Synchronous Generators Questions and Answers

Answer: C

Correct answer: False, since frequency, phase sequence and phase angle must match as well

Matching the voltage magnitude alone leaves three other ways to be wrong. A frequency error means the phase relation is drifting, a phase angle error puts a difference voltage across the machine at the instant of closing, and a sequence error is worst of all - the machine is thrown against a field turning the wrong way.

Answer: D

Correct answer: alternator

Alternator is simply the older name for a synchronous generator, still in common use in engineering practice. An exciter supplies its field, an inverter is a static converter, and an induction generator is an entirely different machine that must draw its magnetising current from the network.

Answer: A

A steady current in the rotating winding produces a field that revolves with the rotor, and it is that revolving field which induces the alternating emf in the stator. The armature on the stator carries the load current.

Answer: B

Bringing tens of thousands of amperes at high voltage out through sliding contacts would be impossible, whereas a few hundred amperes of field current is straightforward. That is why the arrangement is the reverse of the dc machine's.

Answer: C

Correct answer: controlling the speed of the generators collectively

System frequency is the speed of every synchronous machine connected to it, expressed in electrical terms, so holding the frequency means holding that speed - which means matching generation to demand through the governors. Excitation control holds the voltage instead; the two loops are separate and act on different quantities.

Answer: D

Correct answer: slow down, and with it the system frequency

If the electrical load exceeds the mechanical power supplied, the deficit comes out of the rotating kinetic energy of the machine and it decelerates, carrying the frequency down with it. The governor senses the fall and admits more steam or water. On a system, a falling frequency is the signal that generation is short of demand.

Answer: A

A large rotating mass gives the governors time to act before the frequency has moved far, which is why systems dominated by converter-connected generation need synthetic inertia. It contributes nothing to steady-state capacity.

Answer: B

Correct answer: kVA or MVA rather than in kW

The machine’s limits are thermal: the stator current heats the armature and the field current heats the rotor, and both flow whatever the power factor. Since the product of voltage and current is what those limits constrain, the rating is in apparent power. How much of it appears as kilowatts is decided by the load.

Answer: C

Correct answer: the phase voltage is then lower and the neutral is available

Star connection gives a phase voltage \( 1/\sqrt{3} \) of the line value, so the winding insulation need only withstand the smaller figure. It also brings out a neutral, which can be earthed - usually through a resistance - giving a reference for the system and a path for earth fault protection to detect.

Answer: D

Correct answer: appears between line and neutral but not between lines

Third harmonics in the three phases are all in phase with one another, so in a line-to-line measurement they cancel exactly and the output is free of them. Measured from line to neutral they are present. This is one of the practical advantages of the star connection, and in delta they would instead circulate as current round the closed loop.