Author: Noyafa–Cable Tester Suppliers
The relays used in the circuits of relay protection devices and electrical automatic devices are generally made of electromagnetic, inductive, electrodynamic and magnetoelectric principles. Recently, there have been magnetic appliances working with rectified current, magnetic relays and electronic relays with saturated inductance coils, and relays using semiconductors are being studied. The purpose of studying and adopting the relays of these new principles is to simplify the structure of the relay, improve its reliability and improve its parameters.
Use indirect methods to indicate that a fault or improper working state has occurred in the power system. For example, such relays are capable of reacting to gas generation, heat generation, pressure increase, etc. The relay protection device must have the function of correctly distinguishing whether the protected element is in normal operation or has a fault, and whether it is a fault within the protection zone or a fault outside the zone.
In order to realize this function, the protection device needs to be constructed on the basis of the characteristics of electrical and physical quantity changes before and after the failure of the power system. The three-phase/six-phase electric protection tester is developed using the latest digital technology, high-precision electronic devices, microcomputer system and new circuit and cable fault tester structure. It can independently complete device testing in professional fields such as computer protection, relay protection, excitation, metering, cable fault tester fault recording, etc. It is widely used in scientific research and production in electric power, petrochemical, metallurgy, railway, aviation, medical and other industries and electrical test site.
After the power system fails, the main characteristics of the power frequency electrical quantity change 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 source will increase from the load current to greatly exceed the load current. (2) Voltage drop.
When an interphase short circuit and a ground short circuit fault occur, the phase-to-phase voltage or phase-to-phase voltage value at 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 the current and the voltage is the power factor angle of the load, which is generally about 20. In the case of a three-phase short circuit, the phase angle between the current and the voltage of the cable fault tester is determined by the impedance angle of the line, which is generally 60 ~ 85, and when protecting the three-phase short circuit in the opposite direction, the phase angle between the current and the voltage is 180 + (60 ~ 85).
(4) The measurement impedance changes. The measurement impedance is the ratio of the voltage to the 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 is transformed into the line impedance. After the fault, the measured impedance decreases significantly, while the impedance angle increases.
Negative-sequence current and negative-sequence voltage components appear; when single-phase grounding, negative-sequence and zero-sequence current and voltage components appear. These components are not present during normal operation. Using the change of electrical quantity during short-circuit fault, relay protection of various principles can be formed.
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