Explaining Reactive Power

Lynne Kiesling

This conversation raises a challenging point, one that I struggle with constantly — how do we communicate complex technical issues to people who have little or no background knowledge of the subject? Probabilistic issues are a constant source of this challenge. For what I’m working on right now, the primary challenge is in explaining reactive power.

I’ve heard two really good ways of describing reactive power non-technically. One is from a staff presentation at December 15’s regular FERC meeting, where the presenter said that reactive power is like the bouncing up and down that happens when you walk on a trampoline. Because of the nature of the trampoline, that up-down bouncing is an essential part of your forward movement across the trampoline, even though it appears to be movement in the opposite direction.

The second is from the KP Spouse (who, as a physicist and extremely smart Renaissance-style fellow, gives me priceless help in my electricity understanding). Reactive power and real power work together in the way that’s illustrated by this labyrinth puzzle, Labyrintspel:

The description of the objective of the game begins to show why this game represents the relationship between real and reactive power:

The intent is to manipulate a steel ball (1.2cm in diameter) through the maze by rotating the knobs – without letting the ball fall into one of the holes before it reaches the end of the maze. If a ball does fall prematurely into a hole, a slanted floor inside the box returns the ball to the user in the trough on the lower right corner of the box.

So you twist the two knobs to adjust the angle of the platform in two directions, in order to keep the ball rolling through the maze without falling into any holes. Those twists are reactive power, which helps propel the real power through to its ultimate goal, which is delivery to the user. Without reactive power, you fall into holes along the way, which are network failures.

Both of these examples illustrate how important it is to understand the system and how it works in order to meet your objectives effectively. In the Labyrintspel game, if you didn’t take into account the structure of the system (so to speak), winning would be really easy because you would turn one knob all the way in one direction, and the other knob all the way in the other direction, and the ball would merely roll across the platform. If that’s our model of how electricity works, then that would deliver the electrons to the end user. But in the game, on the trampoline, and in the electric power network, the system has more going on that means it’s essential to do things that seem counterintuitive, like bouncing up and down on the trampoline or turning the platform in the game west to avoid the hole to the east, even though you have to go east to win.

In electric power, the counterintuitive thing about reactive power is the idea that you have to use some power along the path to balance the flow of electrons and the circuits. Otherwise, the electricity just flows from the generator to the largest consumer (that’s Kirchoff’s law, basically). In this sense, reactive power is like water pressure in a water network.

And for you techie folks out there, one reason why the Labyrintspel game and the trampoline are good examples is that they capture the fact that mathematically, real power and reactive power are conjugates.