Induction vs. Synchronous Generators

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this. One reason is power factor correction. We will explain this in depth later, but suffice to say at this juncture, induction generation works out to be most favorable when the generated load is small compared to your total plant load. Another reason is that the single stage turbine with an inexpensive, self-contained lubrication system is limited in its power producing capabilities. At higher powers more expensive multi-stage turbines and pressurized lubrication systems are required. The major advantage of the single stage steam turbine induction generator package is its simplicity: no gear, no pressure lube, simple controls, and simple connection to electric grid. Note: connection to grid is required for excitation. All these factors result in a relatively low cost package, which when applied in a simple topping application, results in extremely fast payback on investment.

A synchronous generator runs at a constant speed and draws its excitation from a power source, external to, or independent of, the load or transmission network it is supplying. A synchronous generator has an exciter that enables the synchronous generator to produce its own "reactive" power and to also regulate its voltage.  Synchronous generators can operate in parallel with the utility, or in "stand-alone" or "island" mode. Synchronous generators in the <15 mw power range are only available as 4-pole (1800 RPM) units, are almost always geared, and require large pressure lube systems. Most synchronous packages over 1.0 mw are driven by multistage steam turbines and typically involve more sophisticated controls and monitoring equipment. Although more complex and costly, larger synchronous generators can have a lower cost per kw due to their economy of scale.