affects the operation of the entire energy system.
C) An increase in voltage above the permissible value usually occurs on hydrogenerators when their load is suddenly switched off. The unloaded hydrogenerator increases the rotation speed, which causes an increase in e. d.s. stator to values dangerous for its insulation. Protection in such cases should reduce the excitation current of the generator or turn it off.
An increase in voltage that is dangerous for equipment insulation can also occur when long power lines with high capacitance are turned off or on one-sidedly.
In addition to the noted abnormal modes, there are others, the elimination of which is possible using relay protection.
1-4. BASIC REQUIREMENTS FOR RELAY PROTECTION
/. REQUIREMENTS FOR PROTECTION FROM K. 3.
a) Selectivity
Selectivity or selectivity of protection is the ability of protection to turn off during a short circuit. only the damaged section of the network.
In Fig. 1-4 show examples of selective fault tripping. So, with short circuit at the point TO 1 the protection must disconnect the damaged line with a switch IN V , i.e., the switch closest to the location of the damage. In this case, all consumers, except those fed from the damaged line, remain in operation.
In the case of short circuit at point K 2, with selective action of protection, the damaged line should be disconnected I, line II remains in work. During such a shutdown, all network consumers retain power. This example shows that if a substation is connected to the network by several lines, then selective shutdown of the short circuit. on one of the lines allows you to maintain the connection of this substation with the network, thereby ensuring uninterrupted power supply to consumers.
Thus, selective shutdown of faults is the main condition for ensuring reliable power supply to consumers. Non-selective action of protection leads to the development of accidents. As will be shown below, non-selective shutdowns may be allowed, but only in cases where this is dictated by necessity and does not affect the power supply of consumers.
b) Speed of action
Switching off the short circuit should be carried out as quickly as possible to limit the extent of equipment destruction, increase the efficiency of automatic reconnection of lines and busbars, reduce the duration of voltage reduction for consumers and maintain the stability of the parallel operation of generators, power plants and the power system as a whole. The last of the listed conditions is the main one.
Allowable short-circuit disconnection time (1-2, b) according to the condition of maintaining stability depends on a number of factors. The most important of them is the amount of residual voltage on the buses of power plants and hub substations connecting power plants with the power system. The lower the residual voltage, the more likely the instability is and, therefore, the faster the short circuit needs to be turned off. The most severe in terms of stability conditions are three-phase short circuits. and two-phase short circuits to the ground in a network with a solidly grounded neutral (Fig. 1-2, a and d), since with these damages the greatest decreases in all phase-to-phase voltages occur.
In modern power systems, to maintain stability, a very short short-circuit disconnection time is required. So, for example, on 300-500 kV power lines it is necessary to disconnect the fault within 0.1-0.12 s after it occurs, and in 110-220 kV networks - within 0.15-0.3 s. In 6 and 10 kV distribution networks, separated from power sources by high resistance, short circuit. can be turned off over a period of approximately 1.5-3 s, since they do not cause a dangerous drop in voltage on the generators and therefore do not affect the stability of the system. An accurate assessment of the permissible outage time is made using special stability calculations carried out for this purpose.
As an approximate criterion (measure) of the need to use high-speed protection, Electrical Installation Rules (PUE) [L. 1] 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 quick shutdown should be used to maintain stability. damage, i.e. apply quick-acting protection.
Total damage shutdown time t open consists of the protection operating time t 3 and operating time of the switch t V , breaking the short-circuit current, i.e. t off = t a + t V. Thus, to speed up the shutdown, it is necessary to speed up the action of both the protection and the circuit breakers. The most common switches operate with a time of 0.15-0.06 s.
In order to ensure with such switches the above requirement to disconnect the short circuit, for example, with t =0.2 s, protection should operate with a time of 0.05-0.12 s, and if necessary, turn off with t = 0.12 s and switch action from 0.08 s, the protection operating time should not exceed 0.04 s.
Protections operating for up to 0.1-0.2 s are considered fast-acting. Modern high-speed protection can operate with a time of 0.02-0.04 s.
The requirement for speed is in some cases a determining condition that ensures the stability of parallel operation of power plants and power systems.
Creating selective high-speed protection is an important and difficult task in relay protection technology. These protections are quite complex and expensive, so they should be used only in cases where simpler time-delay protections do not provide the required speed of action.
For the sake of simplicity, it is allowed to use simple high-speed protections that do not provide the necessary selectivity. In this case, to correct non-selectivity, automatic reclosure is used, which quickly turns back on the non-selectively disconnected section of the system.
c) Sensitivity
In order for the protection to respond to deviations from the normal mode that occur during a short circuit. (increase in current, decrease in voltage, etc.), it must have a certain sensitivity within the established zone of its action. Each protection (for example, I in Fig. 1-5) should disable damage in that area AB, for the protection of which it is installed (the first section of protection I), and, in addition, must act in case of short circuit. on the next, second section sun, protected by protection II. The action of protection in the second section is called long-range redundancy. It is necessary to disconnect the short circuit. in the event that protection II
or circuit breaker Sun will not work due to a malfunction. Reserving the next site is an important requirement. If it is not fulfilled, then with a short circuit. Location on Sun and failure of its protection or switch, the damage will remain undisconnected, which will lead to disruption of the operation of consumers throughout the network.
Protection action I at short circuit in the third section is not required, since if the protection of the third section or its switch fails, the protection must operate II. A simultaneous failure of protection in two sections (third and second) is unlikely, and therefore such a case is not taken into account.
Some types of protection, due to the principle of their action, do not work beyond the first section. The sensitivity of such protections should ensure their reliable operation within the first section. To ensure redundancy of the second section in this case, additional protection is installed, called backup.
