IN“The Velveteen Rabbit” by Margery Williams, the Rabbit asks the Skin Horse, “What is Real?” A customer reminded me of this line when he asked a similar question in regard to the parameter watts used to measure electrical power. There was a time when most of the power was real power, which some define as that part of the power supplied by the electrical source that works. Then there is apparent power, which is all of the power supplied by the electrical source.
There also was a time when real and apparent power were able to be used to calculate the third power of the power triangle, which in Figure 1 is called reactive power. When expressed in vector or magnitudes with phase angles associated with them, the apparent power S was equal to the real power P plus the reactive power Q acted on by the imaginary vector j.
The concept of an imaginary vector or phasor can be better thought of as adding or subtracting 90 degrees, depending on the type of load. Inductive loads, such as motors and transformers, have a reactive component that is 90 degrees in one direction, whereas capacitive loads have a reactive component that is 90 degrees in the opposite direction. This is because inductors and capacitors behave exactly opposite of each other with respect to voltage and current. For an inductor, the current will lag the voltage, or the voltage will change first and will be followed some time later by the current. The nature of an inductor is that it does not allow a current change instantaneously, so it lags behind the voltage.
For capacitors, it is the opposite, with current changing first, followed by the change in voltage. In the capacitor’s case, it does not allow a voltage change instantaneously. It requires current to charge it up before the voltage will rise. A perfect capacitor—one without any resistive component to it—would have the current phasor lead the voltage by 90 degrees, the maximum that is possible for a load. In the real world of loads, there are not many perfect capacitors, with most having some resistive component, just as motors and transformers are not perfect inductors, since the wire that makes them up also has a resistive component.
So, why do we care about what is real or not? The electrical source, most often the utility company, has to have the capacity to provide the apparent power value. If consumers have anything other than pure resistive loads, they are using only the real power and, in many cases, are billed for only the real power component.
The reactive component, with its imaginary vector, can be thought of as just flowing back and forth down the wires without doing any real work. The difference between the real and apparent power is another term called the power factor (PF), which is the ratio of the real power divided by the apparent power. Also in a time gone by, the trigonometric function called the cosine of the phase angle of the voltage and current was called the power factor. But with the increase in nonlinear loads, such as PCs and ASDs, this phase angle became the displacement power factor.
The opposite nature of inductors and capacitors is why electric utilities use power factor correction capacitors to bring the power factor back closer to 1, where the real power would equal the apparent power. The negative volt-ampere reactives (VARs) of the one can be used to cancel the positive VARs of the other.
This is most efficient for the supplier of electricity, since the generators and wires have to supply only useful energy, and they get to bill for everything that they supply. There are some rate structures of utilities that they impose PF penalties for “poor” PF, or they can bill on the volt-ampere or the VARs. The combined effect of the opposite nature of the two can be seen in the waveforms of an energized PF cap switching event, where there is a sudden decrease in the voltage as the cap is being charged, followed by a voltage increase as the inductor “objects” to the instantaneous current demand of the capacitor in Figure 2.
So, with all of that background info, let’s get back to the original question: “What is real?” The customer wanted to know what his wattmeter was really reading. Was it accurate with today’s mixture of loads as they interact with today’s unbalanced and distorted voltage supplies? Stay tuned, as I’ll cover that in next month’s article. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.