Continuing to look at power quality’s biggest enemies
As a continuation of last month’s article, here are a couple of additional “pictures” of the most popular PQ criminals for the office wall to help quickly identify what the squiggly lines mean and to find the source of problems.
When the voltage increases between 110 percent and 140 percent of the nominal value for 1/2 cycle to 3 minutes, then it is characterized as a swell. In the distant past, this was also called a surge or overvoltage. Swells usually occur less frequently than sags, but are often the result of the same phenomena (but in reverse). Remember that sags are usually caused by an increase in current that results in a larger voltage drop across the source impedance, leaving less voltage remaining for the loads. Many electric distribution systems have automatic tap changing transformers, which will boost the nominal voltage back up as daily loading of the system drops the voltage down. At the end of the business day, there is often a significant decrease in current on the distribution system, as factories and offices turn off loads between 4 and 5 p.m. This decrease in current results in a decrease in the voltage drop across the source impedance, as shown in Figure 1. Hence, more voltage is then available at the loads, and a swell may result until the automatic tap changer switches the tap to a lower voltage output. This usually occurs around 30 seconds after the increase in voltage is detected and the voltage level remains stable, so that the tap changer is not constantly switching back and forth to react to loads turn on and off rapidly during the daily operation.
Another less recognized cause of a swell results from when there is a single-line-to-ground fault (SLTG) on a high impedance grounded system. The neutral-to-ground voltage will increase significantly, and the phase-to-phase voltage between the faulted phase and either non-faulted phase will decrease significantly. Sometimes the fault progresses to involve the other phases, in which case they experience a sag. If the site is downstream from the distribution breaker, this will then progress to an interruption when the breaker operates to try to clear the fault. What name should be used to call a “swell-that-becomes-a-sag-that-becomes-an-interruption” has been the subject of countless debates in the standards-making committees, and remains unresolved to date.
Though most people don’t consider the voltage fluctuations that result in light flicker to be a serious PQ problem, there can be significant productivity and quality issues that result from such phenomena. Particularly vulnerable are processes that require a constant pressure or torque that can be affected by these rapid voltage changes. For example, processes such as fiber optic extruding, thin sheet film extruding and plastic pipe extruding can have the thickness of the product vary, whereas yarn weaving in the presence of such can result in different tensions causing color changes. For humans, the flicker phenomena can result in discomfort, nausea or even seizures for certain individuals. Perhaps some of what is called “sick building syndrome” is the result of the flicker perception as fluorescent lighting reacts with the 72 Hz scan rates of some computer displays, which causes a 12 Hz side band. This frequency falls near the maximum perception by humans (9.8 Hz), where it only requires a quarter of a percent modulation in the voltage for most people to perceive it. That’s just three-tenths of a Volt on a 120V system.
Flicker is measured with power quality monitors that can produce the parameters Pst and Plt, for perceptibility short term and perceptibility long term, respectively. If the Pst is 1 or greater, most people will perceive such. Typical targets for Plt are 0.8 or lower. Processes that draw large amounts of current in a rapidly changing manner, such as arc welders, arc furnaces, heat pumps, even heating elements with short heating cycles, can result in the light flicker being noticeable and even problematic.
Figure 2 is the Pst recorded about 10 miles away from an arc furnace that produced steel wire. In this case, the lighting in the room at the facility being monitored consisted of 25W candelabra light bulbs on a dimmer switch, making it about the most vulnerable combination there is. The smaller the wattage, the smaller the filament is in an incandescent bulb. The smaller the filament, the less “photonic inertia” that there is for the light output (measured in lumens) to remain constant during these voltage fluctuations. The problem is even more pronounced overseas where there is 240V lighting. There, the filament is one-fourth the size for the same lumen output. Since W = V x V / R, the same wattage bulb at 240V will have 4 times the resistance of one rated for 120V; hence, one-fourth the thickness. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.