Looking into the origin of disturbances
From the load’s point of view, it is not just the generator that is considered the source, it is also the transformers and the wiring between the generator and the load, as well as other loads connected upstream from the point of monitoring. All of this has the potential to modify the pure sine wave, either because of changes in the load or by events that impact the equipment.
The first event in last month’s article was a sag, termed such because the rms of the voltage went below 90 percent of the nominal for longer than half of a cycle (of the 50 or 60Hz sine wave). In this case, it only occurred on a single phase, which indicates that the source was either the energization of a large single-phase load or a single-line-to-ground (SLTG) fault somewhere on the circuits between the generator and the load.
Sags are generally the most common type of power quality phenomena and SLTGs are the most common types of sags originating on the distribution system. But, since most studies show that most problems occur within a facility, one shouldn’t just assume that it is on the utility side of the meter.
In this case, the waveshape of the rms voltage indicated that it was a large motor starting up. Electric motors have a large inrush current that results in a voltage drop across all of the source impedances. This results in less voltage left for the loads or a sag. As the motor starts running, the current gradually decreases to the steady state or running current level required by the motor to carry its load.
If it had been a SLTG on the distribution system, the voltage would have either gone back to nominal when the fault was cleared—either naturally or by the breaker operation—or, if the fault was on the feeder powering the load, an interruption would have resulted when the protection breaker opened.
Instead, the gradual exponential return of the voltage to a steady state level that is somewhat below the original voltage level before the sag (since there is still a smaller drop in the source impedance with the steady state current flowing) is the key telling the story of the source of the problem.
Event number two, classified as a rapid-voltage change, had another characteristic that helped determine the source. The voltage sag occurred near the peak of the sine wave and then went away at the zero crossing of the waveform, such as shown in another similar event in Figure 1.
This indicates that it was most likely caused by a breakdown in an insulator of some kind. It reoccurred on several subsequent half cycles every so often. In this case, it was contact of a distribution wire with a tree branch during windy conditions. At the lower voltage levels on the sine wave, the insulation value of the wire and the tree was high enough to prevent an arc and the subsequent current flow.
Near the peaks, when the voltage potential is at the highest, it was high enough to arc over. A SLTG resulted in current flow, which results in voltage drop in the source impedance, which results in less voltage at the load, characterized as a rapid-voltage change—end of story.
The last event was one where the voltage changes weren’t large enough to even be classified as a rapid-voltage change. However, the change was at a rate or modulation frequency and amplitude sufficient to be perceived in flickering lights.
This is the usually the less disruptive of the three disturbances for equipment and processes, but often the most annoying. Unfortunately, there wasn’t adequate information (such as the current waveforms and description about loads in the vicinity), to determine the story behind this blinking of the lights. Often, high-energy and sporadic loads (such as arc furnaces or welders) are the source.
As the saying goes, “every picture tells a story,” and every waveform graph, rms voltage and current plot can help tell the story behind the power quality event, where the lights blinked or not. The more information that you can find out about the situation, the clearer the story becomes. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.