Easy First Step

摘要

We had to backtrack for some of the attendees to explain what "stiffness" means in the context of electrical systems. It is one of those abstract terms we use (or misuse) and assume everyone knows what we mean. It is also one of the first items we should check in a power quality investigation, but we often overlook it because we assume the electrical supply source is stiff. As we use the term, stiffness reflects the electrical supply's source impedance. The stiffer it is, the less impact changing loads will have on the voltage. This is true both at the power frequency (60 hertz) and at the harmonic frequencies. Like most PQ conditions, there isn't an exact number that makes it good or bad. Exact numbers are not what is needed for most power quality problems. It would involve a lot of complex mathematical equations to get an exact number, and too many unknown values are in the equations. But we know, when it is bad, problems get amplified and are generally more numerous. Rules of thumb based on those same two laws that we keep bringing up-Ohm's and Kirchhoff's Laws-along with a variation on a theme, the power superposition theorem, are all that's needed here.