Alternators Running in Parallel: Effect of Excitation Change

When two or more alternators are running in parallel, they are synchronized to the same frequency and voltage. In this configuration, the overall system frequency is determined by the combined action of the prime movers and the system load, while the voltage magnitude is largely controlled by the total excitation of all machines and the system's reactive load.

Understanding Alternator Excitation

The excitation of an alternator primarily controls the magnitude of its internally generated voltage. This internally generated voltage, when operating in parallel with other machines connected to a grid or busbar, determines the machine's ability to supply or absorb reactive power (\(Q\)).

• Increased Excitation: A machine with increased excitation relative to others will tend to supply more lagging reactive power to the bus. Its power factor will become more lagging.
• Decreased Excitation: A machine with decreased excitation will tend to absorb lagging reactive power (or supply leading reactive power). Its power factor will become more leading or less lagging.

The real power output (\(P\)) of each alternator in parallel is primarily controlled by its prime mover's governor, which adjusts the torque input to maintain the system frequency. Changing the excitation has only a minimal effect on the real power sharing, which is mainly governed by the speed-load characteristics set by the governors.

Analyzing the Effect of Changing Excitation

Consider two alternators running in parallel, supplying power to a common load. If the excitation of one alternator is changed (say, increased), its internal voltage increases. This causes it to take on a larger share of the reactive load, supplying more lagging reactive power to the bus. Consequently, its operating power factor becomes more lagging.

The other alternator, with unchanged excitation, will then supply less lagging reactive power (or absorb more) to maintain the system voltage and total reactive power balance. Its power factor will change accordingly.

The overall system frequency and the total real power supplied to the load are largely unaffected by this change in excitation of one machine. Frequency is maintained by the governors, and total real power is dictated by the load demand and shared based on governor settings, not excitation.

Evaluating the Options

• Option 1: Frequency will change - Incorrect. Frequency in a parallel system is determined by the balance between total mechanical input power (from prime movers) and total electrical output power (to the load), governed by the system's inertia and governor responses. Changing the excitation primarily affects reactive power, not real power balance, so frequency will not significantly change.
• Option 2: Power factor will change - Correct. As explained, changing the excitation of an alternator running in parallel directly alters its reactive power output/absorption, which in turn changes its operating power factor.
• Option 3: Speed will reduce - Incorrect. Speed of a synchronous alternator running in parallel is locked to the system frequency (\(N = \frac{120f}{P}\)). Since the frequency doesn't significantly change due to excitation adjustment, the speed also won't change (it will remain synchronized).
• Option 4: Load demand will change - Incorrect. The load demand is determined by the equipment connected to the system, not by adjustments made to the generators themselves. The alternators must adjust their output to meet the existing load demand.

Therefore, changing the excitation of one alternator running in parallel primarily affects the sharing of reactive power and thus changes its power factor.