Zero Sequence Filtering in Differential Protection

Zero sequence filtering is a concept used to eliminate zero sequence current component from the secondary current of Current Transformer used in Differential Protection of Transformer. This concept is useful in Transformer having earthed Star winding.

What is the Need for Zero Sequence Filtering?

Let us have a detailed discussion on this topic. For better understanding, let us consider a Delta Star Transformer of voltage ratio 1:1 as shown in figure below. Voltage ratio of 1:1 has been considered for the sake of simplicity.

Suppose a single line to ground fault take place in PN phase of Transformer. This ground fault will obviously result in flow of fault current through the R phase which comprises of Zero Sequence I₀, positive sequence I₁ and negative sequence current I₂. Though Zero Sequence current I₀ will flow from the earthed Y side of Transformer, this component of current will not flow in Delta side as Delta winding rather it will circulate in the delta winding.



Therefore in line current of delta side of Transformer, there will not be any zero sequence current. This will create an imbalance in the differential current and may lead to actuation of Differential Relay even in through fault condition. Note that Differential Protection is a unit protection and shall only operate if the fault is within the specified zone of protection. But due to introduction of zero sequence current under fault condition, differential relay may operate even in through fault condition which is definitely not desirable.

Let us understand the above expiation with some numeric data. Let the fault current be 100 A. This fault current will flow through ground and winding PN for star side of Transformer. 

As we know that during single line to ground fault, the fault current has equal positive, negative and zero sequence current component. Therefore, the fault current will have Zero Sequence current I₀ = 1/3(100+0+0) = 100/3 = x (say)

Since 100 A of current is flowing in winding PN of star side, therefore, the current in corresponding delta side of winding AB will also be 100 A as voltage ratio is assumed unity. This current in delta side is already having zero sequence current component of x A. This current of magnitude x A will circulate around the delta loop.

Therefore, line current in AS = (100-x) A.

But the current in the star side of phase PN = 100 A

Thus a differential current of x A will be sensed by the relay and will cause the relay to operate though the fault is not in the zone protected. Therefore in differential protection of Delta Star or Star Delta Transformer, zero sequence filtering is required in star side to make differential relay stable under through fault condition. In Star Star transformer we may or may not need this filtering but it is recommended to enable this.

How Zero Sequence Filtering is achieved?

In earlier design, zero sequence filtering was achieved by proper selection of line CTs connection. Let us discuss this aspect in detail. For better understanding, consider a DELTA STAR Transformer as shown in figure below.

CTs in Delta side of Transformer are connected in STAR while that in STAR side, it is connected in Delta. Why?

It�s quite easy. As zero sequence current cannot flow in Delta side, therefore line current I_A � I_B will not have any zero sequence current components and hence line CTs secondary currents I₁, I₂ and I₃ won�t also have no zero sequence current component.

Next you might ask why we didn�t connect these CTs in delta? Actually in differential protection of Transformer we need to correct the phase as there is a phase change of 30 �in currents from Delta to Star, therefore to match the phase, we connected CTs in STAR fashion.

Similarly in star side of Transformer, CTs are connected in DELTA to have phase correction. This also have one more advantage. In case of single line to ground fault in STAR side, fault current in line will contain zero sequence current component but due to DELTA connection of CTs, this zero sequence current will circulate around the DELTA loop. This will result in no zero sequence current component in I�₁, I�₂ and I�₃.

This is how zero sequence current is eliminated in differential protection of Transformer.

In modern Numerical Relays, this elimination of zero sequence current do not need such connection of line CT secondary. We connect both Delta and Star side CTs in star and fed to relay. What the numerical relay does is that it calculates the zero sequence current as

I₀ = (I_a + I_b +I_c)/3

and subtracts this current I₀ from the corrected phase current  of CT input. If the CT input to relay is Ia, then the input from differential calculation of relay will be (I_a � I₀). The fault current in the example was 100 A. Zero sequence current was x A. Therefore if filtering is enabled, then relay input for differential calculation will be (100 - x) A in Star side of Transformer and that in Delta side is (100 - x) A. This means no differential current and hence relay is stable under through fault.

But mind that for relay to filter out zero sequence current, we need to enable this feature provided in the numerical relay.

Hope you got what I meant to say throughout the post. But if you have anything to ask, please write in comment. Thank you!