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Transformer differential protection

Differential protection may be a unit-type protection for a given zone or piece of kit. it’s supported the actual fact that it’s only within the case of faults internal to the zone that the differential current (difference between input and output currents) are going to be high.

A differential relay is outlined because the relay that operates once the section distinction of 2 or a lot of identical electrical quantities exceeds a planned quantity. …Differential protection provides unit protection.

When a distinction exists (other than because of the voltage regulation) it means that device has developed a fault and also the system is mechanically disconnected by tripping the suitable variety of breakers. … an enclosed fault produces AN unbalance or “spill current that results in the operation of differential protection.

Introduction to Differential Protection

Differential protection may be a theme that compares the present on the first aspect of the protection unit (electrical device) to the secondary aspect. once a distinction exists (other than thanks to the voltage regulation) it means that device has developed a fault and also the system is mechanically disconnected by tripping the suitable range of breakers. it’s largely used and represented in relation of transformers.

The operation principle utilized by electrical device differential protection is that the Merz-price system as shown below. below normal conditions I1 and I2 area unit equal and opposite specified the whole current flowing through the relay is zero. an indoor fault produces AN unbalance or “spill current that ends up in the operation of differential protection.

Feasibility of differential protection.


The practicability of the differential protection is that it’s applicable thereto unit who’s incoming and outgoing terminals are close to. as an example it provides best protection for generator and electrical device, as a result of their incoming and outgoing don’t seem to be faraway from one another. within the case of transmission lines, the ends ar faraway from one another thus standard differential protection will to be directly applied.

Zones for differential protection


Zone of the differential protection is that the space below that it’s effective. really the Differential relay zone is Closed Zone. This zone rectifies the fault between 2 CTs at each extreme ends of the protection

 unit. A typical differential protection with its 2 CTs at every finish is shown within the figure.

For differential protection scheme, all fault which occur outside the close zone of CTs are known as external fault or through fault which are not rectified by the differential relay. Ideally, a differential relay should only responds to the internal fault.

Selection OF CTs for Differential Protection

The selection of CTs is most vital issue within the differential protection. The CTs utilized in the Differential protection ought to have identical properties that are transformation quantitative relation, saturation, excitation characteristics etc. they’re called matching CTs. If the CTs don’t have identical

values then distinction in their output won’t be zero. As a result, spill current can flow even just in case of no fault condition. important worth of the spill current could cause supererogatory tripping. To avoid such state of affairs it,s extremely suggested that each CTs ought to be of same company.

In case once the 2 equalisation CTs disagree within the excitation characteristics or have unequal burden then the transient flux can engineered which may eventually results in the event of spill current. Hence, there’s a consequence risk that relay can operate in healthy circuit, that isn’t acceptable. The remedy is to introduce the stabilising resistance nonparallel with the relay. The procedure of shrewd the worth of stabilising resistance is sometimes enclosed within the manual of the relay.


Design Consideration for differential protection

An ideal theme is needed to be very stable below through fault conditions. It ought to be in no time to control throughout internal fault. the subsequent problems should be thought of whereas coming up with differential protection for any system

  • The matching of CTs ratio.
  • Current imbalance produces by tap changing.
  • Dealing with Zero sequence currents.
  • Phase shift through the transformer.
  • Magnetizing inrush currents.

Current imbalance produces by tap changing.

A electrical device equipped with on-line faucet ever-changing is outline on modification the voltage level because it move the faucet changer . thus throughout ever-changing the faucet the voltage can vary thanks to that variation in current can occur. ever-changing the faucets of electrical device could results in increase in spill current which may cause the supererogatory tripping of circuit. To cater for this downside biased relay is employed.

Dealing With Zero Sequence Currents

Earth Fault of electrical device might bring about to zero sequence currents, relying upon the winding association and also the earth arrangement. Since zero sequence doesn’t passes tho’ the electrical device, it’ll seen on one facet and cause the spill current and there’s risk of the relay to control out of the zone. So, zero sequence current ought to be paid in differential protection.
This is achieved by attaching the secondaries of the CTs in Delta association that are related to main electrical device winding connected in star.

Phase shift through the transformer

Even removing the zero sequence current there square measure still positive and negative current that will expertise section shift and passes through the electrical device depend on the vector cluster of the electrical device. It causes issues in differential protection.

Magnetizing Inrush Currents

When the transformer is 1st energized the result of magnetizing inrush has the result of manufacturing high magnitude currents for a brief amount of your time. this will be seen by the CT on the availability aspect solely and will be wrong taken as fault. the answer is to putting together a time delay feature within the relay and therefore the use of harmonics restrain driven. The second harmonics area unit related to Magnetizing inrush currents.

Application of Differential Protection

Mostly the differential protection is applicable to the following

  • Transformer protection
  • Busbar protection
  • Generator protection

A differential protection theme are often used for the protection of electrical device, generator and bus bars. just in case of electrical device, the CTs ought to be elect by considering the transformation magnitude relation of the electrical device. The CTs are connected across to make a current Current System

Differential relay

The definition of the relay is “one that operates once the vector distinction of 2 or a lot of similar electrical quantities exceeds a pre-determined quantity

This additionally referred to as unit protection because it senses the distinction in current between incoming terminal and outgoing terminal of the unit that is being protected.

Behavior of differential protection during faults

External Fault

The differential relaying theme ought to stay stable (non responsive) for any fault

that is outside the protecting zone. Such faults are called external fault or through fault. within the following figure the present feat or getting into terminal are same. assumptive that the CTs remodel the 2 currents with an equivalent fidelity, there’ll be no current within the spill path and therefore the theme stay constant.


Magnitude of spill current( If,int) flows in differential relay. this exploit the protected zone is I2that is said to current coming into I1 is said as I1= I2+ If,int. this can be as a result of this within the spill path is ( If,int / n) wherever n is that the CT quantitative relation. just in case of internal fault, price of spill current can surpass the pickup price of the relay and differential relay can trip the each breaker. The minimum internal fault current that may cause tripping is
If,min= (CT ratio) (plug setting of the OC relay)=nIps



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