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EE 587 Mid Term

# EE 587 Mid Term - EE 587 Mid Term Electrical Power...

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EE 587 Mid Term Electrical Power Distribution 1 . A 34.5-kV, 60-Hz radial system is shown in figure P-1. Select current tap settings (TS) and time dial settings (TDS) to protect the system from faults. The time-current characteristics of the inverse-time over current relay are depicted in figure 5.6 [IEEE red book]. Assume 0.3-second coordination time interval and the voltage at all buses, during normal operation, is 34.5-kV (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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I 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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EE 587 Mid Term - EE 587 Mid Term Electrical Power...

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