Differential Relay

The relays used in power system protection are of different types. Among them differential relay is very commonly used relay for protecting transformers and generators from localised faults.

Differential relays are very sensitive to the faults occurred within the zone of protection but they are least sensitive to the faults that occur outside the protected zone. Most of the relays operate when any quantity exceeds beyond a predetermined value for example over current relay operates when current through it exceeds predetermined value. But the principle of differential relay is somewhat different. It operates depending upon the difference between two or more similar electrical quantities.

Definition of Differential Relay

The differential relay is one that operates when there is a difference between two or more similar electrical quantities exceeds a predetermined value. In differential relay scheme circuit, there are two currents come from two parts of an electrical power circuit. These two currents meet at a junction point where a relay coil is connected. According to Kirchhoff Current Law, the resultant current flowing through the relay coil is nothing but summation of two currents, coming from two different parts of the electrical power circuit. If the polarity and amplitude of both currents are so adjusted that the phasor sum of these two currents, is zero at normal operating condition. Thereby there will be no current flowing through the relay coil at normal operating conditions. But due to any abnormality in the power circuit, if this balance is broken, that means the phasor sum of these two currents no longer remains zero and there will be non-zero current flowing through the relay coil thereby relay being operated.

In current differential scheme, there are two sets of current transformer each connected to either side of the equipment protected by differential relay. The ratio of the current transformers are so chosen, the secondary currents of both current transformers matches each other in magnitude. The polarity of current transformers are such that the secondary currents of these CTs opposes each other. From the circuit is clear that only if any nonzero difference is created between this to secondary currents, then only this differential current will flow through the operating coil of the relay. If this difference is more than the peak up value of the relay, it will operate to open the circuit breakers to isolate the protected equipment from the system. The relaying element used in differential relay is attracted armature type instantaneously relay since differential scheme is only adapted for clearing the fault inside the protected equipment in other words differential relay should clear only internal fault of the equipment hence the protected equipment should be isolated as soon as any fault occurred inside the equipment itself. They need not be any time delay for coordination with other relays in the system.

Types of Differential Relay

There are mainly two types of differential relay depending upon the principle of operation. 1. Current Balance Differential Relay 2. Voltage Balance Differential Relay

In current differential relay two current transformers are fitted on the either side of the equipment to be protected. The secondary circuits of CTs are connected in series in such a way that the carry secondary CT current in same direction. The operating coil of the relaying element is connected across the CT’s secondary circuit. Under normal operating conditions, the protected equipment (either power transformer or alternator) carries normal current. In this situation, say the secondary current of CT₁ is I₁ and secondary current of CT₂ is I₂. It is also clear from the circuit that the current passing through the relay coil is nothing but I₁-I₂. As we said earlier, the current transformer’s ratio and polarity are so chosen, I₁ = I₂, hence there will be no current flowing through the relay coil. Now if any fault occurs external to the zone covered by the CTs, faulty current passes through primary of the both current transformers and thereby secondary currents of both current transformers remain same as in the case of normal operating conditions. Therefore at that situation the relay will not be operated. But if any ground fault occurred inside the protected equipment as shown, two secondary currents will be no longer equal. At that case the differential relay is being operated to isolate the faulty equipment (transformer or alternator) from the system.
Principally this type of relay systems suffers from some disadvantages

1. There may be a probability of mismatching in cable impedance from CT secondary to the remote relay panel.



2. These pilot cables’ capacitance causes incorrect operation of the relay when large through fault occurs external to the equipment.



3. Accurate matching of characteristics of current transformer cannot be achieved hence there may be spill current flowing through the relay in normal operating conditions.

Percentage Differential Relay

This is designed to response to the differential current in the term of its fractional relation to the current flowing through the protected section. In this type of relay, there are restraining coils in addition to the operating coil of the relay. The restraining coils produce torque opposite to the operating torque. Under normal and through fault conditions, restraining torque is greater than operating torque. Thereby relay remains inactive. When internal fault occurs, the operating force exceeds the bias force and hence the relay is operated. This bias force can be adjusted by varying the number of turns on the restraining coils. As shown in the figure below, if I₁ is the secondary current of CT₁ and I₂ is the secondary current of CT₂ then current through the operating coil is I₁ - I₂ and current through the restraining coil is (I₁+ I₂)/2. In normal and through fault condition, torque produced by restraining coils due to current (I₁+ I₂)/2 is greater than torque produced by operating coil due to current I₁- I₂ but in internal faulty condition these become opposite. And the bias setting is defined as the ratio of (I₁- I₂) to (I₁+ I₂)/2

It is clear from the above explanation, greater the current flowing through the restraining coils, higher the value of the current required for operating coil to be operated. The relay is called percentage relay because the operating current required to trip can be expressed as a percentage of through current.

