The differential protection used for transformers is based on the principle of current circulation. This type of protection is mostly used for transformers as this responds not only to inter turn fault but also provides protection against phase-to-phase faults. Following are the complicated features in transformers and there remedial measures:

1. In a power transformer, the currents in primary and secondary are to be compared.

As these two currents are usually different, therefore the use of identical tr4ansformers

will give differential current and operate the relay even under no load conditions. The difference in magnitude of currents in primary and secondary of power transformers is compensated by different turns ratios of C.T.s. If T is the turn’s ratio of power transformer, then the turns ration of C.T.s on lv side is made T times the turn’s ration of the C.T.s on hv side. When this condition is fulfilled the secondaries of the two C.T.s will carry same current under normal conditions. And thus no current will flow through the relay and it remains inoperative.

2. There is usually a phase difference between the primary and secondary currents of a

3-phase power transformer. Even if C.T.s of proper transformation ratios are used, a differential current will flow through the relay under normal condition and cause relay operation. The correction for phase difference is effected by appropriate connections of C.T.s. the C.T.s on one side of the power transformer are connected in such a way that the resultant current fed into the pilot wires are displaced in phase from the individual phase currents in the same direction as, and by an angle equal to, the phase shift between the power transformers primary and secondary currents. The table below shows the type of connections to be employed for C.T.s in order to compensate for the phase difference in the primary and secondary currents of power transformer

SL.NO	POWER TRANSFORMER CONNECTIONS		CURRENT TRANSFORMER CONNECTIONS
	PRIMARY	SECONDARY	PRIMARY	SECONDARY
1 2 3 4	Star with neutral earthed Delta Star Delta	Delta Delta Star with neutral earthed Star with neutral earthed	Delta Star Delta Star	Star Star Delta Delta

1. Another factor, which has to be considered, is the inrush of magnetizing current.

When the transformer is switched to supply the magnetizing current may assume very high values momentarily and may cause operation of the relay even though they are transient. This can be avoided by using relays with time delay characteristics.

Fig 8 shows the differential protection for transformer. In this the power transformer is delta- star connected. On delta side the C.T.s are connected in star and on the star side the C.T.s are connected in delta as in fig. Under normal working conditions the circulating currents caused by the primary and secondary load current in the relay circuit will balance; but under fault conditions the balance will no longer be there and the relay will be energized to trip the circuit breakers on the primary and secondary side

In order to understand the phase difference in the two sides consider fig 8. The primary is connected in delta and the set of current transformers CT1 is connected in star, while the secondary is connected in star and the set of current transformers CT2 is connected in delta. Fig 9 illustrates the vector diagram in reference to primary and secondary sides of current transformer. In fig 9.a IRP, IYP and IBP are the phase currents in the primary side, while IR is the line current on the same side in line R as shown in fig 9.a, the corresponding secondary current of current transformers CT1 on the primary side is in

phase with IR and is represented as IRS in fig 9.b. the current in the secondary side of the power transformer is represented as IR, IY and IB in fig 9.c, the phase current in the secondary winding of the current transformers CT2 is represented as I’R, I’Y and I’B in fig 9.d. the current in pilot wire of CT2 is represented as IRS. Now when we consider fig 9.b and 9.d its clear that the currents in the pilot wires are in phase.

In order to understand the phase difference in the two sides consider fig 8. The primary is connected in delta and the set of current transformers CT1 is connected in star, while the secondary is connected in star and the set of current transformers CT2 is connected in delta. Fig 9 illustrates the vector diagram in reference to primary and secondary sides of current transformer. In fig 9.a IRP, IYP and IBP are the phase currents in the primary side, while IR is the line current on the same side in line R as shown in fig 9.a, the corresponding secondary current of current transformers CT1 on the primary side is in

phase with IR and is represented as IRS in fig 9.b. the current in the secondary side of the power transformer is represented as IR, IY and IB in fig 9.c, the phase current in the secondary winding of the current transformers CT2 is represented as I’R, I’Y and I’B in fig 9.d. the current in pilot wire of CT2 is represented as IRS. Now when we consider fig 9.b and 9.d its clear that the currents in the pilot wires are in phase.

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