What Is the Function and Principle of Relay Protection?

01 Basic principles

The relay protection device must have the function of correctly distinguishing whether the protected component is in normal operation or has a fault, and whether it is a fault within the protection zone or a fault outside the zone. To achieve this function, the protection device needs to be constructed based on the characteristics of changes in electrical physical quantities before and after a power system failure occurs.

After a power system failure occurs, the main characteristics of changes in power frequency electrical quantities are:

a. Current increases

In the event of a short circuit, the current in the electrical equipment and transmission lines between the fault point and the power supply will increase from the load current to well beyond the load current.

b. Voltage decrease

When a phase-to-phase short circuit or a ground short-circuit fault occurs, the phase-to-phase voltage or phase voltage value at each point of the system drops, and the closer to the short-circuit point, the lower the voltage.

c.Change in phase angle between current and voltage

During normal operation, the phase angle between current and voltage is the power factor angle of the load, which is generally about 20°. When a three-phase short circuit occurs, the phase angle between current and voltage is determined by the impedance angle of the line, which is generally 60°~ 85°, while when protecting a three-phase short circuit in the opposite direction, the phase angle between current and voltage is 180°+ (60°~85°).

d. Measured impedance changes

The measured impedance is the ratio of voltage to current at the measuring point (protection installation location). During normal operation, the measured impedance is the load impedance; when there is a metallic short circuit, the measured impedance changes to the line impedance. After a fault, the measured impedance decreases significantly, while the impedance angle increases.

When an asymmetric short circuit occurs, a phase sequence component appears. For example, when a two-phase or single-phase grounding short circuit occurs, a negative sequence current and a negative sequence voltage component appear. When a single phase is grounded, a negative sequence and zero sequence current and voltage components appear. These components are not present during normal operation. By utilizing the changes in electrical quantities during short-circuit faults, various principles of relay protection can be constructed.

In addition, in addition to the above protection that reflects power frequency electrical quantities, there are also protections that reflect non-power frequency electrical quantities, such as gas protection.

02 Basic requirements

In order to complete its tasks, the relay protection device must technically meet the four basic requirements of selectivity, quick action, sensitivity and reliability. For relay protection that acts on relay tripping, four basic requirements should be met at the same time. For relay protection devices that act on signals and only reflect abnormal operating conditions, some of these four basic requirements can be reduced.

① Selectivity

Selectivity means that when equipment or lines in the power system are short-circuited, the relay protection will only remove the faulty equipment or lines from the power system. When the protection or circuit breaker of the faulty equipment or lines refuses to operate, the relevant relay protection shall The protection of adjacent equipment or lines will remove the fault.

② Quick action

Quick action means that the relay protection device should be able to remove the fault as quickly as possible to reduce the time the equipment and users operate at high current and low voltage, reduce the damage to the equipment, and improve the stability of the system's parallel operation.

Generally, faults that must be removed quickly include:

a. Make the bus voltage of the power plant or important user lower than the effective value (generally 0.7 times the rated voltage).

b. Internal faults in large-capacity generators, transformers and motors.

c. The cross-section of medium and low-voltage line conductors is too small, and delayed removal is not allowed to avoid overheating.

d. Faults that may endanger personal safety and cause strong interference to the communication system.

The fault removal time includes the action time of the protection device and the circuit breaker. Generally, the action time of fast protection is 0.04s~0.08s, and the fastest can reach 0.01s~0.04s. The tripping time of general circuit breakers is 0.06s~0.15s, and the fastest is 0.06s~0.15s. The fastest one can reach 0.02s~0.06s.

For relay protection devices that respond to abnormal operating conditions, rapid action is generally not required, but signals should be sent with delay according to selective conditions.

③Sensitivity

Sensitivity refers to the response ability of the protection device when a short-circuit fault or abnormal operation occurs in the electrical equipment or circuit within the protected range. The sensitivity of the protection device is measured by the sensitivity coefficient.

The relay protection that can meet the sensitivity requirements can respond correctly when a fault occurs within the specified range, regardless of the location and type of the short-circuit point, and whether there is a transition resistance at the short-circuit point. That is, it is required that the system not only operates at its maximum It can operate reliably when a three-phase short circuit occurs in the minimum operating mode of the system, and it can also operate reliably when a two-phase or single-phase short circuit fault occurs through a large transition resistance in the minimum operating mode of the system.

