Voltage Regulation

Frequency is a single number shared by every generator in North America. Voltage is not. It varies by location, drops under heavy load, and cannot be fixed by sending reactive power from far away, because reactive power barely travels before it dissipates.

Voltage must stay within roughly plus or minus 5% of nominal (e.g., 114-126 V on a 120 V circuit). Outside that band, motors overheat, equipment malfunctions, and severe cases trigger voltage collapse that can black out a region in seconds. The tool for managing voltage is reactive power: injecting it raises local voltage; absorbing it lowers it. Capacitor banks inject; inductors absorb. Large generators do both by adjusting excitation current. But reactive power losses on AC lines are high, so injections must happen close to the problem.

A summer afternoon voltage sag. Air conditioners load up a distribution feeder. Heavy current draw sags voltage from 120 V toward 112 V, just below the acceptable floor. The nearest large generator is 50 miles away.

Can that remote generator fix the sag by increasing reactive output?

Not reliably. Reactive power dissipates rapidly with distance; most of what the remote generator produces never reaches the feeder. The practical fix is local: switching in a capacitor bank at the substation itself. The injection is immediate, voltage recovers, no long-distance transmission required.

Because reactive power cannot travel far, every grid region needs its own voltage support resources. This shapes where utilities site equipment and why adding large loads in weak-reactive-support areas creates reliability problems that cannot be solved cheaply from a distance.

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