EE35T - Transformer Differential Protection

A Differential System can be arranged to cover a complete Transformer. The underlying principle of such a protection scheme is shown in the figure below.

The General Idea behind the Differential Protection is that the CT's on the primary and Secondary side must transform the respective Line currents to the same value. For the Bias Coils in the relay to function without any damage, this transformed current may lie between 1A and 5A. Hence the function of the CT's is to transform the line currents to the same magnitude and phase under normal operation of the Transformer.

Should some imbalance occur within the Transformer, such as Interturn Faults within the Windings of the Transformer or Faults on the incoming or outgoing feeders to the transformer, the Line Currents are no longer at the balanced value. Thus, the transformed current at the relay coils is no longer the same at both ends, but different. This causes an imbalance within the differential relay, and as a result, some protection mechanism will be operated, so as to isolate the Transformer and protect it.

Basic Considerations for Transformer Differential Protection

1. Line Current Transformer Primary Ratings

The rated currents on the primary and secondary sides of the transformer depends on the Line Current. This current will be an inverse ratio of the primary or secondary voltages. Consequently, the line current transformers should have primary ratings equal to or greater than the rated line currents of the power transformer to which they are applied. Standard primary ratings will usually limit the choice and availability of the Current Transformers that are used.

For Example.
Consider a two winding, 11kV/132kV, 30MVA Power Transformer.
The rated line current on the 11kV side of the Transformer is:
Irated = MVA / Line Voltage
          = 30000000 / (11000)(sqrt 3)
          = 1574.59A

Thus, choose a CT with a primary rating of 1600/1.

The line current on the 132kV side of the Transformer is:
Irated = 30000000 / (132000)(sqrt 3)
          = 131.22A

Thus choose a CT with a primary rating of 200/1.

Notice that in both cases, the current that the Bias Coils in the relay see is 1A at both ends. Any deviation of this 1A will causes an imbalance within the relay, which will consequently cause protection to operate and isolate the Transformer.

2. Current Transformer Connections

In certain configurations of Power Transformers, the CT's must be connected such that they compensate for phase differences on each side of the Power Transformer. For a WYE-DELTA connected power transformer, the phase shift is 30 degrees. So a CT must be able to compensate for the phase shift on both sides, such that the resultant current that is seen by the relay in of Equal Phase and Magnitude.

So, for a DELTA-WYE connected power transformer, in order for the phase to be to be the same at both the primary and secondary CT, the CT's must be connected as WYE-DELTA. By doing this, the resultant phase that is seen by the relay is effectively Zero. (Similarly, for a WYE-DELTA connected power transformer, the CT's must be connected as DELTA-WYE).

This configuration is shown by following the link here .

With the configuration of CT's shown, the zero sequence current flowing on the star side of the Transformer will not produce current outside the delta on the other side. Thus, the zero sequence is eliminated on the star side by connecting the CT's in a Delta connection, and that on the delta side is connected as star. Also, by connecting the CT's as shown, the phase shift of 30 degrees is removed.

Finally, for the CT's connected in delta, the secondary ratings must reduce to 1/(sqrt 3) times the secondary rating of the Star connected CT's. This is to balance the currents on the delta side with that of the star side of the power transformer.

3. Magnetising Inrush Conditions

Magnetising Inrush produces current input to the energised winding which has no equivalent on the other sides of the transformer. The Inrush appears as an imbalance, not distinguishable from a fault. To prevent this, several methods have been employed:


This is a transient phenomena, and as a result, the stability of the transformer system may be maintained by providing a small time delay after switching on the transformer. The use of an instantaneous kick fuse diverts most of the current. Under transient conditions, the fuse does not blow. Under faulted conditions, the fuse blows and thereby allows the relays to operate.


Tests have shown that the magnetising inrush current has a high second harmonic content. However, this component does not exist in fault currents. As a result, the CT's must be sufficiently large such that the harmonics produced does not delay relay operation.

The filter uses a circuit which extracts the second harmonic current. This differential current is then applied to another circuit which applies a restraining quantity, sufficient to overcome the operating tendency due to the whole of the inrush current which flows in the operating circuit. In effect the second harmonic component is used to prevent the relay from operating under transformer energising conditions.


This relay is used to protect Oil Immersed Transformers. The relay comprises two floats contained in an enclosed housing located in the pipe from the transformer tank to the conservators.

Any fault in the transformer causes the oil to decompose. generating a gas which passes up the pipe towards the conservator tank, and thus trapped in the relay.

In the case of a heavy fault, bulk displacement of the oil takes place. In a two float relay, the upper float responds to the slow accumulation of gas due to mild incipient faults. The lower relay is deflected by oil surge caused by major faults. These floats control contacts, which in the first case generates an alarm, and in the second case, causes isolation of the transformer.

Alarms are given for incipient faults such as

Major faults which will cause isolation of the transformer include