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Relay protection generally consists of three parts: measurement part, logic part and execution part. The function of the measurement part is to measure the physical quantity of the working state of the protected component and compare it with the given setting value to determine whether the protection should be activated. The function of the logic part is to make the protection device work according to a certain logic program based on the size, nature, order of appearance, etc. of each output of the measurement part, and finally pass it to the execution part. The function of the execution part is to complete the task of the protection device according to the signal sent by the logic part. Such as sending a signal, tripping or not acting, etc. Classification of relay protection 1) Classification according to the protected objects: transmission line protection, generator protection, transformer protection, busbar protection, motor protection, etc. 2) Classification according to protection principles: current protection, voltage protection, distance protection, differential protection, direction protection, zero sequence protection, etc. 3) Classification according to the type of faults reflected by the protection: phase-to-phase short-circuit protection, ground short-circuit protection, inter-turn short-circuit protection, disconnection protection, out-of-step protection, loss of excitation protection and over-excitation protection, etc. 4) Classification according to the implementation technology of relay protection devices: electromechanical protection, rectifier protection, transistor protection, integrated circuit protection, and microcomputer protection. 5) Classification of the relationship between relay protection measured value and set value: over-protection (measured value > set value), under-protection (measured value > set value) 6) Classification according to the role of protection: main protection, backup protection, auxiliary protection, etc. Main protection: protection that reflects the fault of the protected component itself and removes the fault in the shortest possible time. Backup protection: Protection used to remove faults when the main protection or circuit breaker refuses to operate. It is further divided into near backup protection and far backup protection. Near backup protection: Install two sets of protection at this component. When the main protection refuses to operate, the other set of protection of this component will act. Remote backup protection: When the main protection or circuit breaker refuses to operate, the backup protection is implemented by the protection of adjacent power equipment or lines.   Principle analysis of relay protection devices Relay protection devices include a measurement part (and a fixed value adjustment part), a logic part, and an execution part. 1. Sampling unit It electrically isolates the physical quantities (parameters) in the operation of the protected power system and converts them into signals that can be accepted by the comparison and identification unit in the relay protection device. It is composed of one or several sensors such as current and voltage transformers. 2. Comparative identification unit Including a given unit, the signal from the sampling unit is compared with the given signal to determine what signal the next-level processing unit sends out. (Normal state, abnormal state or fault state) The comparison and identification unit can be composed of 4 current relays, two of which are quick-break protection and the other two are over-current protection. The setting value of the current relay is the given unit. The current coil of the current relay receives the current signal from the sampling unit (current transformer). When the current signal reaches the current setting value, the current relay acts and passes to the next level through its contacts. The processing unit sends a signal that causes the circuit breaker to eventually trip; if the current signal is less than the set value, the current relay does not act, and the signal transmitted to the downstream unit does not act. The information of "quick break" and "over current" of the identification and comparison signal is sent to the next unit for processing. 3. Processing unit It accepts the signal from the comparison and identification unit, processes it according to the requirements of the comparison and identification unit, and determines whether the protection device should act according to the size, nature, and order of combinations of the outputs of the comparison link; it is composed of time relays, intermediate relays, etc. . Current protection: quick break---intermediate relay action, overcurrent, time relay action. 4. Execution unit Fault handling is implemented through execution units. Execution units are generally divided into two categories: one is a sound and light signal relay; (such as an electric whistle, an electric bell, a flashing signal light, etc.) and the other is the opening coil of the operating mechanism of the circuit breaker to open the circuit breaker. 5. Control and operating power supply The relay protection device requires its own independent AC or DC power supply, and the power of the power supply increases or decreases depending on the number of devices controlled; the AC voltage is generally 220V or 110V.   Read more:  https://www.hemeielectricpower.com/