Relay protection is an important measure to detect the fault or abnormal situation in the power system, so as to send an alarm signal, or directly isolate and remove the fault part.

In order to accomplish its task, relay protection device must meet the four basic requirements of selectivity, quickness, sensitivity and reliability in technology. For relay protection acting on relay trip, four basic requirements should be met at the same time, while for relay protection devices acting on signals and reflecting only abnormal operation, some of these four basic requirements can be reduced.

1) Selectivity

Selectivity refers to when the equipment or line in the power system short circuit, the relay protection will be the fault of the equipment or line from the power system to remove, when the protection of the fault equipment or line or circuit breaker refused to move, should be the protection of the adjacent equipment or line to remove the fault.

2) Quick

Quickness refers to the relay protection device should be able to remove the fault as soon as possible, in order to reduce the equipment and users in the large current, low voltage operation time, reduce the damage to the equipment, improve the stability of the system parallel operation.

Generally, the faults that must be removed quickly are:

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

(2) Large capacity generator, transformer and motor internal failure.

(3) the cross section of medium and low voltage line is too small, in order to avoid overheating, it is not allowed to delay the fault of excision.

(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 general fast protection action time is 0.04s ~ 0.08s, the fastest up to 0.01s ~ 0.04s, the general circuit breaker trip time is 0.06s ~ 0.15s, the fastest up to 0.02s ~ 0.06s.

For the reaction of the abnormal operation of the relay protection device, generally do not require rapid action, but should be in accordance with selective conditions, with delay to send out a signal.

3) Sensitivity

Sensitivity refers to the reaction ability of the protective device in case of short circuit fault or abnormal operation of the electrical equipment or line within the protected scope.

Can meet the sensitivity of the requirements of the relay protection, in the specified range of fault, regardless of the position of the short circuit point and the type of the short circuit, and the short circuit point whether there is a transition resistance, can respond to the correct action, that is, the requirements not only in the system under the maximum operation mode of three-phase short circuit can be reliable action, And in the system under the minimum operation mode after a larger transition resistance two-phase or single-phase short circuit fault can also be reliable action.

The maximum operation mode of the system: when the end of the protected line is short-circuited, the equivalent impedance of the system is the minimum, and the short-circuit current through the protective device is the maximum operation mode;

System operation mode: in the same case of short circuit fault, the equivalent impedance of the system is the maximum, the short circuit current through the protection device is the minimum operation mode.

The sensitivity of a protective device is measured by the sensitivity factor.

4) Reliability

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

Safety: Relay protection is required to be reliable when it does not need to act, that is, no misaction.

Reliability: Relay protection is required to act reliably when a fault occurs within the specified protection scope, that is, it does not refuse to move.

The misoperation and refusal of relay protection will bring serious harm to power system.

Even for the same power components, the impact of protection on the system will change with the development of the grid.

The above four basic requirements are the basis of design, configuration and maintenance of relay protection, as well as the basis of analysis and evaluation of relay protection. These four basic requirements are interrelated, but there are often contradictions. Therefore, in practice, we should unify the power grid dialectically according to the structure of the power grid and the nature of the users.