Most of the articles in this column have focused on how to find what type of power quality demons are lurking in your power system. However, the other aspect of PQ is how to get rid of them. Otherwise, it is sort of like a doctor who only diagnoses your illness, but can't offer any treatment. Some of the treatments for PQ illness don't require any additional equipment or devices: tightening ground bolts, removing illegal neutral-to-ground bonds and relocating PQ generating loads to their own circuits or at least circuits that don't have the susceptible loads on them.
But when “two aspirins and drink plenty of fluids” don't work, there are still a plethora of solutions or mitigating devices on the market for these types of power quality related problems. Choosing which one is the best for a particular facility and its problems is similar to a doctor prescribing the right medicine for a certain illness. Sometimes the medicine can have adverse effects and actually can make the situation worse. This is not only a waste of money but can do far more damage than having done nothing. This is true in power quality solutions, also. For example, there have been instances where harmonic filters have been placed in circuits that result in resonance conditions where the distortion levels are much higher after putting in the filters.
One of the keys is to match the solution to the problem, which lends itself to knowing what the problem is before just trying out solutions. You wouldn't want to have a doctor tell you to try out different medicines until you happen on the right one for your illness. But unfortunately, some people try that approach with PQ illness.
One of the most common solutions that people use is TVSS or transient- voltage-surge-suppressors. These can range from the multi-outlet strip to the large service-entrance modules. Their purpose is to provide a low impedance path to shunt to ground of the high frequency transient voltages that can be very damaging to semiconductor and other devices. If the energy is large enough, a fuse will blow and isolate the equipment from the fault. Using surge suppressors that are not approved by a regulatory agency (such as UL) can create a potential fire hazard if they fail to do large surge voltages, such as from nearby lightning strikes. The joule or energy rating of the box needs to match the application. Some of them don't even have a fuse or circuit breaker inside to trip when it becomes excessive and the MOV fails in a short circuit, where the current went to more than 35 Arms.
If your problem with voltage is sags from the start of large motors, which causes other equipment to trip offline on low voltage, why install surge suppressors? According to most published PQ surveys, the overwhelming majority of the power quality problems are sags lasting less than one second. What will a surge suppressor do for resolving this condition, when the voltage is reduced for a short duration? The answer is absolutely nothing, besides make the vendor of such devices wealthier.
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 (UPS), to high-speed static transfer switches (STS) that can switch between two different power sources within a quarter of a cycle. Having a battery-backed UPS without a proper maintenance program in place for the batteries is an ineffective solution.
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 PLC that controls some process equipment on the factor floor may a waste of money. It may turn out that the most vulnerable part of the system is the electric photo-cell safety switch on the machine, which trips the process off on just a two-cycle sag to 90 percent of nominal voltage, whereas the PLC can tolerate a much deeper sag for much longer to 47 percent for 37 cycles.
There is a story circulating the industry of a large semiconductor fab plant, where the facility manager decided to have a UPS unit placed on each piece of equipment in each production line, ending up with nearly a thousand UPS units in the plant. Given the MTBF (mean time between failure) of 1,000 UPS units, they ended up having a problem on the line every couple of months from a UPS problem, instead of once or twice per year from the electric utility supply.
Remember, what it is really important is that the characteristics of the power required by the load equipment is compatible with the characteristics of the power being supplied, from the utility or through a mitigation device. Know your equipment's susceptibility, monitor your electrical distribution system within your facility and find the solution that works for you. And don't forget, as soon as you add more load or make some other change to the electrical infrastructure, its time to do it all over again. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.