18 The Primary Conductor In Figure 102 Is One

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18. The primary conductor in Figure 10.2 is one phase of a three-phase trans- mission line operat…

18. The primary conductor in Figure 10.2 is one phase of a three-phase trans- mission line operating at 345 kV, 700 MVA, 0.95 power factor lagging. The CT ratio is 1200:5, and the VT ratio is 3000:1. Determine the CT second- ary current and the VT secondary voltage V. Assume zero CT error. A CO-8 relay with a current tap setting of 5 amperes is used with the 100:5 CT in Example 10.1. The CT secondary current I' is the input to the relay operating coil. The CO-8 relay burden is shown in the following table for various relay input currents CO-8 relay input current l'. A 5 8 10 13 15 CO-8 relay burden Z, 2 05 08 1.0 13 1.5 Primary current and CT error are computed in Example 10.1 for the 5-,8-, and 15-A relay input currents. Compute the primary current and CT error for (a) I'=1° A and ZB-1.0Ω, and for (b) I' =13Aand Zs-1.3Ω. (c) Plot I' versus I for the above five values of I'. (d) For reliable relay operation, the fault-to-pickup current ratio with minimum fault current should be greater than two. Determine the minimum fault current for application of this CT and relay with 5-A tap setting.

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