Differential protective relaying is the most positive in selectivity and in action. It operates on the principle of comparison between the phase angle and magnitudes of two or more-similar electrical quantities. Comparing two electrical quantities in a circuit by means of differential relays is simple in application and positive in action.
For example, in comparison of the current entering a line and the current leaving it, if more current enters the protected line than leaves it, the extra current must flow in the fault. The difference between the two electrical quantities can operate a relay to isolate the circuit.
A differential relay is defined as the relay that operates when the phasor difference of two or more similar electrical quantities exceeds a predetermined amount.
This means that for a differential relay, it should have:
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(i) Two or more similar electrical quantities and
(ii) These quantities should have phase displacement (normally approximately 180°) for the operation of the relay.
Almost any type of relay, when connected in a certain way, can be made to operate as a differential relay. In other words, it is not so much the relay construction as the way the relay is connected in a circuit that makes it a differential relay. Most of the differential relays are of the “current differential” type in which phasor difference between the current entering the winding and current leaving the winding is used for sensing and relay operation.
Differential protection is generally unit protection. The protected zone is exactly determined by the location of CTs and PTs. The phasor difference is achieved by suitable connections of secondaries of CTs or PTs.
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Differential protection principle is employed for the protection of generators, generator-transformer units, transformers, feeders (transmission lines), large motors and bus-bars.
Types of Differential Relays:
1. Current Differential Relays:
An arrangement of an overcurrent relay connected to operate as differential relay is shown in Fig. 3.57. The dotted line represents the system element that is to be protected by the differential relay. This system element might be a length of circuit, a portion of the bus or a winding of a generator or that of a transformer. A pair of CTs are fitted on either end of the section to be protected. The secondaries of CTs are connected in series with the help of pilot wires in such a way that they carry the induced currents in the same direction. The operating coil of an overcurrent relay is connected across the CT secondary circuit, as shown in Fig. 3.57.
Normally when there is no fault or there is an external fault [Fig. 3.57 (a)] the currents in the two CT’s secondaries are equal and the relay operating coil, therefore, does not carry any current.
But should a short circuit develop anywhere between internal fault the two CTs, the conditions will exist as shown in Fig. 3.57 (b). If the current flows to the fault from both sides as shown, the sum of the CT secondary currents will flow through the differential relay. It is not necessary that fault current flows to the fault from both sides to cause secondary current to flow to the differential relay.
A flow on one side only, or even some current flowing out of one side while a larger current entering the other side will cause a differential current to flow through the relay operating coil. In the other words, the differential relay current will be proportional to the phasor difference between the currents entering and leaving the protected element; and, if the differential current exceeds the relay’s pick-up value, the relay will operate.
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Differential Protection for 3-Phase Circuits:
The above principle can be extended to a system element having several connections. A three-phase circuit is only necessary, as before, that all the CTs have the same ratio, and that they be connected so that the differential relay carries no current when the total current entering the circuit is vectorially equal to that current leaving the circuit.
During normal operating conditions the three secondary currents of CTs are balanced and no current flows through the relay coil. But during fault in the protected zone, the balance is disturbed and differential current flows through the relay operating coil and when the differential current exceeds the relay’s pick-up value, the relay operates.
The principle can still be applied for the protection of a 3-phase power transformer, but in this case, the ratios and connections of the CTs on the opposite sides of the power transformer must be such as to compensate for the magnitude and phase-angle change between the power transformer currents on either side.
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Difficulties Associated with Differential Protection:
1. Difference in Length of Pilot Wires:
The power system element under protection and CTs are located at different places and normally it is not possible to connect the relay operating coil to the equipotential points. However, this difficulty can be overcome by connecting adjustable resistors in series with the pilot wires.
2. CT Ratio Errors during Short Circuits:
The CTs used may have almost equal ratio at normal currents, but during short-circuit conditions, the primary currents are unduly large and the ratio errors of CTs on either side differ. This is due to (i) inherent difference in characteristic of CTs arising out of difference in magnetic circuit, saturation conditions etc. (ii) unequal dc components in the short-circuit currents.
Saturation of Magnetic Circuits of CTs under Short-Circuit Condition:
The differential relay of the type explained above is likely to operate inaccurately with heavy through (i.e., external) faults. The relay may lose its stability for through faults. This drawback is overcome by using percentage-differential relay or biased-differential relay.
Magnetizing Current Inrush at the Switching Instant:
When the power transformer is connected to the supply, a large current (about 6 to 10 times full-load current) inrush takes place. The differential relay operates due to such inrush current, though the transformer has no fault. However, this difficulty is overcome by providing harmonic restraint for the differential relay. This relay filters the harmonic component from the inrush current and supplies it to the restraining coil. The harmonic content of the magnetizing current is used to obtain restraining torque during switching-in of transformer.
Tap-Changing:
Transformer transformation ratio is changed whenever the taps are changed. Due to this CT ratios do not match with the new-tap settings and result in flow of current in pilot wires even during healthy condition. However, this problem is also overcome by employing biased- differential relay.
2. Biased or Percentage-Differential Relay:
The most extensively used form of differential relay is the percentage-differential or beam biased relay. This is essentially the same as the overcurrent type of current- balance relay, but it is connected in a differential circuit, as illustrated in Fig. 3.59 (a). Schematic arrangement is shown in Fig. 3.59 (a) while equivalent circuit is shown in Fig. 3.59 (b).
