If the excitation of an alternator operating in parallel with another alternator is increased above the normal value of excitation, its ________.

Question

Prepp Team · Accepted Answer

Analyzing Alternator Operation in Parallel

When an alternator operates in parallel with other alternators or a large power system (often called an infinite bus), its behavior is different from when it operates standalone. In parallel operation, the terminal voltage and frequency are essentially fixed by the grid or the system. The alternator must synchronize its generated voltage and frequency with the system before connecting.

Once synchronized and connected, the alternator contributes to the total load. This load has both real power (kW) and reactive power (kVAR) components. The division of these components among the parallel alternators is controlled by different parameters:

Real Power (kW): The amount of real power supplied by a specific alternator is primarily controlled by its prime mover input (the mechanical power supplied by the turbine or engine driving it). Increasing the prime mover input tends to increase the alternator's output kW.
Reactive Power (kVAR) and Power Factor: The amount of reactive power supplied by an alternator is primarily controlled by its excitation level (the DC current supplied to the field winding). Adjusting the excitation changes the magnitude of the generated electromotive force (EMF).

Impact of Increased Excitation in Parallel Alternators

Let's consider what happens when the excitation of one alternator operating in parallel is increased, while the prime mover input is kept constant:

The increased excitation causes the magnitude of the generated EMF ($E_f$) to increase.
Since the terminal voltage ($V_t$) is fixed by the parallel system, the increased $E_f$ creates a voltage difference within the machine impedance.
This voltage difference drives a current. In parallel operation, this current is largely reactive.
An overexcited synchronous machine (which is what an alternator is) operates at a lagging power factor and supplies reactive power to the system.
Conversely, an underexcited synchronous machine operates at a leading power factor and absorbs reactive power from the system.

Therefore, increasing the excitation above its normal value makes the alternator overexcited. An overexcited alternator operating in parallel delivers more reactive power to the grid, causing its output power factor to become more lagging.

The real power output (kW) is mainly determined by the prime mover and does not significantly change with excitation adjustments, assuming the prime mover input remains constant. The total output current will increase because it now consists of the original real current component plus a larger lagging reactive current component.

Analyzing the Given Options

Based on the understanding of parallel operation and excitation control:

Option 1: Output current decreases. Incorrect. Increasing excitation increases the reactive current component, thus increasing the total output current.
Option 2: Power factor becomes more leading. Incorrect. Increasing excitation makes the machine overexcited, leading to a more lagging power factor.
Option 3: Output kW decreases. Incorrect. kW output is controlled by the prime mover, not excitation. Assuming prime mover input is constant, kW remains relatively unchanged.
Option 4: Power factor becomes more lagging. Correct. Increased excitation in a parallel alternator leads to overexcitation and a lagging power factor, where the alternator supplies reactive power.

Conclusion

Increasing the excitation of an alternator operating in parallel primarily affects its reactive power output and power factor. It causes the alternator to supply more reactive power to the system, resulting in a more lagging power factor.

Parameter	Effect of Increasing Excitation (Parallel Operation)
Generated EMF ($E_f$)	Increases
Terminal Voltage ($V_t$)	Remains essentially constant (set by the grid)
Real Power (kW)	Remains essentially constant (controlled by prime mover)
Reactive Power (kVAR)	Increases (supplied to the grid)
Power Factor	Becomes more lagging
Output Current	Increases

Revision Table: Alternator Parallel Operation Excitation

Key Concept	Explanation
Parallel Operation	Connecting alternators to a common electrical grid or busbar.
Excitation Control	Adjusting DC current in the field winding to control generated EMF and reactive power.
Prime Mover Control	Adjusting mechanical input power (e.g., fuel/steam) to control real power output.
Overexcitation	Higher than normal excitation; causes lagging power factor, supplies kVAR.
Underexcitation	Lower than normal excitation; causes leading power factor, absorbs kVAR.

Additional Information: Alternator Power Factor

The power factor of an alternator operating in parallel indicates whether it is supplying or absorbing reactive power. A lagging power factor means the alternator is supplying reactive power to the load or the grid, which is necessary to maintain the voltage across inductive loads. A leading power factor means the alternator is absorbing reactive power from the grid, which might be needed if the grid has excess capacitive reactance or another alternator is supplying too much kVAR.

In parallel operation, excitation control is the primary method used to manage the reactive power flow and thus the power factor of an alternator, independent of its real power output (which is controlled by the prime mover).