EXPERIMENT 29
29.1Object: Study of the Percentage Differential Relay
29.2An Overcurrent Relay Can Be Used as a Differential Relay
An overcurrent relay can be used as a differential relay if connected as shown:
The arrangement offers protection to that part of the system which lies between the two current transformers. If at the two ends A and B, currents are equal with the normal conditions or for external fault conditions, the current transformers’ secondary currents are also equal (assuming identical C.T.s are used) and they will flow in the circuit as shown; for the instantaneous polarities of the C.T.s as given; if obviously current through the relay will be zero and hence it will not operate.
If the fault occurs within the protected zone, then only one of the C.T.s will be energized (neglecting the load current flowing through the second C.T.) and hence most of the secondary current will pass through the relay and operate it (provided this current is greater than the pickup value). Although we said that the relay is inoperative for external fault, in practice it is possible that the relay might operate at high values of fault, owing to the difference in performance of two C.T.s.
To prevent this malfunction a coil known as a restraining coil is connected as shown.
The pilot current flowing through the restraining coil tends to restrain the operation of relay, and unbalanced current through the operating coil tends to operate the relay, yet under external fault conditions the whole restraining coil carries the current and the unbalanced current through the operating coil is very small. This causes the restraining coil torque to become greater than the operating torque, and hence the relay does not operate.
Figure 29.1
Under internal fault conditions only half of the restraining coil carries current. Neglecting the small current through the other half, unbalanced current through the operating coil increases. This causes the operating torque to become greater than the restraining torque and hence the relay operates.
29.3Characteristics
If the characteristics are plotted for both the restraining current and the operating current which will just operate the relay, the bias characteristics will be obtained.
The characteristics show that except at low current, the ratio of differential operating current is a fixed ratio. This is due to a slight effect of the control spring at the low current.
29.4Questions and Answers on the Experiment
Q1. What are the difficulties encountered in the differential protection of transformers?
Ans. The difficulties encountered in the differential percentages of transformers are:
1.Different ratio current transformers are required at the primary and secondary so as to balance the current and phase angle. This will cause difficulties in calculations.
2.The C.T. ratio should be changed for different types of transformer connections.
Figure 29.2
3.In case of a top changing transformer, the C.T. ratio should be changed for every tapping.
4.At the time of switching the transformer, the magnetizing current is very high; this will create the unbalancing, and so the relay will trip.
5.The C.T. and its ratio may be changed with different types of connections.
Q2. Why is the operating coil connected to the midpoint of the restraining coil?
Ans. The operating coil is connected to the midpoint of the restraining coil so it will not operate by occurrence of any fault outside the protected zone, because the secondary currents of both transformers will be the same and no current will flow through the relay. This is the case when normal conditions exist.
Q3. In how many ways can the slope of the bias characteristic be changed?
Ans. The restraining factor,
where
Nr = No. of turns in the restraining coil.
No = No. of turns in the operating coil.
So the slope of the characteristic can be changed by changing the restraining factor Kr, which can be changed either by increasing Nr or by reducing No.
Figure 29.3
The slope can be changed by changing the operating current (I1 – I2), but (I1 – I2) is always constant. There are only possibilities of changing the A.T. in the restraining coil.