This system consists of an additional restraining coil connected in the pilot wires, as shown in the figure and current induced in both CTs flows through it. The operating coil is connected to the mid-point of the restraining coil. The reason for using this modification in circulating current differential relay is to overcome the trouble arising out of differences in CT ratios for high values of external short-circuit currents.
The torque due to restraining coil prevents the closing of trip circuit contacts, while the torque due to operating coil tends to close the trip circuit contacts. Under normal operating conditions and through load conditions, the torque developed by restraining coil is greater than the operating coil torque. Thus the relay remains inoperative. When an internal fault occurs, the operating torque exceeds the restraining torque. Consequently the trip circuit contacts are closed to open the circuit breaker. The restraining torque may be adjusted by varying the number of turns of the restraining coil.
The differential current required to operate this relay is a variable quantity, owing to the effect of the restraining coil. The differential current in the operating coil is proportional to (I1 – I2) and the equivalent current in the restraining coil is proportional to [(I1 + I2)/2] as the operating coil is connected to the mid-point of the restraining coil.
The torque developed by the operating coil is proportional to the ampere-turns i.e., T0 ∝ (I1 – I2) N0 where N0 is the number of turns on the operating coil. The torque due to restraining coil T ∝ [(I1 + I2)/2]N
Where, N is the number of turns on the restraining coil. For external faults both I1 and I2 increase and thereby the restraining torque increases which prevents the mal-operation.
It is clear from the operating characteristic of the relay, that except for the effect of the control spring at low currents, the ratio of the differential operating current to the average restraining current is a fixed percentage. This is why it is known as percentage-differential relay. The relay is also called the biased-differential relay because the restraining coil is also called a biased coil as it provides additional flux.
The percentage or biased differential relay has a rising pick-up characteristic. So with the increase of magnitude of through current, the relay is restrained against mal-operation.
Figure 3.61 shows the comparison of a simple overcurrent relay with a percentage-differential relay under such conditions. An overcurrent relay having the same minimum pick-up as a percentage-differential relay would operate undesirably when the differential current merely exceeded the value X, while there would be no tendency for the percentage-differential relay to operate.
Three-Terminal System-Application of a Percentage-Differential Relay:
Percentage-differential relay protection can be applied to the system elements having more than two terminals, as in the three-terminal application shown in Fig. 3.62. Each of the three restraining coils has the same number of turns, and each coil develops a torque independently of others. Their torques is added arithmetically.
The percent slope characteristic for such a relay will vary with the distribution of currents between the three restraining coils. Percentage-differential relays are generally instantaneous or high speed. Time delay is not required for selectivity because the percentage-differential characteristic makes these relays immune to the effects of transients when the relays are properly applied.
Induction Type Biased Differential Relay:
In the simplest electromagnetic form the relay is shown in Fig. 3.63. Such a relay consists of a pivoted disc free to rotate in the air gaps of two electromagnets, a portion of each pole of which is fitted with a copper shading ring. This ring can be moved further into, or out of the pole.
The disc experiences two torques—one due to the operating element and the other due to restraining element. If the shading rings were in the same position on each element, the resulting torque experienced by the disc would be zero. But if the shading rings of restraining element were moved further into the iron core, the torque exerted by the restraint element will exceed than that of the operating element.
The advantages of such a relay over the beam type are:
(i) The induction element is not susceptible to operation due to transients and
(ii) A slight time delay can be obtained and the biasing feature can be finally adjusted merely by changing the position of the shading rings on either or both elements.
3. Voltage Balance Differential Relay:
The differential relays are known as current balance relays. Such relays are convenient where both ends of the protected element are close together e.g., with generator or transformer protection but do not suit for the protection of feeders. If such relays are used for feeder protection of several km length, the secondary emfs of the CTs would be required to circulate about 1 or 5 A at full load or several times the current during external fault conditions, round a pilot loop of fairly high impedance. Such a burden is impracticable for any economic design of CT. Another class of relays is the voltage balance differential relays, which are preferred for the feeder protection.
In this arrangement, two similar current transformers are connected at either end of the system element under protection (such as a feeder) by means of pilot wires. The relays are connected in series with the pilot wires, one at each end. The relative polarity of the CTs is such that there is no current through the relay under normal operating conditions and through fault conditions. The CTs used in such protection should be such that they should induce voltages in the secondary linearly with respect to the current. Since the magnitude of fault current is very large, in order that the voltage should be a linear function of such large currents the CTs should be air-cored.
When a fault occurs in the protected zone, the currents in the two primaries will differ from one another and so voltages induced in the secondaries of the CTs will differ and circulating current will flow through the operating coils of the relays. Thus the trip circuit will be closed and the circuit breaker will open.
To provide for capacity currents, the relays employed may be overcurrent type which should operate only when the difference of the currents on both sides exceeds certain value.
In this system no restraining coil or balancing resistance or overload coil is required.
Though this method is more reliable than current balance or circulating current system but has great disadvantage that CTs do not carry current so acts as open circuited and inserts high impedance in the circuit. This method may be employed for protection of feeders, alternators and transformers. For use on transformers, the turn-ratio of power transformers must be kept in view.