For transformer equipment, the differential protection of the dry transformer is the main protection of the transformer, which is installed according to the principle of circulating current. Transformer differential protection is mainly used to protect the interphase short circuit faults occurring in the inner winding of double-winding or three-winding transformer and its lead line, and it can also be used to protect the short-circuit faults between single-phase turns of transformer.
Current transformers are installed on both sides of the winding transformer, and the secondary side is wired according to the circulating current method. That is, if the same-stage ends of the current transformers on both sides are facing the bus side, the same-stage terminals will be connected, and the current relay will be connected in parallel between the two wires. The current flowing through the relay coil is the secondary current difference between the current transformers on both sides, that is to say, the differential relay is connected to the differential circuit.
Theoretically speaking, normal operation and external fault, the differential circuit current is zero. In fact, due to the reasons that the characteristics of current transformers on both sides cannot be exactly the same, 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
Unbalanced flow is required to be as small as possible to ensure that the relay will not operate by mistake.
When interphase short circuit fault occurs inside the transformer, I2 changes direction or is equal to zero in the differential circuit (no power side), which is the sum of the current flowing through the relay I1 and I2, namely IK =I1+I2= IUMB
Can make the relay reliable operation.
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. Because the differential protection will not act for the fault outside the protection zone, the differential protection does not need to cooperate with the adjacent component protection outside the protection zone in the action value and action time, so when the fault inside the zone, it can act instantaneously
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