The differential protection is the main protection of the transformer and is installed according to the principle of circulating current.

　　 is mainly used to protect various phase-to-phase short-circuit faults that occur in the windings of dual-winding or three-winding transformers and their lead wires. It can also be used to protect single-phase inter-turn short-circuit faults of the transformer.

Current transformers are installed on both sides of the winding transformer, and its secondary side is connected according to the circulating current method, that is, if the same polarity ends of the current transformers on both sides are facing the bus side, the same polarity terminals are connected, and the Connect a current relay in parallel between the two wires. The current flowing in the relay coil is the difference between the secondary currents of the current transformers on both sides, which means that the differential relay is connected to the differential circuit.

　　Theoretically, the differential circuit current is zero during normal operation and external faults. In fact, due to the fact that the characteristics of the current transformers on both sides cannot be completely consistent, etc., during normal operation and external short circuit, there is still an unbalanced current Iumb flowing in the differential circuit. At this time, the current IK flowing through the relay is Ik=I1 -I2=Iumb

　　 requires that the unbalanced current should be as small as possible to ensure that the relay will not malfunction.

　　 When a phase-to-phase short-circuit fault occurs inside the transformer, in the differential circuit because I2 changes direction or is equal to zero (no power supply side), this is the current flowing through the relay is the sum of I1 and I2, namely

　　Ik=I1+I2=Iumb

　　 can make the relay operate reliably.

　　The scope of transformer differential protection is the electrical equipment between the current transformers that constitute the transformer differential protection, and the wires connecting these equipment. Since the differential protection does not act on faults outside the protection zone, the differential protection does not need to cooperate with the adjacent component protection outside the protection zone on the action value and action time limit, so it can act instantaneously when the zone fails.

　　 Differential protection is a protection device that reflects the current difference between the two sides of the protected element (or area) and acts. Differential protection is to protect the transformer from internal short-circuit faults. Current transformers are installed on both sides of the transformer. In normal load conditions or external short-circuits, the current flowing into the differential relay is an unbalanced current. Choose the current mutual inductance on both sides appropriately. Under the conditions of transformer transformation ratio and wiring mode, the unbalanced current value is very small and less than the operating current of the differential protection, so the protection does not operate; when a short circuit occurs inside the transformer, the current flowing into the relay is greater than that of the differential protection With operating current, the differential protection operates on tripping.