The differential protection is based on the difference in current variation within the protected area.It is widely used to protect large-capacity power transformers,substation bus,high-voltage motors and so on.As shown on the right is the power transformer differential protection shematic.
The area between the current transformers TA1 and TA2 is the differential protection zone.When the short circuit fault occurs in the protection area,that is,inside the transformer(such as d1 point),the current relay KA will generate a large starting current to protect the device,when the protection area outside the short circuit,that is,outside the transformer,such as(d2 points),the current relay only flows through a small non-smooth current,the protection device will not move.
The so-called transformer differential coupling protection,refers to the transformer by the secondary and secondary current values and phase comparison of the composition of the protection.Pilot differential protection device,generally used to protect the transformer coil and lead line occurred in the phase short circuit and high current grounding sustem in the single-phase ground short circuit.For the transformer coil inter-turn short circuit and other internal failures,usually only for backup protection.
The longitudinal differential protection device consists of current transformers and relays on both sides of the transformer.The two current transformers are connected in series to form a loop and the current relay is connected to the loop.Therefore,the current relay current is equal to the current on both sides of the current transformer secondary side of the difference.Under normal circumstances or outside the scope of protection failure,both sides of the current transformer secondary side of the current equal size,the same phase,so the current flow through the relay is zero,but if the fault occurred in the protection zone,through the relay the current is no longer zero,so the relay will act,so that the circuit breaker trip,which play a protective role.Transformer differential protection
is in accordance with the principle of circulating current composition,the principle of transformer differential protection requirements of the transformer in the normal operation and longitudinal protecion zone(longitudinal protection zone for the current transformer TA1,TA2 range)outside the fault,the inflow the current in the differential relay is zero to ensure that the differential protection does not operate.However,due to the transformer high-voltage side and low-voltage side of the rated current is different,therefore,in order to ensure the correct work of differential protection,it is necessary to properly select the two sides of the current transformer ratio,making the normal operation and external failure,the two current equal.