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This post was last edited by chenjinfeng on 2020-3-27 at 14:12. Differential protection is a type of protection device for electrical equipment that operates based on changes in the current difference between the incoming and outgoing currents at the ends of the equipment being protected. It is generally divided into longitudinal differential protection and transverse differential protection. Transformer differential protection belongs to longitudinal differential protection, while transverse differential protection is commonly used for the protection of busbar equipment in substations. In line protection, longitudinal protection involves using some communication channel to connect the protection devices at both ends of the transmission line in a longitudinal manner, transmitting electrical parameters (such as current and the direction of power) from one end to the other. By comparing these electrical parameters, it is possible to determine whether a fault lies within the scope of the line or outside it, thereby deciding whether to disconnect the protected line. Therefore, in theory, this series protection possesses absolute selectivity. Characteristics of line longitudinal differential protection: Since longitudinal differential protection operates only in the event of a short circuit within its protection zone, there is no issue of selective coordination with the protections of adjacent components in the system. As a result, it is possible to quickly eliminate short circuits at any point within the protection zone, which is its valuable advantage. However, in order to construct a longitudinal differential protection device, current transformers must be installed at both ends of the component to be protected, and their secondary coils must be connected together with auxiliary wires to the differential relay. Due to the limitations imposed by the conditions of the auxiliary conductors, longitudinal differential protection is limited to use on short circuits; whereas for generators, transformers, and busbars, longitudinal differential protection can be widely employed as the primary protection mechanism. The application of longitudinal differential protection in transformers and its principle: The longitudinal differential protection device consists of current transformers and relays located on both sides of the transformer. The two current transformers are connected in series to form a loop, with the current relay connected in parallel to this loop. Therefore, the current of the current relay is equal to the difference between the secondary currents of the current transformers on both sides. Under normal conditions or in the event of a fault outside the protection area, the currents on the secondary sides of the current transformers on both sides are equal in magnitude and have the same phase; therefore, the differential current flowing through the relay is zero. However, if a short-circuit fault occurs within the protection area, the differential current flowing through the relay is no longer zero, and as a result the relay will activate, causing the circuit breaker to trip and thus providing protection. Transformer differential protection is based on the principle of circulating current. The principle of transformer longitudinal differential protection requires that, during normal operation as well as in the event of a fault outside the longitudinal differential protection zone (which is the area between current transformers TA1 and TA2), the current flowing into the differential relay be zero, thereby ensuring that the longitudinal differential protection does not activate. However, since the rated currents on the high-voltage side and low-voltage side of the transformer are different, in order to ensure the proper operation of the differential protection, it is necessary to appropriately select the turns ratios of the current transformers on both sides so that the currents remain equal under normal operating conditions as well as in the event of external faults.