One of my responsibilities is to provide new employees with an abbreviated power quality course, so they know enough to relate the basic concepts to their particular job function. I do not want to overwhelm them by trying to turn them into power quality engineers. While I have also taught many power quality seminars to groups of 20–100 people of various backgrounds, the one-on-one approach seems to be the most effective, especially when trying to educate someone about power quality who has never heard the names Georg Ohm and Gustav Kirchhoff.
So, to provide someone with the power quality essentials, you must first understand what they plan on using the information for and what, if any, knowledge (correct or incorrect) they have acquired already. A marketing person who is writing press releases and an electrician going out to troubleshoot a piece of equipment require different degrees of detail, but they have one need in common: an understanding of Ohm’s and Kirchhoff’s Laws. Though I once was told that I will lose my audience if I show any equations, Ohm’s and Kirchhoff’s Laws can be solved with the basics of high school algebra, and they are essential for understanding the power quality phenomena, from transients to sags and swells to harmonics to unbalance and even light flicker.
It also is helpful to use physical examples to represent the electrical forces at play, such as comparing the flow of electricity to the flow of water. The large water storage towers (or pumps) provide the potential to send water where desired, just as voltage provides the potential for electricity. The potential is there even if there is no water flowing in the pipes or no electrons in the wires. The size of the pipe and its construction determine how easily the water can flow through the pipes, just like the impedance of the wiring and loads affects the flow of electrons. The relationship between the potential of water flow and the pipe to determine the water pressure is a bit more complex than in electricity, where Ohm’s Law states that volts (potential represented as V) divided by impedance (restriction of flow represented by Z) equals the current (pressure represented as I). Algebra lets us rearrange the formula to the more commonly known expression V = I x Z.
Kirchhoff’s Laws can also be related to water flow analogies, though the relationship may not be quite as obvious or simple to the layman who isn’t likely to have taken a fluid dynamics course. All of the water that enters from the main supply line must equal the sum of all of the water through the various branches that tee-off the main (assuming no leaks). One of Kirchhoff’s Laws states that the sum of all the currents at a node (connection point of two or more wires) must equal zero. Another way to look at this is that the current entering the node from the direction of the voltage source will equal the sum of all the currents in the various wires leaving that node (I source = I load1 + I load2 + I load3 . . .).
The second law applies to closed-loop water systems, unlike in one’s house or facility where the water comes in from the main but leaves through the faucet and doesn’t return back to the source directly. The sum of all the drops in water pressure around the loop will equal the pressure at the source. In electrical terms, the voltage drops across each of the impedances in the circuit, from the wiring to the protective devices to the transformers to the loads, will equal the voltage potential of the source. In demonstrating the principle, we often lump all of the impedances between the source and the load into one, so the equation looks something like this: Vgeneration = Vsource_impedance + V load.
The simple circuit below demonstrates these principles. The voltage across the source impedance is equal to the current of the source multiplied by the impedance of the source or Vs = Is Zs (Ohm’s Law). The sum of the currents at node 1 equal zero, or Is = Iz1 + Iz2 + Iz3. And lastly, Vg = Vs + V load.
These same equations will hold, whether we are trying to determine the source of harmonics or voltage fluctuations that result in light flicker or the drop in voltage from a large motor starting up that results in a sag, causing equipment to malfunction.
Next month, I will apply these simple rules in several examples to illustrate how a basic understanding of these three rules can help you get your job done.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.