USCG Exam PracticeElectrical measuring devices and instrumentation
A switchboard wattmeter suddenly deflects to a negative (reverse) reading on a generator that was previously sharing load normally with another machine. What does this indication most likely mean, and what is the required action?
- A. The generator's voltage regulator has failed high; reduce field excitation to bring the reading positive
- ✓ The generator is now motoring — absorbing power from the bus rather than supplying it — and must be tripped to protect its prime moverCorrect
- C. The wattmeter's current coil leads are reversed; swap the CT secondary connections to correct the polarity
- D. The power factor has dropped below 0.5 lagging, causing the wattmeter to read apparent power in the negative direction
Why B is correct
A negative wattmeter deflection on a paralleled generator indicates reverse power flow: the machine is being motored by the bus, meaning the bus is driving the prime mover rather than the prime mover driving the generator. This condition can damage the prime mover and requires the generator to be tripped off the bus. Reverse power relays are fitted specifically to detect and respond to this condition.
Cited:NEETS Mod. 3 §1-4