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When a single synchronous generator is operated alone adding a load on a synchronous generator the following happens:

Adding inductive loads, |Eo| will increase, however, δ will not vary.

Adding resistive loads, |Eo| will NOT change, however, E will reduce. Because only resistive loads consume real power and therefore δ will increase.

Adding capacitive loads, |Eo| will decrease, however, δ will not vary.

δ = torque angle (angle of Eo with respect to E)

Eo = line-to-neutral internally induced stator voltage

E = line-to-neutral terminal voltage

Is this correct?

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2 Answers 2

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Think of the mechanical equivalent at the drive shaft. A capacitive load would be like a flywheel, and a resistive load would be like brake/friction.

Resistive loads consume a steady rate of power. Capacitive loads consume power initially, but gradually consume less as they become energized.

In both cases the time component is important.

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The described effect would seem to be the case when the generator's voltage regulator maintains a constant terminal voltage, E (E2 in the diagram below). The diagram below is the capacitive load addition case. Angles are measured with respect to the terminal voltage. The output current, I is reduced and rotated to reduce the angle between the current and the terminal voltage. The regulator adjusts the excitation to reduce the internal generated voltage to keep the terminal voltage constant.

For addition of inductive load, the effect would be similar. The reactive VA would increase rather than decrease and the generator impedance voltage drop vectors would rotate in the opposite direction.

enter image description here Image (less red markings) from Fitzgerald, Kingsley, Umans, Electric Machinery 4nd ed.

For addition of resistive load, the real component of the voltage drop vectors will increase and the vectors will rotate as shown below, but the torque angle will need to change because the real power increase must be reflected in generator torque. It is assumed that the generator speed regulator will prevent the speed from changing. The generator voltage regulator will adjust the excitation as necessary to prevent the terminal voltage from changing.

This description is for changes in steady-state operation for a new load condition as compared to a previous load condition. The system behavior during the transition is not covered. That would require consideration of the system inertia and energy storage aspects of the circuit elements.

enter image description here Image (less red markings) from Fitzgerald, Kingsley, Umans, Electric Machinery 4nd ed.

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