The relay protection device must have the function of correctly distinguishing whether the protected element is in a normal operating state or has a fault, a fault in 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 the changes in electrical physical quantities before and after the failure of the power system.

After the power system fails, the main characteristics of power frequency electrical quantity changes are:

1) Increased current

When the short circuit occurs, the current between the electrical equipment and the transmission line between the fault point and the power supply will increase from the load current to greatly exceed the load current.

2) Voltage drop

When the phase-to-phase short-circuit and ground-to-ground short-circuit faults occur, the phase-to-phase voltage or phase voltage value of each point of the system drops, and the closer to the short-circuit point, the lower the voltage.

3) The phase angle between current and voltage changes

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

4) Measurement impedance changes

The measurement impedance is the ratio of the voltage and current at the measurement point (protection installation). During normal operation, the measured impedance is the load impedance; when the metal is short-circuited, the measured impedance changes to the line impedance, the measured impedance decreases significantly after the fault, and the impedance angle increases.

When an asymmetric short circuit occurs, a phase sequence component occurs, such as a negative sequence current and a negative sequence voltage component when a two-phase and single-phase ground short circuit occurs; when a single phase is grounded, a negative sequence and zero sequence current and voltage components occur. These components do not appear during normal operation. The use of changes in electrical quantities in the event of a short-circuit fault can constitute relay protection of various principles.

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