EE 587 Mid Term
Electrical Power Distribution
1
. A 34.5kV, 60Hz radial system is shown in figure P1. Select current tap settings (TS) and
time dial settings (TDS) to protect the system from faults. The timecurrent characteristics of the
inversetime over current relay are depicted in figure 5.6 [IEEE red book]. Assume 0.3second
coordination time interval and the voltage at all buses, during normal operation, is 34.5kV
(lineto line). Assume available current tap settings, in amperes, are 0.5, 0.6, 0.8, 1.0, 1.5, 2.0,
2.5, 3, 3.5, 4, 5, 6, 7, 8, 10 and 12.
Given that the radial system is operated at 34.5kV (line to line) and 60Hz as shown in the figure.
Solution:
Given that the radial system is operated at 34.5kV (line to line) and 60Hz as shown in the figure.
Let us assume that the fault occurred between bus 2 (B2) and bus 3 (B3) be P1 and between
bus 1 (B1) and bus 2 (B2) be P2.
First we have to select the Tap Settings (TS) such that the
relays do not operate for maximum load currents.
Calculation of TS3:
Now for maximum load L3, starting at B3 the primary and secondary CT currents are
calculated as follows:
I
L3
= S
L3
/(V
3
*√3)
= 9*10
6
/(34.5*10
3
*√3)
= 150.6 A
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L3
’ = 150.6/(200/5)
=
3.765A
Therefore we have to select the TS for the relay B3 choosing the lowest TS above 3.765A
from the given TS. Hence
4A
is the lowest above 3.765A.
Calculation of TS2:
Since it is given that the load power factors are identical,

S
L2
+S
L3

=

S
L2

+

S
L3

. Now at B2, the
primary and secondary CT currents for maximum load can be calculated as follows.
I
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 Spring '11
 Dr.MohammedSafiuddin
 at bus, time dial settings, phase delay angle

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