What are the tasks and requirements of relay protection?

Update:06-02-2021
1. The task of relay protection
In order to ensure the safe operation of the power supply and distribution system and avoid the impact of overcurrent caused by overload and short circuit on the system, different types of overcurrent protection devices should be installed in the power supply and distribution system. Commonly used over-current protection devices include fuse protection, low-voltage circuit breaker protection and latching relay protection. Among them, relay protection is widely used in high-voltage power supply and distribution systems. It has many protection functions and is the basis for realizing power supply and distribution automation.
The relay protection device refers to an automatic device that can reflect the fault or abnormal working state of the electrical equipment in the power supply and distribution system, and can act on the circuit breaker to trip or start the signal device to send out a forecast signal.
The main tasks of relay protection are as follows:
(1) Automatically, quickly and selectively remove faulty components from the power supply and distribution system, so that other non-faulty parts can quickly resume normal power supply;
(2) It can correctly reflect the abnormal operation status of electrical equipment and send out a forecast signal so that the operator can take measures to restore the normal operation of electrical equipment;
(3) Cooperate with automatic devices of the power supply and distribution system (such as automatic reclosing devices, automatic backup power input devices, etc.) to improve the operational reliability of the power supply system.
The relay protection of the building power supply and distribution system is simple, effective, reliable, and has strong anti-interference ability.
2. Basic requirements for relay protection
The design of relay protection should be based on reasonable operation modes and possible fault types, and should meet the four basic requirements of selectivity, quick action, reliability and sensitivity.
(1) Selectivity
Selectivity means that the fault is first removed by the protection of the faulty equipment or the line itself. When a short-circuit fault occurs in the power supply system, the relay protection device will act to remove only the faulty components and minimize the power outage range to reduce the impact of the power failure. The protection device's ability to select faulty components is called protection selectivity.
(2) Quick mobility
In order to reduce the loss caused by the fault, reduce the user's working time under low voltage when the fault occurs, and improve the stability of the power system operation, the relay protection device is required to act as soon as possible and remove the fault when a fault occurs. Quickly removing the faulty part can prevent the fault from expanding, reduce the damage of the fault current to the electrical equipment, accelerate the recovery of the voltage of the power supply system, and improve the reliability of the power supply system.
(3) Reliability
Reliability means that the protection device will act when it should act, and not act when it should not. In order to ensure reliability, the simplest possible protection method should be selected, reliable components and the simplest possible loop should be adopted to form a device with good performance, and necessary measures such as detection, blocking and dualization should be provided. The protection device should be convenient for setting, debugging and operation and maintenance.
(4) Sensitivity
Sensitivity refers to the ability of the relay protection to respond to failures or abnormal working conditions within its protection range. The sensitivity SP of overcurrent protection uses the ratio of the minimum short-circuit current Ik,min in the protection area when the power system is in the minimum operating mode and the primary operating current of the protection device, that is, the value of the protection device operating current converted to the primary circuit, OP,1 Means that SP=Ik, min/IOP, 1
The required sensitivities are different for different protective devices and protected equipment, which are stipulated in the "Technical Specifications for the Design of Relay Protection and Automatic Devices of Power Installations" (GB50062-1992).
In addition, the four basic requirements described above are not necessarily equally important to a specific protection device, but should be emphasized. For example, power transformers are the most critical equipment in the power supply and distribution system, and the sensitivity of their protection devices is high; while for the protection devices of general power lines, they require high selectivity.