What is the basic principle and composition of relay protection?

The protective relay must have the function to correctly distinguish whether the protected element is in normal operation or has failed, and whether it is a fault in the protected area or outside the zone. In order to realize this function, the protection device needs to be composed based on the characteristics of the change of electrical and physical quantities before and after the failure of the power system.

After the failure of the power system, the main characteristics of the change of power frequency electrical quantity are:

1) The current increases. In the event of a short circuit, the electrical equipment between the fault point and the power supply and the current on the transmission line will increase from the load current to a significant excess of the load current.

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

3) Change in phase angle between current and voltage. The phase angle between the current and the voltage is the power factor angle of the load during normal operation, which is generally about 20°, and when the three-phase short circuit, the phase angle between the current and the voltage is determined by the impedance angle of the line, which is generally 60 ° 85 °, and when the three-phase short circuit in the opposite direction of protection, the phase angle between the current and the voltage is 180 ° + (60 ° 85 °).

4) The measurement impedance changes. The measurement impedance is the ratio of voltage to current at the measurement point (protection installation). During normal operation, the measured impedance is the load impedance; In the case of a metallic short circuit, the measured impedance is transformed into line impedance, and the measured impedance decreases significantly after the fault, while the impedance angle increases.

In the case of asymmetric short circuit, the phase sequence component appears, such as the negative sequence current and negative sequence voltage component in the case of two-phase and single-phase ground short circuit; When a single phase is grounded, negative and zero sequence current and voltage components occur. These components do not appear during normal operation.

By taking advantage of the change in the amount of electrical power in the event of a short-circuit fault, it is possible to construct relay protection based on various principles.

In addition, in addition to the above-mentioned protection of the reaction power frequency electrical quantity, there is also the protection of the reaction non-power frequency electrical quantity, such as gas protection.

Substation relay protection.

1) The role of relay protection in substation.

Substation relay protection can quickly and selectively issue trip commands to cut off faults or send alarms in the event of failures (three-phase short circuit, two-phase short circuit, single-phase grounding, etc.) and abnormal phenomena (overload, overvoltage, low voltage, low frequency, gas, overtemperature, control and measurement circuit disconnection, etc.) during the operation of the substation, so as to reduce the scope of power outages caused by faults and the degree of damage to electrical equipment, and ensure the stable operation of the power system.

2) The basic working principle of relay protection in substation and distribution station.

Substation relay protection is based on the current increase, voltage increase or decrease, frequency reduction, gas, temperature increase and other phenomena that occur during the operation of the substation exceed the setting value (given value) or over-limit value of the relay protection, and selectively issue a trip command or alarm signal within the setting time.

The inverse time limit is the inverse time limit for selective tripping according to the current value, and the higher the current value, the faster the trip. Selective tripping according to time is called time-limited protection, and the time-limited time limit occurs only after the fault current exceeds the set value, and the time given by the time fixed value appears. Gas and temperature are non-electric protection.

The reliability coefficient is an empirical data, and when calculating the relay protection action value, the calculation result should be multiplied by the reliability coefficient to ensure the accuracy and reliability of the relay protection action, and its range is.

The ratio of the minimum value in the event of a fault to the operating value of the protection is the sensitivity factor of the relay protection, which is generally selected according to the design specifications.

3) Substation relay protection is classified according to the nature of protection.

4) Substation relay protection is classified according to the protected object.

Analyze the composition and principle of relay protection device.

Rationale: The protective relay must have the function to correctly distinguish whether the protected element is in normal operation or faulty, and whether it is a fault in the protected area or outside the zone. In order to realize this function, the protection device needs to be composed on the basis of the characteristics of the change of electrical and physical quantities before and after the failure of the power system.

Composition: 1. The measurement comparison part is to measure the physical parameters of the electrical components that pass through the protection, and compare them with the value of the given shouting rules, and according to the results of the comparison, give a set of logical signals of the nature of "yes" and "non", so as to judge whether the protection device should be started.

2. The logic part makes the protection device judge the type and scope of the fault according to a certain logical relationship, and finally determines whether the circuit breaker should trip, send out a signal or whether it is active and whether it is delayed, etc., and pass the corresponding instructions to the execution output part.

3. Execute the instructions transmitted by the output part according to the logic, and finally complete the task undertaken by the protection device. For example, it acts in tripping when it fails, sends a signal when it is not operating normally, and does not act during normal operation.