For the system shown in Figure 3: assume the current transformers do not saturat

Question

For the system shown in Figure 3: assume the current transformers do not saturate

a. Determine the equivalent (Thevenin) circuit at the fault point (FX)

b. Determine the operation time of the four overcurrent elements at the 115 kV side of the transformer, for a phase a-to-ground fault at the point indicated on the 13.8kV line, very close to the bus. These relays are connected to CT’s with CTR=200/5. The phase relays have Ipick-up=5 A, TD=2; and the ground element has Ipick-up=1 A and TD=3. All the relays have U.S. very inverse curve.

13.8 kV 115 kV - Transformer 0115/ 13.8 kV D/GY 30 MVA Short-circuit current: 3-ph: 8 kA 1-ph-gr: 6 kA Assume Z,=Z, and neglect resistance FX XR-10 Figure 3

Explanation / Answer

neglet resistance=9%

Z1=Z2 115=13.8

101.2kv

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