Differential Protection of Transformer- Working Principle, Differential relay

The transformer is the main device of the power system and its reliability is more important. Hence protection must be provided to the transformer. Buchholz relay is used to provide protection to the transformer against fault that occurs inside the transformer tank. Differential protection scheme is also used to protect the transformer. 

Generally, transformers are protected against short circuits and overheating. To provide short circuit protection to the transformer percentage differential protection is used. this protection is recommended for a transformer rated more than 1MVA. 

The differential protection scheme of the transformer is base on Merz Price's circulating current principle. 

For the protection of 3 phase delta-delta connected power transformer against phase to phase and phase to ground fault. CTs on the two sides of the power transformers are connected in star. This arrangement compensates phase difference between transformer primary and secondary winding. 

CTs are connected by pilot wires and a relay is used for each pair of CTs. 

Principle of operation of differential relay

  • Differential relay is actuate when the phase difference of two or more similar electrical quantity exceeds a predetermined value. 
  • The differential relay operates only for internal faults. 
The principle of such protection is the comparison of the currents entering and leaving the ends of a Transformer. 

The vector difference of current (I1-I2) passes through the operating coil and the average current (I1+I2)/2 flows through the restraining coil. 

In normal operating conditions, the two currents at two ends of the transformer are equal and balance. No current flows through the operating coil and hence operating coil remains Inoperative , but when an internal fault occurs in the transformer like phase to phase fault or phase to ground. The differential current passes through the operating coil due to which the differential protection relay operates and it trips the circuit breaker. 

Differential protection for star delta transformer 

The diagram shows the connection of differential protection for star delta power transformer 
differential-protection-transformer-diagram

The primary of power transformer is connected in star while the secondary is connected in delta. Hence to compensate the phase difference, the CT secondaries on the primary side I.e the star side of the transformer must be connected in delta and the CT secondaries of the secondary side I.e the delta side of the transformer must be connected in star. 

The star point of the transformer primary and star point of star-connected secondaries of CT must be grounded. 

The restraining coils are connected across the secondaries of CT while operating coils are connected between the Tapping point on restraining coils and the star point of CT secondaries. 

During normal operation, the CT secondaries carry an equal current which is in phase under normal operating conditions. Therefore the current entering and leaving the pilot wire at both ends are the same and no current flows through the operating coil of the relay and the relay is inoperative. 

When an internal fault occurs in power transformer winding the balance in CTs gets disturbed and the current in secondaries of CTs will no longer be the same and the differential current starts flowing through the operating coil and the relay circuit will trip the breaker on both sides of the transformer. 

The protection provided by the differential protection scheme is limited to the region between CTs on the high voltage side and the CTs on the low voltage side of the power transformer. 

Problem with differential protection of transformer

Some time differential relay operates for the through fault or even if there is no fault. 

saturation of CTs core

differential and phase angle error for the CT1 and CT2 

Wrong connection of pilot wire

If the pilot wire is connected wrongly on one side of the relay then the relay will operate. If both side CTs pilot wires reverse or are connected wrongly then the relay will not operate. 

Magnetic Inrush Current

During the time of transformer charging initial charging current of the transformer is very high which is 5 to 10 times the full load current known as magnetic inrush current. Due to this current relay operates without fault. To compensate with this current harmonic restrain relay is used which produces additional restarting torque for the operating coil of the relay so that the differential protection relay does not operate. 

Tap Changing transfoemer

At the timing of changing of taps of on-load tap changer transformer. The primary and secondary voltage of the power transformer changes so that current changes and hence CT current also changes and hence sometimes this cause malfunction of the differential protection relay. 

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