Last month, I reduced the vast array of phenomena that create power quality problems to the four most common categories and a set of causes-and-effects within those categories. Now, I solve them. An important step to remember comes from the Hippocratic Oath in the medicine world: don’t make the problem worse. Beating down one problem shouldn’t make a different one pop up.
Harmonics and interharmonics: Since it is relatively uncommon for a properly functioning generation and electrical distribution system to create harmonics, that leaves loads as the likely source. With a little shopping around, it isn’t too difficult to find replacements for high-harmonic-current loads in your facility. Rectified input, switch mode power supplies used in most electronic equipment, such as IT equipment and adjustable speed drives, now can use higher switching frequencies with internal zigzag transformers, which have transformer(s) with interconnected star winding connections that cancel out the common triplen harmonics. Twelve and 24-pole or pulse converters generally have much lower harmonic currents.
If you can’t find or afford the new equipment, a zigzag transformer can be used on the circuit with the harmonic-generating equipment to reduce the spread of harmonic currents. If that is not an option, the operating point of the equipment can be derated to allow it to survive the heating effects of the harmonic currents. Insulated motor bearings can prevent the fluting effect from harmonic currents. Properly designed and installed harmonic filters can snub out the harmonic currents; remember the “properly” part so as to not cause other problems.
Voltage transients: Since transients can be very fast and full of energy, the solution must be able to react quickly and absorb or divert the energy, repeatedly. The most common solution for transients is probably the most common mitigation equipment deployed, surge protection devices (SPDs), formerly called transient voltage surge suppressors, also known as “power strips.” Just like with harmonic filters, these need to be properly selected and installed.
The amount of energy that can be absorbed is listed under the joule rating, which—along with the surge rating and clearing time—are key parameters. A protection device, such as a fuse, should be part of it because when the suppression components finally fail, they can create a short circuit that can do even more damage if not protected against. Be sure that the SPD is listed by a national recognized testing laboratory, such as meeting the requirements of UL1449 Third Edition. The wiring, particular of the equipment grounding conductor, should be adequately sized and a low-enough impedance path to the grounding conductor.
Voltage rms variation: The first step is to determine if most of the problems originate upstream from the facility or within it. If the former, contact the local utility to see what can be done, especially in ongoing situations. If the latter, find the offending equipment, often a large load or loads, and either electrically isolate them from critical equipment or sequence their start-ups.
Since most voltage rms variations are reductions in voltage, the next step is to supply the missing voltage adequately to keep the load properly powered. A common solution is the uninterruptible power supply (UPS). Properly selecting the size (usually in kVA) and installing are keys for success. Part of the installation is knowing which loads are critical to keep operating, and what circuit that they are on. Also, know what the noncritical loads are on—really on—not just showing up an outdated schematic. Facility personnel make assumptions and end up wasting the power capacity of the UPS on unintended loads while not powering critical loads.
Critical loads aren’t just IT equipment or production-line equipment. A fire alarm system can shut down a process facility if it falsely trips, as can the contactor on a motor feeding a waste treatment pump, while the vending machines in the cafeteria can afford to be down for a minute (or an hour). Note that SPDs can’t prevent sags, though many a person has tried.
Unbalance: Since unbalance basically means unequal, the solution is to make it equal. Most often, phase currents get unbalanced as loads are changed or moved from one circuit to another. Periodically (or continually) measuring the phase currents during various times of day and the year is the first step, followed by finding which equipment are the heavy hitters. If an individual three-phase load has unbalanced current, contact the manufacturer to see if that is normal. A partial failure or impedance increase within the equipment may be the source. Check that all of the connections in the panels and equipment are clean and properly torqued. Unequal impedance in the wiring works in the math just like unequal currents in the phases to create voltage unbalance.
There are plenty of magic-mitigating claims on the market, but take time to think the solution through after understanding the problem before spending time and money, lest you violate the PQ Hippocratic Oath.
About The Author
BINGHAM, a contributing editor for power quality, can be reached at 908.499.5321.