The Synchronous Generator

Almost all industrial and oilfield generation uses the synchronous generator: a rotor carrying a DC-excited field, turned by a prime mover, inducing three-phase voltage in the stator windings.

Speed and frequency

Output frequency is rigidly tied to shaft speed and pole number. A two-pole machine must turn at 3000 revolutions per minute for 50 hertz, a four-pole at 1500. There is no slip, which is what distinguishes a synchronous machine from an induction machine.

Excitation

The rotor field is supplied by an excitation system. Modern machines use brushless excitation: a small AC exciter on the same shaft feeds a rotating rectifier, so there are no slip rings or brushes to maintain. Older and some special machines use static excitation through slip rings, which offers faster response.

The automatic voltage regulator controls excitation to hold terminal voltage at setpoint. Increasing excitation raises the voltage and, when connected to a system, increases the reactive power the machine exports. Decreasing it does the reverse, and taking excitation too low risks loss of synchronism and can cause the machine to absorb reactive power to the point of instability — which is why underexcitation limiters exist.

The two controls

This is the central idea in generator operation and it is worth stating plainly:

• The governor, controlling fuel to the prime mover, determines real power (kilowatts) when running in parallel, and frequency when running alone.
• The AVR, controlling excitation, determines reactive power (kilovolt-amperes reactive) when running in parallel, and voltage when running alone.

Operators who understand this diagnose most generator problems quickly; those who do not tend to adjust the wrong control.

The capability chart

A generator's operating envelope is bounded by stator current heating, rotor field heating at high excitation, and stability and end-core heating limits at low excitation, plus the prime mover's power limit. The capability chart plots these, and the machine must be operated inside it. Running at very low power factor loads the machine thermally without delivering useful output.