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.
When we increase the prime mover input to the generator, power (kW) produced by the generator will also increase. However, the reactive power (kvars) output will decrease in response to an increase in the bus voltage. This change in reactive power output or bus voltage is not always readily observable, since the magnitude of the bus voltage increase depends on many factors (i.e., the machine's kW output with respect to the load at the bus). Also, analog instruments don't provide the resolution to see these changes.
The change in output power results from the increase in the generator current's resistive component. Correspondingly, the decrease in the current's reactive component results in a change in reactive power. The vector sum (Eq. 2) of these two components results in an overall increase in the magnitude of the line current.
Because the bus voltage (voltage at the machine's terminals) has increased and the voltage drop across the winding (jIXS) has increased due to the increase in line current (IL), the internal voltage EF (excitation) of the machine must also increase. (See Eq. 1.) These voltages are phasors and cannot be summed through linear addition. The main idea: If you increase the generator power (kW), you'll decrease generator reactive power (kvars) output.
Changing excitation during power generation paralleled to an infinite bus Eq. 1 represents the mathematical model of our simple synchronous generator. If you increase the machine excitation and leave the prime mover input (kW) constant, reactive power (kvars) of the generator will increase. This change in kVAR output results from only the increase in the current's reactive component. In turn, the vector sum of the reactive and resistive components increases (Eq. 2).
The only difference between changing excitation against an infinite bus and that of the real world is that with an infinite bus, voltage, and frequency never change regardless of generator output. In reality, because of the increase in excitation, more kvars are produced locally at the bus and, therefore, less come from the utility. This decreases the voltage drop from the utility supply (which is usually the dominant factor in determining busvoltage), thereby increasing the local bus voltage.Assume you have a "black st art" synchronous generator (one providing power to the regulator/exciter) that maintains a given kW load while paralleled to an infinite bus. If the excitation were increased, with no operator change to the prime mover input (i.e. kW), the actual output power of the generator would decrease slightly. (You may not be able to see it, due to instrumentation resolution.) This decrease in output power results from the increase in exciter power demand and additional kW lost in the windings as a result of increased magnitude of current they are carrying. Eq. 5 describes this. (in original text)
Isolated bus You can more readily understand voltage regulator and prime mover control interaction when a generator is supplying loads on an isolated bus. An infinite bus dictates the machine speed (frequency) and terminal voltage. Operator-controlled setpoints determine isolated bus frequency and voltage. On an isolated bus, the controller output continually adjusts to keep the measured frequency and voltage at their respective setpoints. The block diagrams [at right] (in original text) show the basic control loops for automatic voltage and frequency regulation while operating on an isolated bus.
As loads are added to the bus, the bus voltage decreases. This creates an error between the setpoint and measured voltage. In turn, the regulator increases excitation to maintain the bus voltage at the desired setpoint.
Prime mover response is somewhat different in that when loads (especially motors) are added, the machine initially imparts rotational energy (like a spinning flywheel as it slows) to accelerate the motors and their driven loads. Because there is a finite amount of time required for the controller and prime mover to react, the prime mover speed will decrease, creating an error between the setpoint and measured speed (frequency). The controller responds to the error by increasing the prime mover input to maintain the desired speed under given load conditions. We can conclude from this that a machine attached to an isolated bus requires a unique value of prime mover input and excitation for each unique set of load conditions.
The close interrelationship between speed and voltage during generator loading (on an isolated bus) can make it difficult to determine if a malfunction is due to the prime mover or voltage regulator. A malfunctioning voltage regulator causes the bus voltage to oscillate continually. This results in changing power demand conditions as described in Eq. 6 (in original text). These changing conditions require prime mover input changes to maintain desired speed.
Conversely, an oscillating prime mover controller will result in frequency swings, which result in a varying reactive load. This load causes generator current swings, which result in bus voltage fluctuations. And those fluctuations require regulator changes to maintain the desired voltage.
What are we saying here? Prime mover input and voltage regulator output essentially change only as a result of manual action- when paralleled to a stable grid (infinite bus). When on an isolated bus, however, the change is a dynamic function as a result of load changes. When attached to an infinite bus, the prime mover and voltage regulator are independent entities whose effects from malfunctions are much more discernible.
Power system instability A good understanding of prime mover and regulator control interaction will help you develop solutions to problems when they arise. In particular, those problems are the effects of power system instability on voltage regulators/exciters and prime mover controllers.
When a machine is paralleled with a large utility (infinite bus) but incapable of producing rated kvars, suspect one of the following: *Instrumentation problems; *Regulator malfunctions; or *Excessively high grid/bus voltage.
Instrumentation problems are the least likely to occur, but often they are the first problems you should check for. Once you have verified your instrumentsare giving you correct readings, then you can proceed with troubleshooting for the other two problems.
Don't assume regulator malfunctions are limited to the regulator itself. Expand your troubleshooting to include input component or any device failure that would prevent you from obtaining rated field current.
If the inability to obtain rated kW is the problem, suspect the prime mover, its control devices, and anything associated with its ability to produce rated horsepower.
In Part 2, we will look at some fundamental concepts of prime mover and voltage regulator/exciter control systems. You can use these concepts to isolate machine malfunctions. And that means faster, more accurate repair of your generator system.
EC&M books: Pocket Electrician, Order #5992. Illustrated Electrical Calculations, Order #4622. Practical Guide to Emergency Standby and Other Auxiliary Power Systems, Order #6034. Ugly's Electrical Reference, Order #293X. Stallcup's Electrical Calculations Simplified, Order #113X. For ordering information, call 1-800-543-7771 or fax 1-800-633-6219.
