Prescribing the Right Medicine

A plethora of solutions, often called “mitigating devices,” is being marketed for power quality-related problems. Choosing the best one for a particular facility and its problems is similar to prescribing the right medicine for an illness. It starts with the proper diagnosis, which has been written about in previous articles. Here we will focus on the medicine. Sometimes the medicine has adverse effects. The wrong medicine can be a waste of money and far more damaging than doing nothing. This is true for power quality solutions, also. For example, some harmonic filters placed in circuits have resulted in resonance conditions where the distortion levels are much higher. Match the solution to the problem. One of the most common solutions is transient voltage surge suppressors (TVSSs). These can range from Radio Shack or Comp-USA outlet strips to large service-entrance modules. Their purpose is to provide a low-impedance path to shunt to ground of the high-frequency transient voltages, which can be very damaging to semiconductors and other devices. Using surge suppressors that are not approved by a regulatory agency (such as UL) can create a fire hazard if they fail to handle large surge voltages, such as those from nearby lightning strikes. The joule or energy rating of the box needs to match the application. Here is another thing to consider with these solutions: Are they solving anything? According to a recent survey, over 90 percent of the power quality problems are sags lasting less than one second. What will a surge suppressor do to resolve this condition, when the voltage is reduced for a short duration? The answer is absolutely nothing, besides make the vendor of such devices wealthier. While there is a place for such devices, some vendors’claims can be overzealous. National Institute of Standards Technology (NIST), formerly the National Bureau of Standards, conducted a test showing that, if some of these claims were true, then incandescent light bulbs would be blowing up regularly from transients. How often have you seen a light bulb fail once it is already turned on and running? Voltage reductions, either sags, interruptions, or undervoltage conditions require a solution that makes up for the missing energy. These solutions can range from automatic tap changing transformers, to battery-powered uninterruptible power supplies (UPSs), to high-speed static transfer switches (STSs) that can switch between two different power sources within a quarter of a cycle. Of course, having a quarter-cycle STS is a waste of money, unless you can convince the electric utility company to provide two feeds from different substations. Otherwise, you will probably just be switching from one problem to another. Having a battery-backed UPS without a proper battery maintenance program in place is equally ineffective. Sometimes the expensive solution is just that: expensive. The screwdriver is a very effective tool in fixing many power quality problems, by tightening loose connections, or wiring up outlets and distribution panels correctly. Putting a UPS on the Programmable Logic Controller (PLC) that controls some process equipment on the factor floor may a waste of money. The most vulnerable part of the system may be the electric photocell safety switch on the machine. That trips the process off on just a two-cycle sag to 90 percent of nominal voltage, whereas the PLC can tolerate much deeper sag for much longer to 47 percent for 37 cycles, as the figure shows. What it is really important is that the characteristics of the power required by the load equipment are compatible with those of the power being supplied from the utility or through a mitigation device. BINGHAM, manager of products and technology for Dranetz-BMI in Edison, N.J., can be reached at (732) 287-3680.

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