EEET2336 Power System Protection
Tutorial Problem Sheet 1
Problem 1
For the radial power system shown in Fig. 1.1, design simple overcurrent protection using IDMT relays
with the characteristics given in Fig. 1.2. Assume that the relay of the circuit breaker B3 is set for the
fastest operation available, and the coordination time is in the commonly used range. The maximum fault
level in MVA-s on each busbar is given in Table 1.1. The available pick-up current settings for the relays
are 1 A to 12 A, in steps of 1 A. The available Time Dial Settings for the relays are 0.4 to 10, in steps of
0.2. The available ratios of current transformers to be used are given in Table 1.2.
Determine the CT ratio and the settings for each relay.
Show that the time grading computed for the maximum fault levels results in protection selectivity at
lower fault currents too. What is the condition for the above to be fulfilled automatically?
Busbar No.
Max. Fault MVA
1
2
3
4
130
90
60
40
Table 1.1 Busbar Fault Levels
CT No.
Ratio
1
2
3
4
150/5
200/5
300/5
400/5
5
6
500/5
7
600/5
8
700/5
9
800/5
1000/5
Table 1.2 Available CT Ratios
1

3
2
C/B
B1
B2
4
B3
11 kV
5 MVA
Load
6 MVA
Load
5 MVA
Load
Fig: 1.1 Radial 11-kV distribution feeder
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0.4
Fig. 1.2
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Problem 2
The minimum current at which an overcurrent relay operates is 10 A. The relay is connected to
a CT with a CT ratio of 200: 5. The data for the CT is included in Fig. Q4 and Table 1.
Determine the minimum primary fault current that will cause the relay to operate, if the CT
burden is
(a) 1.0 Ω,
(b) 4.0 Ω and
(c) 5.0 Ω.
Calculate the CT error for each case.
Table 1
CT ratio
50:5
100:5
150:5
200:5
250:5
300:5
400:5
450:5
500:5
600:5
Secondary
impedance [Ω}
0.061
0.082
0.104
0.125
0.146
0.168
0.211
0.230
0.242
0.296
Secondary magnetizing
voltage [V]
Secondary magnetizing current [A]
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