Question

You are given that the system shown in Figure 4.25 has a 110/220 kV autotransformer. The positive- and zero-sequence impedances in ohms or percent are as shown in the figure, the zero-sequence impedances being in parentheses. Assume that the low- voltage system is solidly grounded. For a phase-a-to-ground fault at the midpoint of the transmission line, calculate the transformer current In in the neutral and the phase a currents Ia and I'a on the high and low sides of the transformer. If the source on the low-voltage side is to be grounded through a reactance, determine the value of the grounding reactance for which the transformer neutral current becomes zero. As the grounding reactance changes around this value, the direction of the neutral current will reverse, and will affect the polarizing capability of the neutral current for ground faults on the high side. Can faults on the low-voltage side ever cause the neutral current to reverse?

20%
20%
4j40
(10 j80)
(10%
at 100 MVA
(10%
at 100 MVA
F1
l'a XT =40%
XLT 20%
XHL 10%
110/220 kV
Figure 4.25 System diagram for Problem 4.10

Image Transcription

20% 20% 4j40 (10 j80) (10% at 100 MVA (10% at 100 MVA F1 l'a XT =40% XLT 20% XHL 10% 110/220 kV Figure 4.25 System diagram for Problem 4.10

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