RMIT University ©
School of Engineering
Power System Protection
Tutorial Problem Sheet 2
Problem 1
Consider the system shown in Fig. 1. Circuit breakers Q12, Q21, Q23, Q32, Q34 and Q43 are tripped by directional
overcurrent relays. Circuit breakers Q1 and Q4 are tripped by overcurrent relays only.
a. Which relays will not pick up if a fault occurs at P1?
b. Repeat the above for a fault at P2.
c. Repeat the above for a fault at P3.
d. What is the relationship between the operating times of all relays, if the relays are properly coordinated?
e. Illustrate the above with plots of the relay operating time v. the fault location. Assume that all relays have
definite-time trip characteristics. The minimum trip time is 0.2 s. The coordination time is 0.3 s.
f. Is the system protected against bus faults? Justify your answer.
1

Q1
Q21
Q12
Q32
Q23
P1
L1
4
3
2
P2
Q43 Q4
Q34
P3
L3
L2

L4
Fig. 1
Problem 2
A bus protective zone is shown in Fig. 2. The zone is protected by three numerical differential relays, one for each
phase. All current transformers are the same and have a ratio of 100/5.
If a fault occurs inside the zone or close to it, the feeder fault currents are of the same phase and their magnitudes
are as follows:
IAF = 1000 A, IBF = 1200 A, ICF = 800A, IDF = 900 A, IEF = 1000A, IFF = 800 A.
1. Show how the current transformers in one phase are connected to the relay. Indicate the dotted ends of the
primary and the secondary of each current transformer.
2. During the normal operation of the system, the feeder currents are IA = 90 A, IB = 100 A, IC = 95 A, ID = 90
A, IE = 95 A, IF = 90 A. What are the values of the differential current Idiff and the restraint current Irest, as a
multiple of the CT rated current In during normal operation?
3. Calculate the differential current and the restraint current, as a multiple of the CT rated current, if a fault
occurs at point F1. Assume ideal CT-s.
4. Calculate the differential current and the restraint current, as a multiple of the CT rated current, if a fault
occurs at point F2. Assume ideal CT-s.
5. On a graph of Idiff/In vs. Irest/In, indicate the points of operation of the relay corresponding to normal
operation, a fault at F1 and a fault at F2.
1
RMIT University ©
School of Engineering
ID
IE
I1
IF
I2
I3
I4
Differential
Protection
Relay
F1
I5
IA
F2
IC
IB
I6
Fig. 2
Problem 3
The circuit breakers in the power system shown in Fig. 3 are set to provide zone protection.
1. Draw the boundaries of protective zones in the power system shown in Fig. 3.
2. Which circuit breakers should open as primary protection for a fault at P1?
3. Which circuit breakers should open as primary protection for a fault at P2?
4. Which circuit breakers should open as primary protection, for a fault at P3?
1
4
2
Q21a
Q12a
Q24a
Q1

P1
Q12b
Q42a
Q46
Q21b
Q13
Q24b
1
Q23
Q42b
1
P3
P2
Q31
Q32
3
Q35
5
Q5

Fig. 3
2