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For the transformer equipment, the differential protection of the dry type transformer is the main protection of the transformer and is installed according to the principle of circulating current. Transformer differential protection is mainly used to protect all kinds of interphase short circuit faults in the two winding or three winding transformer winding and the extraction line. It can also be used to protect the transformer’s single-phase inter turn short circuit fault.

The two sides of the winding transformer are equipped with a current transformer, and the secondary side is connected according to the cyclic current method. That is, if the side ends of both sides of the current transformer are all facing to the bus side, they will be connected to the same level terminals, and the current relay will be connected in parallel between the two wires. The current in the relay coil is the two current difference between the two sides of the current transformer, that is to say, the differential relay is connected to the differential circuit

In theory, the current of the differential circuit is zero when the normal operation and the external fault are operated. In fact, due to the two current transformers can not be exactly the same as in normal operation and external short circuit, differential circuit still has the unbalanced current Iumb flows, the current through the relay IK is: Ik=I1- I2=Iumb does not require equilibrium flow should be as small as possible, to ensure that the relay will not malfunction. When the interphase short circuit fault occurs in the transformer, in the differential loop, I2 changes the direction or is equal to zero (no power supply side) due to the change of the I2, which is the sum of the current flowing through the relay is I1 and I2, that is, the relay can operate reliably.

The range of transformer differential protection is the electrical equipment between the current transformers that form the differential protection of the transformer and the wires that connect these devices. Because the differential protection does not act on the fault outside the protection area, so the differential protection does not need to cooperate with the adjacent components outside the protection area to cooperate with each other in the action value and the action time limit, so it can act instantaneously in the district fault.

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