Electrical Power and Machines Depart.
Power System Protection Application
Tanta University Faculty of Engineering
Sheet 1: Differential Protection
1. A generator, having normal current = 100 A is to be protected by current differential relays using
stabilizing resistor. The through fault required is up to 15 times full load current; assuming one
end of the CT gets completely saturated, CT ratio = 100/1. CT resistance = 0.046 ohm, and total
lead burden = 1 ohm, determine the required stabilizing resistor and the stability ratio. Given also
the relay pickup current = 0.1 A, relay operating voltage = 12 volts, and the magnetizing current of
CT = 200 mA for induced voltage of 15.6 volt.
2. A differential protection circulating current is used to protect a generator having a full-load current
of 600 A, CT ratio 2000/5 A and the distance between the CTs at opposite ends of the machine is
200 yds. Under straight through fault condition of 15 times full-load the CTs at one end have a
voltage of 80% of that of other end. The relay having an impedance of 100 ohm is connected across
the physical midpoint of the pilots. Determine (1) at what distance from the physical midpoint will
zero voltage be located. (2) At what current the relay will have to be set to give a stability factor
of 3. Magnetizing current of CT = 100 mA.
3. In differential relay using an electromagnetic comparator, the relay pickup current is 0.4 A. When
a through fault current of 10 times full load passes through the generator winding, the secondary
currents at the two ends of the CTs are 50 A and 47.5 A respectively. Assuming the slope of the
differential relay to be 5% determine whether through fault stability will be achieved. In case this
is not achieved what modification will have to be done in the relay so as to achieve stability,
keeping the relay pickup current to be the same.
4. Consider the power system shown in Figure 1 which represents a unit-connected generator prior
to being synchronized to the system and protected with an overcurrent relay connected as a
differential relay. Determine the maximum load, select a CT ratio for the generator differential,
calculate the relay operating currents for a three-phase fault at F1 and F2 and set the relay.
Assume there is no CT error and the relay has the CO-11 time–current characteristics.
Fig 1 . One line diagram for Problem 7
Electrical Power and Machines Depart.
Power System Protection Application
Tanta University Faculty of Engineering
5. Repeat problem 7 assuming that the line-side CT has an error of (a) 1%, (b) 5% and (c) 10% of its
secondary current. Set the overcurrent relay so it will not operate for an external fault.
6. Repeat problem 7 for a phase-to-phase fault at F1.
7. Figure 2 shows a percentage differential relay applied for the protection of a generator winding.
The relay has a 0.1 A minimum pickup and a 10% slope. A high-resistance ground fault has occurred
as shown near the grounded-neutral end of the generator winding while it is carrying load with
the currents flowing at each end of the generator as shown. Assume that the CT ratios are as
shown in the figure and they have no error. Will the relay operate to trip the generator under this
condition? Would the relay operate if the generator were carrying no load with its breaker open?
Draw the relay operating characteristic and the points that represent the operating and restraining
currents in the relay for the two conditions.
