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Answer each one before you open the explanation. On the exam there's no explanation, and you have about 77 seconds a question on RC and BT, 90 on TO and BI.
Compared with the line's normal load current, the current during a three-phase Fault close to a strong source bus is
- A. slightly larger, and set by the load being served at that moment
- B. about equal, because the conductor rating limits what can flow
- C. many times larger, and set by the impedance back to the sources
- D. smaller, because voltage at the faulted point collapses to zero
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Answer C many times larger, and set by the impedance back to the sources
Fault current is driven by source voltage acting on the small impedance between the sources and the Fault, so it reaches many times load current and barely depends on what the line was carrying beforehand. Conductor ratings are thermal limits for sustained load; they do not restrain the current a Fault draws.
Reference: Power system fundamentals; NERC Glossary - Fault
Compared with a shunt capacitor bank at the same substation, a series capacitor installed in a transmission line mainly
- A. supplies Reactive Power that varies with the bus voltage squared
- B. holds the receiving-end voltage at a scheduled set point
- C. absorbs Reactive Power whenever the line is lightly loaded
- D. reduces the net reactance between the ends of that line
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Answer D reduces the net reactance between the ends of that line
A series capacitor cancels part of the line's inductive reactance, which raises the power a given angle can push through it and lowers the angle needed for a given flow. Injecting Reactive Power that rises with the square of bus voltage describes the shunt bank, and absorbing Reactive Power on a lightly loaded line describes a shunt reactor.
Reference: Power system fundamentals; NERC Glossary - Reactive Power
