Basic principle \x0d\ The relay protection device must have the function of correctly distinguishing whether the protected element is in normal operation or faulty, and whether it is in the protected area or out of the protected area. In order to realize this function, the protection device needs to be based on the characteristics of the changes of electrical physical quantities before and after the power system failure. \x0d\ After power system failure, the main characteristics of power frequency electric quantity change are: \x0d\ (1) current increases. During short circuit, the current on the electrical equipment and transmission line between the fault point and the power supply will increase from the load current to greatly exceed the load current. \x0d\ (2) Voltage drops. When phase-to-phase short circuit and grounding short circuit occur, the phase voltage or phase voltage value of each point in the system decreases, and the closer to the short circuit point, the lower the voltage. \x0d\ (3) Phase angle change between current and voltage. During normal operation, the phase angle between current and voltage is the power factor angle of the load, which is generally about 20. In three-phase short circuit, the phase angle of current and voltage is determined by the impedance angle of the line, which is generally 60 ~ 85, while in three-phase short circuit protection, the phase angle of current and voltage is180+(60 ~ 85). \x0d\ (4) Measured impedance change. The measured impedance is the ratio of voltage to current at the measuring point (where protection is installed). In normal operation, the measured impedance is the load impedance; In the case of metal short circuit, the measured impedance is converted into line impedance, and the measured impedance is significantly reduced after the fault, while the impedance angle is increased. \x0d\ Phase sequence components occur during asymmetric short circuit, such as negative sequence current and negative sequence voltage components during two-phase and single-phase grounding short circuit; When single-phase grounding occurs, negative sequence and zero sequence current and voltage components appear. These components will not appear during normal operation. \x0d\ Various principles of relay protection can be formed by using the change of electrical quantity during short circuit fault. \x0d\ In addition, in addition to the above-mentioned reactive power frequency electric quantity protection, there is also reactive power non-power frequency electric quantity protection. \ x0d \ x0d \ relay protection can be classified in the following four ways. \x0d\ ① According to the protection objects, there are transmission line protection and main equipment protection (such as generators, transformers, buses, reactors, capacitors, etc. ). \x0d\ ② According to the protection function, there are 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-excitation protection, out-of-step protection, low-frequency protection, open-phase operation protection and so on. \x0d\ ③ There are analog protection and digital protection, which are classified according to the comparison of protection devices and arithmetic processing signals. All electromechanical, rectifier, transistor and integrated circuit (operational amplifier) protection devices that directly reflect the continuous analog quantity of input signals belong to analog protection; Microprocessor and microcomputer protection device are used to reflect the discrete digital quantity after analog sampling and analog-to-digital conversion. This is digital protection. \x0d\ ④ According to the protection principle, there are overcurrent protection, undervoltage protection, overvoltage protection, power direction protection, distance protection, differential protection and high frequency (carrier) protection.