CT Ratio and Connection for Differential Relay

This simple thumb rule is that the current transformers on any star winding should be connected in delta and the current transformers on any delta winding should be connected in star. This is so done to eliminate zero sequence current in the relay circuit.

If the CTs are connected in star, the CT ratio will be I_n/1 or 5 A CTs to be connected in delta, the CT ratio will be I_n/0.5775 or 5X0.5775 A

Voltage Balance Differential Relay

In this arrangement the current transformer are connected either side of the equipment in such a manner that EMF induced in the secondary of both current transformers will oppose each other. That means the secondary of the current transformers from both sides of the equipment are connected in series with opposite polarity. The differential relay coil is inserted somewhere in the loop created by series connection of secondary of current transformers as shown in the figure. In normal operating conditions and also in through fault conditions, the EMFs induced in both of the CT secondary are equal and opposite of each other and hence there would be no current flowing through the relay coil. But as soon as any internal fault occurs in the equipment under protection, these EMFs are no longer balanced hence current starts flowing through the relay coil thereby trips circuit breaker.

There are some disadvantages in the voltage balance differential relay such as A multy tap transformer construction is required to accurate balance between current transformer pairs. The system is suitable for protection of cables of relatively short length otherwise capacitance of pilot wires disturbs the performance. On long cables the charging current will be sufficient to operate the relay even if a perfect balance of current transformer achieved.
These disadvantages can be eliminated from the system by introducing Translay system which is nothing but modified balance voltage differential relay system. Translay scheme is mainly applied for differential protection of feeders. Here, two sets of current transformers are connected either end of the feeder. Secondary of each current transformer is fitted with individual double winding induction type relay. The secondary of each current transformer feeds primary circuit of double winding induction type relay. The secondary circuit of each relay is connected in series to form a closed loop by means of pilot wires. The connection should be such that, the induced voltage in secondary coil of one relay will oppose same of other. The compensating device neutralises the effect of pilot wires capacitance currents and effect of inherent lack of balance between the two current transformers. Under normal conditions and through fault conditions, the current at two ends of the feeder is same thereby the current induced in the CT’s secondary would also be equal. Due to these equal currents in the CT’s secondary, the primary of each relay induce same EMF. Consequently, the EMF induced in the secondaries of the relays are also same but the coils are so connected, these EMFs are in opposite direction. As a result, no current will flow through the pilot loop and thereby no operating torque is produced either of the relays. But if any fault occurs in the feeder within the zone in between current transformers, the current leaving the feeder will be different from the current entering into the feeder. Consequently, there will be no equality between the currents in both CT secondaries. These unequal secondary CT currents will produce unbalanced secondary induced voltage in both of the relays. Therefore, current starts circulating in the pilot loop and hence torque is produced in both of the relays. As the direction of secondary current is opposite into relays, therefore, the torque in one relay will tend to close the trip contacts and at the same time torque produced in other relay will tend to hold the movement of the trip contacts in normal un-operated position. The operating torque depends upon the position and nature of faults in the protected zone of feeder. The faulty portion of the feeder is separated from healthy portion when at least one element of either relay operates.
This can be noted that in Translay protection scheme, a closed copper ring is fitted with the Central limb of primary core of the relay. These rings are utilised to neutralise the effect of pilot capacity currents. Capacity currents lead the voltage impressed of the pilot by 90° and when they flow in low inductive operating winding, produce flux that also leads the pilot voltage by 90°. Since the pilot voltage is that induced in the secondary coils of the relay, it lags by a substantial angle behind the flux in the field magnetic air gap. The closed copper rings are so adjusted that the angle is approximately 90°. In this way fluxes acting on the disk are in phase and hence no torque is exerted in the relay disc.