With businesses under constant pressure to increase profitability by increasing productivity, maximizing assets utilization and doing more with less (fewer people, fewer materials, less time), they continue to rely more heavily on information technology (IT) equipment, such as personal computers (PCs), networks and electronic communication equipment. To better control the processes and reduce electrical energy consumption, programmable logic controllers (PLCs) and adjustable speed drives (ASDs) are commonplace in today’s commercial and industrial environments. Such equipment “can be both a contributor to and a victim of powering and grounding incompatibilities in the power system,” as the Emerald Book (IEEE Std 1100) states.
Having more problems that must be solved in a shorter time period with fewer people has forced many companies to outsource some or all of their electrical equipment preventative and emergency maintenance work to electrical contractors. At the same time, electric utilities have gone through waves of restructuring, with the staffing of their power quality departments being reduced. This translates into numerous opportunities for electrical contractors with good fundamentals in power quality.
Power quality phenomena (disturbances) usually are defined in terms of the effect on the supply voltage, which in turn can affect the loads powered. The disturbances can be broken down into three basic categories: rms voltage variations (sags and swells), voltage transients and voltage waveform distortion. The voltage usually is what the standards mention; however, the current can provide important clues as well, as Ohm’s and Kirchoff’s Laws relate.
Sags are short-duration reductions in the nominal voltage, usually defined as below 90 percent down to 10 percent. Faults on the electrical grid, such as a downed wire or an arcing wire in a tree, create large currents that result in voltage drops in the distribution and/or transmission wires until the fault protection (breakers or fuses) open to clear the fault. Hence, many sags originating on the electrical utility distribution system are readily identifiable, lasting four to 10 cycles long with the voltage dropping down and recovering in a step fashion, and no real dramatic change in current at the monitoring point within a facility.
In contrast, sags originating within a facility often are caused by load changes, electrical system faults or improper wiring. The signature of the sag often is characteristic of the type of load. For example, a motor start usually has a large inrush current (six to 10 times nominal), followed by a gradual decay to the nominal value. The resulting sag is the opposite of that, as shown in Figure 2, where the voltage decreases abruptly, then slowly recovers towards nominal as the current decreases.
Interruptions usually are the result of being downstream from the fault protection in either a distribution substation, transformer on a pole or the breakers within a facility. When the breaker or fuse opens to stop the overcurrent condition, everyone downstream without a uninterruptible power supply (UPS) will see an interruption, and everyone upstream will return to normal. Interruptions also can be caused by loose connections of wires or bus bars.
Since sags tend to be unpredictable and last only for milliseconds, a DVM usually is not of much value for determining when it happened and the source. There are a range of voltage loggers and power quality monitors on the market that a contractor can either purchase or rent. It doesn’t take too long for the return on investment to turn positive. While voltage loggers give an indication of when it happened and possibly the severity of the sag, monitors—which provide the rms timeplot, the waveform of the voltage and current—are more valuable for determining the source.
The magnitude and duration are key parameters, along with whether the current increases, decreases or stays about the same when the voltage sags. If the current increases significantly when the voltage sags, then the source of the problem probably is downstream from where the monitor is located. If the current decreases to near zero when the sag occurs, then the circuit loads probably are rectified input switching power supplies, since the storage capacitor will be at a higher charge level than the incoming voltage. However, if the sag lasts longer the capacitor charge can supply the load, then the voltage output of the power supply may drop too low for proper regulation, and the equipment will malfunction.
Since sags are about missing energy, mitigation devices, such as transient-voltage suppressors, are of little value. The mitigation equipment must provide enough energy to make up for what is missing, relative to the load’s requirements. Most often, a UPS is used. These come in sizes for hundreds of volt-amperes (VA) to millions of VA. Many of these use batteries as the storage source of missing energy, which need a preventative maintenance program, as well as a planned replacement schedule, which is a service that can be another opportunity.
Harmonics often cause the second most common power quality disturbance. In today’s electrical power systems, voltages and currents generally are not pure sine waves consisting of only a fundamental frequency component (60 Hz in North America, 50 Hz in Europe). Harmonics are frequencies that are integer multiples of the fundamental frequency (120 Hz, 180 Hz, 240 Hz, etc.). Many of the power supplies in equipment manufactured today—such as lighting ballasts, adjustable speed drives, PCs, laser printers and programmable logic controllers—draw current during only part of the sine wave, resulting in harmonic currents (Figure 3), which lead to voltage harmonic distortion. The distorted voltage waveform can then affect other equipment powered from the same source.
Harmonics can cause overheating in electromagnetic loads, such as motors and transformers, due to losses that increase proportionally to the square of the harmonic number. In four-wire wye circuits, the triplen harmonics (3, 6, 9, 12, etc.) are additive in the neutral conductor, rather than canceling each other out, which also can cause overheating. Some combinations of harmonics can cause the top of the voltage waveform to be reduced, also called flat topping. This means there is less energy to charge those capacitors mentioned before, making them more vulnerable to sags.
The harmonic spectrum also helps determine what is the source as well as the possible solutions. For example, having the largest current harmonics being the 3rd, followed by 5th, then 7th, probably means the source are single-phase loads with rectified switching power supplies, such as printers and PCs. If it was the 5th, 7th, followed by 11th and 13th, then the sources are three-phase, full-wave rectified loads such as ASDs. The harmonic current trail will lead back to the source, and then the best options for mitigation can be determined. Sometimes the best option may be separating the offending loads to separate circuits, sizing and installing harmonic filters properly or changing the loads to less polluting equipment.
These are just two of the many types of power quality phenomena that equipment can be susceptible to. You can offer your customers a preventative maintenance power quality program, where you look for changes and trends that can be corrected before they result in a process interruption and financial losses for the customer. Or, you can be the PQ detective, providing a troubleshooting service after customers experience a problem. This often is much more difficult to do, since the source of the problem may not occur again for days, weeks or maybe years. As the cost of downtime goes up, the potential financial reward for electrical contractors goes up right along with it. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.