When power components in the power system (such as generators, lines, etc.) or the power system itself fail and endanger the safe operation of the power system, automated measures and equipment that can promptly inform operation duty personnel to halt the development of these events are required. The complete set of equipment implementing such automated measures is generally referred to as relay protection devices. This special topic will provide a detailed introduction to the basic principles, basic requirements, basic tasks, classification, and common fault analysis and handling of relay protection. Relay protection devices must have the function to correctly distinguish whether the protected component is in normal operation or has failed, and whether the fault is within the protection zone or outside it. To achieve this function, protection devices need to be based on the characteristics of changes in electrical physical quantities before and after faults in the power system. During a short circuit, the current on electrical equipment and transmission lines between the fault point and the power source will increase from load current to significantly exceed load current. When phase-to-phase short circuits and ground short circuits occur, the phase-to-phase voltage or phase voltage at various points in the system decreases, and the closer to the short circuit point, the lower the voltage. During normal operation, the phase angle between current and voltage is the load power factor angle, generally about 20°. During a three-phase short circuit, the phase angle between current and voltage is determined by the line impedance angle, generally 60° to 85°. In the case of a reverse three-phase short circuit in the protection zone, the phase angle between current and voltage is 180°+(60° to 85°). The measured impedance is the ratio of voltage to current at the measurement point (where protection is installed). During normal operation, the measured impedance is the load impedance; during a metallic short circuit, the measured impedance transforms to line impedance, and after a fault, the measured impedance significantly decreases while the impedance angle increases. During asymmetrical short circuits, sequence components appear, such as negative sequence current and voltage components during two-phase and single-phase ground short circuits; and negative sequence and zero sequence current and voltage components during single-phase ground faults. These components do not appear during normal operation. By utilizing the changes in electrical quantities during short circuit faults, various principles of relay protection can be constructed. In addition to the protections responding to power frequency electrical quantities mentioned above, there are also protections responding to non-power frequency electrical quantities, such as gas protection. To accomplish its tasks, relay protection devices must technically meet four basic requirements: selectivity, speed, sensitivity, and reliability. For relay protections that act on relay tripping, all four basic requirements should be met simultaneously, while for relay protection devices that act on signals and only respond to abnormal operating conditions, some of these four basic requirements can be relaxed. Selectivity means that when equipment or lines in the power system experience a short circuit, the relay protection only removes the faulty equipment or line from the power system. When the protection or circuit breaker of the faulty equipment or line refuses to operate, the protection of adjacent equipment or lines should remove the fault. Speed means that relay protection devices should remove faults as quickly as possible to reduce damage to equipment and users under high current, reduce equipment damage, and improve the stability of parallel operation of the system. Fault clearing time includes the operating time of the protection device and the circuit breaker. Generally, the operating time of fast protection is 0.04s to 0.08s, with the fastest reaching 0.01s to 0.04s. The tripping time of general circuit breakers is 0.06s to 0.15s, with the fastest reaching 0.02s to 0.06s. For relay protection devices responding to abnormal operating conditions, fast action is generally not required; instead, they should send signals with a delay according to selectivity conditions. Sensitivity refers to the ability of the protection device to respond when a short circuit fault or abnormal operating condition occurs within the protected range of electrical equipment or lines. The sensitivity of a protection device is measured by the sensitivity coefficient. A relay protection that meets sensitivity requirements can correctly respond and act for faults within the specified range, regardless of the location of the short circuit point, the type of short circuit, or whether there is transition resistance at the short circuit point. That is, it should reliably operate not only for three-phase short circuits under the maximum system operation mode but also for two-phase or single-phase short circuit faults with significant transition resistance under the minimum system operation mode. When a short circuit occurs at the end of the protected line, the system equivalent impedance is minimal, and the short circuit current through the protection device is the maximum operation mode. Under the same short circuit fault conditions, the system equivalent impedance is maximal, and the short circuit current through the protection device is the minimum operation mode. It is required that the relay protection reliably operates when a fault that should trigger action occurs within the specified protection range, i.e., it does not refuse to operate. Both misoperation and refusal to operate of relay protection can cause serious harm to the power system. Even for the same power components, with the development of the power grid, the impact of protection not misoperating and not refusing to operate on the system may change. The above four basic requirements are the basis for designing, configuring, and maintaining relay protection, and also the foundation for analyzing and evaluating relay protection. These four basic requirements are interrelated but often contradictory. Therefore, in practical work, they should be dialectically unified according to the structure of the power grid and the nature of users. 1. Automatically, quickly, and selectively remove faulty components from the power system to prevent further damage to the faulty components and ensure that other non-faulty parts quickly return to normal operation. 2. Respond to abnormal operating states of electrical components, and act on signals according to operation and maintenance conditions (such as whether there are duty personnel), so that duty personnel can handle them promptly, or the device can automatically adjust, or remove electrical equipment that would cause damage or develop into accidents if continued operation. At this time, rapid action of the protection is generally not required, but a certain delay is specified according to the degree of harm to the power system and its components, to avoid unnecessary actions and interference caused by transient fluctuations. 