Governor droop is provided on a generator so that
Answer: C
A falling speed-power characteristic lets each machine take a share of a load increase in proportion to its droop setting. An isochronous machine holding frequency exactly cannot share load with another of the same kind.
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Synchronising power of a machine on a network is
Answer: D
Any departure from the equilibrium angle produces a power that pulls the rotor back, and its magnitude per radian is the synchronising power coefficient. A machine with a small coefficient is loosely tied to the system and swings a great deal.
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The synchronising power coefficient of a machine is largest when the machine is
Answer: A
Correct answer: lightly loaded, with a small load angle
The synchronising power coefficient is \( dP/d\delta \), and since \( P = (EV/X)\sin\delta \) it varies as \( \cos\delta \). It is greatest at small load angles and falls to zero at ninety degrees, which is precisely the pull-out point - a machine loaded close to its limit has almost no restoring stiffness left, which is why it falls out of step so readily.
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Two generators sharing a load will divide a further increase according to
Answer: B
Correct answer: their governor droop settings
Droop is the fall in governor speed setting with load, and it is what decides how a shared increase is split: on a common system frequency, each machine picks up load in inverse proportion to its droop. Excitation would only redistribute the reactive power, leaving the real power split unchanged.
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A generator connected to an infinite busbar has its terminal voltage and frequency
Answer: C
An infinite bus is by definition unaffected by anything one machine does, so the machine can control only what it exports. That is why the analysis of one machine on a large system is so much simpler than of an isolated set.
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An isolated generator supplying its own load controls its frequency by
Answer: D
With nothing else to set the frequency, the governor holds the speed and therefore the frequency at whatever the setting demands. On a network the same governor controls power instead, the frequency being fixed by everyone together.
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Loss of excitation on a generator connected to a network causes it to
Answer: A
With the rotor field gone the pull-out power collapses, the machine slips poles and it draws its magnetising current from the network as an induction machine would. Loss of field protection is fitted on any machine of size.
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The capability curve of a synchronous generator shows the limits imposed by
Answer: B
Armature current limits the apparent power, field current limits the lagging output, and the stability margin and end-region heating limit the leading. The permissible operating region is the area bounded by all three.
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A four-pole alternator is to generate at 50 Hz. It must be driven at
Answer: C
Correct answer: 1,500 rpm
\[ N = \frac{120f}{P} = \frac{120 \times 50}{4} = 1500\ \mathrm{rpm}. \] Unlike an induction machine, a synchronous machine runs at exactly this speed under all loads - it is the frequency that fixes the speed, and any departure from it means the machine has lost synchronism.
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A hydro-generator running at 125 rpm on a 50 Hz system has
Answer: D
Correct answer: 48 poles
\[ P = \frac{120f}{N} = \frac{120 \times 50}{125} = 48\ \mathrm{poles}. \] A pole count this high can only be built as a salient pole rotor of large diameter, which is exactly the form a hydro-generator takes - and the reason such machines are wide and shallow rather than long and slender.
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