Each protection must operate not only with a metal short circuit, but also with short circuits through a transition resistance caused by an electric arc.
The sensitivity of the protection must be such that it can act in the event of a short circuit. in minimum system modes, i.e. in such modes when the change in the value to which the protection responds (current, voltage, etc.) will be the smallest. For example, if at station A (Fig. 1-5) one or more generators are turned off, then the short-circuit current. will decrease, but the sensitivity of the protection should be sufficient to operate in this minimum mode.
Thus, 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 system mode and during short circuits through an electric arc.
The sensitivity of protection is usually characterized by the sensitivity coefficient To h :
For protections that respond to short-circuit current,
d) Reliability
The reliability requirement is that protection mustwork reliably during short circuit. within the limits established for itzones and should not operate incorrectly in modes in whichher work is not envisaged.
The requirement of reliability is very important. Failure to operate or incorrect operation of any protection always leads to additional shutdowns, and sometimes to accidents of system significance.
For example, with short circuit at the point TO(Fig. 1-6) and protection failure IN 1 protection will work VZ, as a result, substations // and /// are additionally switched off, and in case of incorrect operation in normal protection mode AT 4 as a result of line disconnection L4 consumers of substations /, //, /// and IV.
Thus, unreliable protection itself becomes a source of accidents. 
Reliability of protection is ensured by the simplicity of the circuit, the reduction in the number of relays and contacts in it, the simplicity of the design and the quality of manufacture of the relays and other equipment, the quality of installation materials, the installation itself and contact connections, as well as care for it during operation.
Recently, methods have been developed for assessing and analyzing the reliability of relay protection devices using probability theory [L. 33],
In the USSR, the general principles of relay protection are regulated by the PUE [L. 1, typical relay protection schemes and their calculation - “Guidelines for relay protection” [L. 2-61.
II. REQUIREMENTS FOR PROTECTION AGAINST ABNORMAL ALPSXMODES
These protections, as well as short-circuit protection, must have selectivity, sufficient sensitivity and reliability. But speed of action from these protections, as a rule, is not required.
The duration of protection against abnormal conditions depends on the nature of the mode and its consequences. Often abnormal conditions are short-term in nature and are eliminated on their own, for example, a short-term overload when starting an asynchronous electric motor. In such cases, rapid shutdown is not only unnecessary, but may cause harm to consumers. Therefore, equipment shutdown in abnormal mode should be carried out only when there is a real danger to the protected equipment, i.e. in most cases with a time delay.
In cases where the elimination of abnormal conditions can be carried out by personnel on duty, protection against abnormal conditions can be carried out with an effect only on the signal.
1-5. PROTECTION ELEMENTS, RELAYS AND THEIR VARIETIES
Typically, relay protection devices consist of several relays connected to each other according to a specific circuit.
The relay is automatic device, which comes into action (triggered) at a certain value of the input quantity acting on it.
In relay technology, relays with contacts are used - electromechanical, contactless - on semiconductors or on ferromagnetic elements. The first ones close or open contacts when triggered. For the second - at a certain value input quantity X the output value changes abruptly y, for example voltage (Fig. 1-7, A).
Each protection set and its circuit are divided into two parts: reactive and logical.
The reacting (or measuring) part is the main one, it consists of main relays that continuously receive information about the state of the protected element and react to damage or abnormal conditions by sending appropriate commands to the logical part of the protection.
The logical part (or operational) is auxiliary; it perceives the commands of the reacting part and, if their value, sequence and combination correspond to a given program, performs pre-programmed operations and supplies a control pulse to turn off the circuit breakers. The logical part can be implemented using electromechanical relays or circuits using electronic devices - tube or semiconductor.
In accordance with this division of protective devices, relays are also divided into two groups: the main ones, which respond to damage, and the auxiliary ones, which act on the command of the former and are used in the logical part of the circuit.
A sign of the appearance of a short circuit. can serve as an increase in current I, voltage drop U and a decrease in the resistance of the protected area, characterized by the ratio of voltage to current at a given point in the network: z= U/ I.
Accordingly, the following are used as responsive relays: current relays that respond to the current value; voltage relays, which respond to voltage levels, and resistance relays, which respond to changes in resistance.
In combination with the indicated relays, power relays are often used that respond to the magnitude and direction (sign) of the short-circuit power passing through the protection installation site.
Relays that operate when the value to which they react increases are called maximum, and relays that operate when this value decreases are called min.
For protection against abnormal conditions, as well as for protection against short circuits, current and voltage relays are used. The former serve as relays that respond to overload, and the latter - to a dangerous increase or decrease in voltage in the network. In addition, a number of special relays are used, for example, frequency relays that operate in the event of an unacceptable decrease or increase in frequency; thermal relays, responding to an increase in heat generated by current during overloads, and some others.
Auxiliary relays include: time relays, which serve to slow down the protection; indicator relays - for signaling and recording the protection action; intermediate relays, transmitting the action of the main relays to open circuit breakers and serving for mutual communication between protection elements.
Each relay can be divided into two parts: sensing and executing. The sensing element in electromechanical structures has a winding that is powered by current or voltage of the protected element, depending on the type of relay (current or voltage).
Power relays and resistance relays have two windings (current and voltage). Through the relay windings it perceives a change in the electrical quantity, to which it reacts.
The actuator element of an electromechanical relay is a moving system that, moving under the influence of forces created by the sensing element, acts on the relay contacts, causing them to close or open.
There are also relays in which the moving system acts directly mechanically to open the switch; such relays do not have contacts.