Protection
Question #1
a. List the four important stages required for the protection of network
equipment.
b. The characteristics curves of an overcurrent relay, used as part of the
protection system of a generator, is shown in Figure #4. The
characteristics of a multi-ratio current transformer are shown in Figure
#3. By selecting an appropriate CT ratio, determine the relay setting that
will allow the generator to operate at maximum load, indicated in table 1,
without tripping.
c. Based on your selections in section "b" estimate the fastest operating
time for the relay to trip during the passage of the minimum and
maximum fault currents.
CURRENT
RATIO
100:5
150:5
200:5
250:5
300:5
400:5
500:5
600:5
700:5
800:5
900:5
1000:5
1200:5
TURN
RATIO
20:1
30:1
40:1
50:1
60:1
80:1
100:1
120:1
140:1
160:1
180:1
200:1
240:1
SEC.
RMS
EXCITING
VOLTS
0.05
0.08
0.11
0.15
0.20
0.29
0.39
0.49
0.59
0.70
0.81
0.92
1.14
1000
100
10
0.001
0.01
0.1
1.0
SECONDARY RMS EXCITING AMPS - I
Figure #3
Table #2
Minimum Load
215 A
Maximum Load
420 A
Minimum Fault
1800 A
Maximum
Fault
3850
7
6
5
4
3
2
1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Relay operating time (seconds)
Relay input current in multiples of current tap setting
Typical time curves
type CO-8
over current relay
50-60 hertz
Current
tap
setting
A
0.5
0.6
0.8
1.0
1.5
2.0
2.5
3
3.5
4
4.5
5
6
7
8
10
12
11 time dial setting