NERC ~ RC (NP3 Transmission) questions with correct answers

Q32618 When a high voltage transmission line circuit breaker trips open as a result of a fault, a synchronous check relay will monitor certain values on both the line side and the bus side of the open circuit breaker. Before the synchronous check relay re-closes the open circuit breaker, which of the following values has to be within a certain limit before the circuit breaker will close automatically? CORRECT ANSWER •) Frequency •) Voltage Magnitudes •) Phase Angle between the voltages •) ALL of the above FB: The synchronizing relay protects the open circuit breaker from closing when two areas of the power system have large differing values. For example, if the line was closed into a substation bus that had different phase angles or huge voltage differences, large amounts of current and reactive power would flow from one part of the system to another. This can cause high voltage swings, bumping or shaking and possibly knock generators off line and damage equipment. Q32616 A car hits a power pole on 13kv Circuit, causing its A and B phases to wrap around each other. The relay at the substation fails to trip the 13kv circuit breaker feeding the troubled line. Instead, a 51 relay senses the fault and trips the 138/13 kv transformer feeding the 13 kv system to protect it from damage. What type of relay is the 51 relay ? CORRECT ANSWER Timed Overcurrent Relay FB: High voltage transformers are normally protected by differential relays for problems occurring to the transformer itself. However, a differential relay would not operate as the result of high current flow passing through the transformer feeding a fault on the distribution system. To protect the transformer from high fault current, overcurrent relays are installed t