https://www.transformer-tester.com/secondary-and-primary-current-injection-test-set/secondary-current-injection-test-set/The relay protection tester device must have the function of correctly distinguishing whether the protected component is in a normal working state or has a failure, whether the protection area has a failure, and whether a failure occurs outside the area. In order to realize this function, it is necessary to construct a protection device according to the characteristics of electrical physical quantity changes before and after the power system failure.
After the power system fails, the main characteristics of power frequency power changes are:
(1) The current increases.
When a short circuit occurs, the current on the electrical equipment and the transmission line between the fault point and the power supply will increase from the load current to a level that greatly exceeds the load current.
(2) Voltage drop.
When a phase-to-phase short circuit and a grounding short-circuit fault occur, the phase-to-phase voltage or phase voltage value of each point of the system will decrease, and the closer to the short-circuit point, the lower the temperature and the voltage.
(3) The phase angle change between current and voltage.
The phase angle between current and voltage during normal operation is the power factor angle of the load, usually about 20°. When a three-phase short circuit occurs, the phase angle between current and voltage is determined by the impedance angle of the line, usually 60°~85°, and the phase angle between current and voltage is 180°+ (60°~85°) protection The three phases in the opposite direction are short-circuited.
(4) The measured impedance changes.
The measured impedance is the ratio of voltage to current at the measuring point (protective installation). In normal operation, the measured impedance is the load impedance; when the metal is short-circuited, the measured impedance becomes the line impedance. After the fault, the measured impedance decreases significantly and the impedance angle increases.
When an asymmetric short circuit occurs, phase sequence components such as negative sequence current and negative sequence voltage components will appear when two-phase and single-phase grounding are short-circuited. And single-phase grounding, negative sequence and zero sequence current and voltage components. These components do not exist during normal operation.
By using the change of the electric quantity during the short-circuit fault, various principle relay protection tester devices can be formed.
