After the failure of the power system, the main features of changes in the frequency of electrical power are:
(1) The current increases. In the event of a short circuit, the current between the fault point and the power supply on the electrical equipment and on the transmission line will increase from the load current to much more than the load current.
(2) The voltage is reduced. When phase-to-phase short circuit and ground short circuit fault occur, the phase-to-phase voltage or phase voltage at each point of the system decreases, and the closer to the short-circuit point, the lower the voltage.
(3) The phase angle between current and voltage changes. In 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 short circuit occurs, the phase angle between the current and the voltage is determined by the impedance angle of the line, which is generally 60°. 85°, and when the three-phase short circuit in the opposite direction of protection, the phase angle between the current and voltage is 180°+(60°~85°).
(4) Measured impedance changes. The impedance is measured as the ratio of voltage to current at the point of measurement (protection installation). In normal operation, the measured impedance is the load impedance; in the case of a metallic short-circuit, the measured impedance is converted into the line impedance, and the measured impedance significantly decreases after the fault, and the impedance angle increases.
In the case of asymmetrical short-circuiting, phase sequence components appear, such as a negative-sequence current and a negative-sequence voltage component when two-phase and single-phase grounding are short-circuited, and negative-sequence and zero-sequence current and voltage components appear when a single-phase ground is applied. These components do not appear during normal operation.
With the change of the electrical quantity when the short-circuit fault occurs, relay protection of various principles can be constituted.
In addition, in addition to the protection of the above-mentioned reaction power frequency electric quantity, there is also protection against non-frequency electric quantity.
Classification relay protection can be classified in the following four ways.
1 According to the classification of protected objects, there are power line protection and main equipment protection (such as generators, transformers, busbars, reactors, capacitors, etc.).
2 According to the protection function classification, there is short-circuit fault protection and abnormal operation protection. The former can be divided into main protection, backup protection and auxiliary protection; the latter can be divided into overload protection, loss of magnetic protection, out of step protection, low-frequency protection, non-full-phase operation protection.
3 According to the protection device to compare and calculate the processing of the signal classification, analog protection and digital protection. All electromechanical, rectifier, transistor and integrated circuit (op amp) protection devices, which directly reflect the continuous analog input signal, are analog protection; using microprocessors and microcomputer protection devices, they respond It is the digital protection after analog sampling and A/D conversion.
4 According to the protection action principle classification, there are over current protection, low voltage protection, over voltage protection, power direction protection, distance protection, differential protection, high frequency (carrier) protection.
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