Maximum operating mode of the system:

When the end of the protected line is short-circuited, the equivalent impedance of the system is the smallest, and the short-circuit current passing through the protection device is the maximum operating mode.

Minimum operating mode of the system:

In the same short-circuit fault situation, the system equivalent impedance is the maximum and the short-circuit current through the protection device is the minimum operation mode.

④Reliability

Reliability includes safety and reliability, which is the most fundamental requirement for relay protection.

a.Safety

The relay protection is required to reliably not operate when it is not required to operate, that is, no malfunction will occur.

b.Reliability

The relay protection is required to act reliably when a fault that should act occurs within the specified protection range, that is, it does not refuse to act.

Both malfunction and rejection of relay protection will bring serious harm to the power system. Even for the same power components, with the development of the power grid, the impact of protection on the system will change.

The above four basic requirements are the basis for designing, configuring and maintaining relay protection, and are also the basis for analyzing and evaluating relay protection. These four basic requirements are interrelated, but often contradictory. Therefore, in actual work, it is necessary to dialectically unify according to the structure of the power grid and the nature of users.

03 Basic tasks

The basic tasks of power system relay protection are:

① Automatically, quickly and selectively remove faulty components from the power system to prevent faulty components from further damage and ensure that other non-faulty parts can quickly resume normal operation.

②Reflect the abnormal operating status of electrical components, and act on the signal according to the conditions of operation and maintenance (such as whether there are regular personnel on duty), so that the personnel on duty can handle it in time, or the device can automatically adjust it, or change those that will cause problems if they continue to operate. Electrical equipment that is damaged or develops into an accident shall be removed. At this time, the protection is generally not required to act quickly, but a certain delay is specified based on the degree of harm to the power system and its components to avoid unnecessary movements and interference caused by temporary operating fluctuations and misoperations.

③The relay protection device can also cooperate with other automation devices in the power system. When conditions permit, predetermined measures can be taken to shorten the time of accidental power outage and restore power supply as soon as possible, thereby improving the reliability of power system operation.

04 Category

Relay protection can be classified in the following 4 ways:

①Classification by protected objects

There are transmission line protection and main equipment protection (such as generator, transformer, busbar, reactor, capacitor, etc. protection).

②Classification by protection function

There is short circuit fault protection and abnormal operation protection. The former can be divided into main protection, backup protection and auxiliary protection; the latter can be divided into overload protection, loss of excitation protection, out-of-step protection, low-frequency protection, non-full-phase operation protection, etc.

③ Classification of signals compared and processed by protection devices

There are analog protection and digital protection. All electromechanical, rectifier, transistor and integrated circuit (operational amplifier) protection devices, which directly reflect the continuous analog quantity of the input signal, are analog protection; they use microprocessors and Microcomputer protection devices, they respond to discrete digital quantities after sampling and analog/digital conversion of analog quantities. This is digital protection.

④Classification according to protection action principle

There are overcurrent protection, low voltage protection, overvoltage protection, power direction protection, distance protection, differential protection, longitudinal protection, gas protection, etc.

05 Abnormal

When abnormalities or defects are found in the operation of relay protection, in addition to strengthening monitoring, remove the outlet pressure plate of the protection that can cause malfunction, and then contact relay protection personnel for handling.

If there are any of the following abnormal situations, you should exit in time:

①Mother differential protection

When the "bus differential AC disconnection" or "bus differential DC voltage disappears" signals are sent; when the bus differential unbalanced current is not zero; the bus tie switch without a dedicated bypass bus is in series line operation and recovery switching operation.

②High frequency protection

When the DC power supply disappears; when the regular channel test parameters do not meet the requirements; when the device malfunctions or the channel abnormal signal is sent and cannot be reset; when the parent line switch operation is in progress.

③Distance protection

When the PT used is out of operation or the three-phase voltage circuit is disconnected; under normal circumstances, the magnetizing current is too large or too small; when the load current exceeds the corresponding section of the protection allowable current.

