Prime Mover of generator
AT A GLANCE: AC power generation is no longer the exclusive domain of utilities. Many contractors and non-utility maintenance people now have responsibility for generator systems. Suppose you see fluctuations in frequency, voltage, kW, or kvar. Do you know how to isolate the problem quickly?
How do you know if your generator control problems result from a deficiency in the prime mover or in the voltage regulator/exciter? Perhaps it's neither; maybe you're just facing grid fluctuations. But how do you know?
When it comes to malfunctions in voltage regulators/exciters and prime movers (and their effects), there's not much information out there for maintenance engineers or electricians. And to make matters worse, the information that is out there is often unnecessarily complicated and hard to follow.
To help solve your problems, we'll look at prime mover control and voltage regulator control for a single synchronous generator connected to an infinite bus (large utility) and isolated bus. By knowing the basic relationships between the prime mover and voltage regulator/excitation system during power generation, you'll have a better understanding of how machine malfunctions relate to one or the other, regardless of size. Of course, it helps to be familiar with terms like those listed in "Terms To Know" [at right] (in original text)
The principles described here are the same whether the machine is a single- or three-phase generator. However, our discussion primarily focuses on the operation of a three-phase generator supplying a balanced load. Also, because the inductance of the stator or generator windings is significantly greater than the resistance, we'll assume, for simplicity, the windings are all inductive in our equations and diagrams.
Power generation with an infinite bus
When looking at power generation with an infinite bus, remember: If you increase the prime mover input to the machine, (i.e. you add more fuel to the engine or steam to the turbine), generator watts increase. This means the resistive portion of the output current has increased and, therefore, the line current will increase.
Because we have an "infinite bus, " there is no change in the bus voltage and, therefore, no change in kvars, regardless of the load (kW) changes.
Let's begin with the effect of a prime mover input change on a synchronous generator with automatic voltage regulation paralleled to an infinite bus. Eq. 1, in the sidebar "Important Equations For Understanding Power Generation, " (on page 38) (in original text) describes this generator.
When you increase the prime mover input to the generator, both the power output of the generator and resistive portion of the current increase-thus increasing the line current. (See Eq. 2.) (in original text)
Eq. 3 (in original text) shows us that an increase in line current results in an increase in the total volt-amperes of the generator. To find the old and new power factors, take the measured voltage, line current, and watt readings before and after the prime mover input increase. Then, calculate the power factors using Eqs. 3 and 4. (in original text) Power factor will go up with an increase in power output.
Real world power generation In reality, there's no such thing as the "infinite bus" previously described. During power generation with a large utility, slight bus voltage changes can sometimes be observed while frequency changes are not typically discernible.