After the power system fails, the main characteristics of the power frequency electrical quantity change are:
a. The current increases
In the event of a short circuit, the current on the electrical equipment and transmission lines between the fault point and the power supply will increase from the load current to greatly exceed the load current.
b. Voltage drop
When a phase-to-phase short-circuit and a ground-to-ground short-circuit fault 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.
c. The phase angle between current and voltage changes
The phase angle between the current and the voltage during normal operation is the power factor angle of the load, generally about 20 degree . When a three-phase short circuit, the phase angle between the current and the voltage is determined by the impedance angle of the line, generally 60 degree 85 degree , and when the three-phase short circuit in the opposite direction is protected, the phase angle between the current and the voltage is 180 degree plus (60 degree -85 degree ).
d. The measured impedance changes
The measured impedance is the ratio of the voltage to the current at the measuring point (protection installation place). In normal operation, the measured impedance is the load impedance; in the case of a metallic short circuit, the measured impedance changes to the line impedance. After a fault, the measured impedance decreases significantly, while the impedance angle increases.
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