3. Relay protection devices can also cooperate with other automation devices in the power system, and when conditions permit, take predetermined measures to shorten the duration of power outages, restore power supply as soon as possible, thereby improving the reliability of power system operation. There are transmission line protections and main equipment protections (such as generators, transformers, busbars, reactors, capacitors, etc.). There are short circuit fault protections and abnormal operation protections. The former can be further 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, non-full-phase operation protection, etc. There are analog protections and digital protections. All electromechanical, rectifier, transistor, and integrated circuit (operational amplifier) protection devices that directly reflect continuous analog input signals are analog protections; protection devices using microprocessors and microcomputers are digital protections. There are overcurrent protection, undervoltage protection, overvoltage protection, power direction protection, distance protection, differential protection, pilot protection, gas protection, etc. When abnormalities or defects are found in relay protection operation, in addition to strengthening monitoring, for protections that may cause misoperation, the exit pressure plate should be removed, and then relay protection personnel should be contacted for handling. When the “bus differential AC disconnection” or “bus differential DC voltage loss” signals are issued; when the bus differential unbalanced current is not zero; during the operation of the bus tie switch replacing a line without a dedicated bypass bus and during restoration switching operations. When the DC power supply disappears; when the periodic channel test parameters do not meet requirements; when the device fault or channel abnormal signal is issued and cannot be reset; during the operation of the bypass bus replacing the line switch. When the PT used is taken out of operation or the three-phase voltage circuit is disconnected; when the excitation current is too large or too small under normal conditions; when the load current exceeds the corresponding section allowed by the protection current. When the total alarm light is on, and one of the four protections (high frequency, distance, zero sequence, comprehensive reclosure) alarm light is on, exit the corresponding protection; if two CPUs are faulty, exit all protections of the device; if all signal lights of the alarm plug-in are off, and if the power indicator is off, it indicates DC loss, exit the outlet pressure plate, and re-energize after restoring DC power; when the total alarm light and call light are on, and the print shows CPU×ERR signal, if the CPU is normal, it indicates a communication loop abnormality between the protection and the interface CPU, exit the CPU patrol switch for handling; if the signal cannot be reset, it indicates a fatal defect in the CPU, exit the protection outlet pressure plate and disconnect the patrol switch for handling. During oil addition, oil filtration, or silica gel replacement during transformer operation; after the submersible oil pump or oil cooler (radiator) oil drain and maintenance are put into operation; when it is necessary to open the vent valve or oil drain plug of the breathing system, or clean the moisture absorber; when personnel work on the oil circuit of the on-load tap changer. The saturation of current transformers has a significant impact on power system relay protection. With the continuous increase in load capacity of distribution system terminal equipment, if a short circuit occurs, the short circuit current can be very large. If a short circuit occurs near the terminal equipment area of the system, the current may reach or approach more than 100 times the single rated current of the current transformer. Under normal short circuit conditions, the error of a larger current transformer increases with the increase of the primary short circuit current multiple. When the instantaneous overcurrent protection sensitivity decreases, it may prevent action. When a line short circuit occurs, due to the saturation of the current transformer current, the induced secondary current is small or close to zero, which can also cause the definite-time overcurrent protection device to fail to operate. When the overcurrent protection at the outlet line of the distribution system refuses to operate, causing the protection at the inlet line of the distribution substation to operate, the entire distribution system may experience a power outage. The selection of switch protection equipment is a very important task. Most current distribution systems establish switch stations in high-load density areas, adopting the power supply and transmission mode of substation-switch station-distribution transformer. In switch stations that have not implemented relay protection automation, we should more often use load switches or combined relay equipment systems as switch protection equipment. Replace components identified as faulty with intact components to judge their condition, which can quickly narrow down the fault search range; by comparing relevant technical parameters of normal and abnormal equipment, find the fault point of the abnormal equipment. This method is mainly used to check wiring errors and faults where the test value significantly differs from the expected value during calibration verification. When the secondary wiring cannot be correctly restored after renovation and equipment replacement, refer to the wiring of similar equipment. During relay calibration, if it is found that the test value of a certain relay differs significantly from the set value, it should not be easily concluded that the relay characteristics are poor. Adjust the scale value on the relay, and use the same meter to measure other similar relays in the same circuit for comparison; short-circuit a section or part of the circuit with a shorting wire to determine whether the fault exists within the shorted range or elsewhere, thus determining the fault scope. This method is mainly used for electromagnetic lock failure, current circuit open circuit, switching relay failure, and judging whether the contacts of control and other transfer switches are intact. Reasonable personnel allocation enables smooth personnel dispatch and assistance, clarifies personnel work goals, and ensures normal power operation; improve rules and regulations, based on the characteristics of relay protection, improve and perfect the rules and regulations for the operation and management of protection devices. The archives of relay protection equipment ledgers, operation and maintenance, accident analysis, periodic calibration, defect handling, etc., should gradually adopt computer management for tracking inspection, strict assessment, and rewards and penalties; implement condition monitoring methods for secondary equipment. For integrated automated substations, it is easy to implement relay protection condition monitoring.