In order to complete its task, the relay protection device must technically meet the four basic requirements of selectivity, snap action, sensitivity and reliability. For the relay protection that acts on the relay trip, four basic requirements should be met at the same time, and for the relay protection device that acts on the signal and only reflects abnormal operating conditions, some of these four basic requirements can be reduced.

1. Selectivity

Selectivity means that when a device or line in the power system is short-circuited, its relay protection only removes the faulty device or line from the power system. When the protection of the faulty device or line or the circuit breaker refuses to operate, the The protection of the adjacent equipment or line will remove the fault.

2. Quickness

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

Generally, the faults that must be quickly removed are:

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

2) Internal failure of large-capacity generators, transformers and motors.

3) The cross section of the medium and low voltage lines is too small. In order to avoid overheating, the fault of delay removal is not allowed.

4) Failures that may endanger personal safety and cause strong interference to the communication system.

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

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

3. Sensitivity

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

Relay protection that can meet the sensitivity requirements. When a fault occurs within the specified range, regardless of the position of the short-circuit point and the type of short-circuit, and whether the short-circuit point has a transition resistance, it can react correctly. Under the mode, the three-phase short circuit can operate reliably, and in the minimum operating mode of the system, it can also operate reliably when the two-phase or single-phase short circuit fault passes through a large transition resistance.

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 through the protection device is the maximum operating mode.

The minimum operating mode of the system:

In the case of the same short-circuit fault, the equivalent impedance of the system is the largest, and the short-circuit current through the protection device is the smallest operating mode.

4. Reliability

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

1) Security

It is required that the relay protection does not operate reliably when it is not needed, that is, it does not malfunction.

2) Reliability

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

The malfunction and refusal 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 the protection against erroneous operation and refusal will also change.