Synchronous generator

4-1. Why is the frequency of a synchronous generator locked into its rate of shaft

rotation?

4-2. Why does an alternator’s voltage drop sharply when it is loaded down with a lag-

ging load?

4-3. Why does an alternator’s voltage rise when it is loaded down with a leading load?

4-4. Sketch the phasor diagrams and magnetic field relationships for a synchronous gen-

erator operating at (a) unity power factor, (b) lagging power factor, (c) leading

power factor.

4-5. Explain just how the synchronous impedance and armature resistance can be deter-

mined in a synchronous generator.

4-6. Why must a 60-Hz generator be derated if it is to be operated at 50 Hz? How much

derating must be done?

4-7. Would you expect a 400-Hz generator to be larger or smaller than a 60-Hz genera-

tor of the same power and voltage rating? Why?

4-8. What conditions are necessary for paralleling two synchronous generators?

4-9. Why must the oncoming generator on a power system be paralleled at a higher fre-

quency than that of the running system?

4-10. What is an infinite bus? What constraints does it impose on a generator paralleled

with it?

4-11. How can the real power sharing between two generators be controlled without af-

fecting the system’s frequency? How can the reactive power sharing between two

generators be controlled without affecting the system’s terminal voltage?

4-12. How can the system frequency of a large power system be adjusted without affect-

ing the power sharing among the system’s generators?

4-13. How can the concepts of Section 4.9 be expanded to calculate the system frequency

and power sharing among three or more generators operating in parallel?

4-14. Why is overheating such a serious matter for a generator?

4-15. Explain in detail the concept behind capability curves.

4-16. What are short-time ratings? Why are they important in regular generator operation?

Answers to Synchronous Generator Questions

1. Frequency Lock: The frequency of a synchronous generator is directly tied to the rate of shaft rotation, meaning that as the rotor spins at a constant speed, it generates electricity at a fixed frequency, determined by the number of poles in the generator and its rotational speed.

2. Voltage Drop with Lagging Load: An alternator experiences a sharp voltage drop when loaded down with a lagging load because the inductive load causes a phase shift between the current and voltage, leading to increased reactive power demand and reduced terminal voltage.

3. Voltage Rise with Leading Load: Conversely, when an alternator is loaded with a leading load, the capacitive nature of the load causes the current to lead the voltage, which can result in an increase in terminal voltage due to the reduction in the overall reactive power demand from the generator.

4. Phasor Diagrams:

   • Unity Power Factor: Voltage and current are in phase, resulting in maximum power transfer.

   • Lagging Power Factor: Current lags behind voltage, indicated by a phase angle representing inductive reactance.

   • Leading Power Factor: Current leads voltage, represented by a phase angle indicating capacitive reactance.

5. Determining Synchronous Impedance and Armature Resistance: The synchronous impedance can be determined by performing a short circuit test, while armature resistance is calculated through a no-load test.

6. Derating for 60-Hz Generator Operating at 50 Hz: A 60-Hz generator must be derated to avoid overloading; typically, a reduction in power output proportional to the frequency difference is necessary (e.g., operating at 83.33% of its rated capacity).

7. 400-Hz vs. 60-Hz Generator Size: A 400-Hz generator is generally smaller than a 60-Hz generator of the same power and voltage rating due to reduced size of core and winding material, allowing for greater efficiency at higher frequencies.

8. Conditions for Paralleling Synchronous Generators: To parallel two synchronous generators, they must match in voltage, frequency, and phase sequence.

9. Higher Frequency Paralleling: The oncoming generator must be synchronized at a higher frequency to ensure it properly matches and locks onto the running system without causing disturbances.

10. Infinite Bus: An infinite bus is a theoretical power system with unlimited capacity and a constant voltage and frequency that imposes constraints on generation power and reactive power—any generator paralleled with an infinite bus must operate at the same voltage and frequency as the bus.

11. Real Power Sharing Control: Real power sharing can be controlled through adjustment of the mechanical input to each generator while maintaining constant frequency by managing their outputs; reactive power can be shared by adjusting excitation levels without voltage alteration.

12. System Frequency Adjustment: System frequency can be adjusted through the control of generator outputs or by changing the speed of generators without affecting power sharing among them.

13. Calculating Frequency and Power Sharing: For three or more generators, the concepts of synchronous operation expand to utilizing the load sharing methods, taking into account their speed and excitation levels to ensure balanced output.

14. Overheating Risks: Overheating in generators can lead to insulation damage, reduced efficiency, and potential mechanical failure, thus it's crucial to monitor thermal conditions closely.

15. Capability Curves: Capability curves illustrate the operational limits of a generator under different loading conditions, helping operators understand maximum output while maintaining safety and efficiency.

16. Short-Time Ratings: Short-time ratings reflect the ability of a generator to withstand overloads for brief periods, important for managing peak demands and preventing system failures.