④Microcomputer protection

When the main alarm light is on and one of the four protection (high frequency, distance, zero sequence, comprehensive weight) alarm lights is on, exit the corresponding protection; if two CPUs fail, all protections of the device should be exited; all signal lights of the alarm plug-in do not light up. If the power indicator light goes out, it means that the DC has disappeared. You should exit the outlet pressure plate and put it back in after restoring the DC power supply; the main alarm light and the calling light are on, and the CPU×ERR signal is printed and displayed. If the CPU is normal, it means that the communication loop between the protection and interface CPU is protected. Abnormal, exit the CPU inspection switch processing. If the signal cannot be reset, it means that the CPU has a fatal flaw. You should exit the protection outlet pressure plate and disconnect the inspection switch processing.

⑤Gas protection

When refueling, filtering oil or changing silica gel while the transformer is in operation; when putting the submersible pump or oil cooler (radiator) into operation after draining the oil for maintenance; when it is necessary to open the vent door or oil drain plug of the breathing system, or when cleaning the moisture absorber; when there is a load When someone is working on the oil circuit of the pressure regulating switch.

06 Analysis of common faults in relay protection

①Current mutual inductance saturation fault

The saturation of the current transformer has a great impact on the relay protection of the power system. As the terminal load of power distribution system equipment continues to increase, if a short circuit occurs, the short circuit current will be very large. If a short circuit occurs in the system close to the terminal equipment area, the current may reach or be close to more than 100 times the single rated current of the current transformer. Under normal short circuit conditions, the larger the current transformer error is, the greater the primary short-circuit current multiple increases. When the current quick-break protection reduces the sensitivity, it may prevent the operation. When a line is short-circuited, the current of the current transformer is saturated and the secondary current induced again is small or close to zero, which will also cause the definite time overcurrent protection device to be unable to operate. When the overcurrent protection of the outlet line of the power distribution system refuses to operate, causing the inlet line protection of the power distribution station to operate, the entire power distribution system will experience a power outage.

②Improper selection of switch protection equipment

The selection of switch protection equipment is a very important task. Most of the current power distribution systems have established switching stations in areas with high load density, that is, using the power supply and transmission model of substation-switching station-distribution transformer. In switch stations where relay protection automation is not implemented, we should use more load switches or relay equipment systems combined with them as switch protection equipment.

07 Treatment methods and measures for relay protection faults

① How to deal with common relay protection faults

a. Substitution method

Replacing the faulty component with a good component to judge whether it is good or bad can quickly narrow down the fault search scope;

b.Reference method

By comparing the relevant technical parameters of normal equipment and abnormal equipment, we can find out the fault points of abnormal equipment. This method is mainly used to check for wiring errors and faults where the test value is significantly different from the expected value during the fixed value verification process. When the secondary wiring cannot be restored correctly after modification or equipment replacement, refer to the wiring of similar equipment. And during the relay setting value verification, if it is found that the test value of a certain relay is far different from the setting value, at this time, you cannot easily make a judgment and judge that the characteristics of the relay are not good, and you should adjust the scale value on the relay. You can use the same meter to measure other similar relays in the same circuit for comparison;

c. Short circuit method

Short-circuit a section or part of the circuit with a short wire to determine whether the fault exists within the short wire range or elsewhere, so as to determine the fault scope. This method is mainly used when the electromagnetic lock fails, the current loop is open, the switching relay does not operate, and it is used to determine whether the contacts of the control switch are intact.

② Measures to ensure normal operation of relay protection

Reasonable staffing ensures that personnel scheduling and assistance can be carried out smoothly, clear personnel work goals, and ensure the normal operation of power; improve rules and regulations, and improve and improve the rules and regulations for the operation and management of protection devices and relay protection equipment based on the characteristics of relay protection. Accounts, operation and maintenance, accident analysis, regular calibration, defect handling and other files should gradually adopt computer management tracking inspection, strict assessment, and implementation of rewards and punishments; the status monitoring method should be implemented for secondary equipment. For comprehensive automated substations, it is easy to implement continuous Electrical protection status monitoring.

HZJB-I 3 Phase Relay Protection Tester