In a recent discussion about the virtues of power quality (PQ) monitoring with some less-than-knowledgeable business people, the point was raised that, “Why bother to figure out whose fault it is if you can’t fix it?” I responded to this comment by showing them some of the latest software tools available that can determine the likely source of a PQ disturbance and the direction from which it originated. (Is it upstream or downstream from where the monitor is located?)
For example, the chart below shows a series of power factor (PF) capacitor switching transients that were recorded at the service entrance of an office complex, with the indication that it was an upstream event, originating back towards the generator or source. In this case, it is the utility-owned PF cap banks that kick on every morning at the same time to improve the voltage as inductive loads come to life in the facilities in the industrial park. Such transients can have the potential to cause adjustable speed drives to trip off line by “confusing” the control logic by making it “think” that an overcurrent situation exists when it really doesn’t.
Whereas it may not be cost-effective for the facilities to mitigate this common transient, they have a much better chance of getting action from the local electric utility distribution company when armed with the facts. Documenting the severity of the transient and the effect on the facility is the first step.
An uncharged capacitor is like a balloon connected to an air supply line. The air will flow into the path of least resistance, which would be the balloon, instead of trying to turn the air-powered tool. Once it begins to fill up, the pressure required to fill the balloon becomes greater than the force required to turn the tool, so the air power then goes back to powering the tool. The same situation applies when energizing a capacitor, relative to the electric current. (See article in September 2001 issue for more details on PF cap switching transients.)
Presenting the data from your site to the utility’s customer account representative is the first step in getting the situation remedied. The odds are that your facility isn’t the only one being affected. There are PF cap banks with “smart switches” in them that will turn on at the zero crossings. At the zero crossings, there is no voltage present; hence, there is no energy to cause the negative transient, and the cap just charges in a more orderly fashion as the sine wave increases over the next quarter cycle.
Another solution is for the cap banks to be energized when most facilities aren’t running, as is the case in the examples shown below. The transient still exists, but no one is negatively affected by it. Fixing this problem at the source can have a positive effect for many of the facilities downstream. But if no one bothers, nothing will change. EC
BINGHAM, manager of products and technology for Dranetz-BMI in Edison, N.J., can be reached at (732) 287-3680.