The upstream sags and interruptions caused by faults on the electric utility system are often blamed for being the root of all evil, yet studies have shown that most power quality (PQ) problems originate within the facility itself, as indicated from the following sources:
• “Customer power quality problems are far more likely to originate inside the customer facility than on the distribution system.”
—A Guide to Monitoring Distribution Power Quality EPRI Search Project RP3098-1 EPRI Interim Phase 1 Report, August 1993
• “Wiring and ground errors and disruptive loads within a facility cause up to 80 percent of all power quality problems reported by customers”.
—EPRI CU.2026.3.90 Wiring and Grounding Quality, via PQA 95 Conference, Investigating PQ Problems, Allen Morinec. CEA study at the PCC
• “The average number of voltage sags monitored on the secondary side of the industrial customers facilities were significantly higher that those monitored on the primary side [double], implying that the majority of the voltage sags originated on the industrial secondary system.”
—CEA Power Quality Survey, “Canadian National Power Quality Survey of Industrial and Commercial Voltage Sags,” Don Koval a, IEEE Transactions on Industry Applications, May/June 1997
This further emphasizes the importance of having a PQ monitor at the service entrance. Using the rules of directivity discussed in previous articles, the origin of the sag can usually be determined by monitoring the voltage and current simultaneously. Remember if the current increases significantly when the voltage sags or decreases, then the source of the disturbance is most likely downstream from the monitoring point. Figures 1 and 2 show sags originating from downstream and upstream sources, respectively.
Faults within a facility occur less frequently than on the distribution system, as the exposure to the weather elements, trees, and critters is much lower. Sudden changes in load current within facilities resulting in sags are common occurrences. Finding the source of those is easier if both the voltage and current signatures during the sag are analyzed.
The start-up of a large-hp electric motor will result in significant in-rush current flowing for up to several seconds. This inrush current is typically six to 10 times higher in magnitude than the steady-state current level. The sag of the voltage is shown in Figure 3a, and the increase in current is shown in Figure 3b. the envelope of the motor start current has an exponentially decaying curve, as overlaid in Figure 3b.
Going back to Ohm’s and Kirchoff’s Laws, the increase in current will result in six to 10 times the voltage drop in the wiring. If we assume 0.5-ohm source impedance and a 10A nominal on a 480V system, the in-rush current can result in a drop of 30 to 50V, or a sag from nominal 475V at the load down to 430V. This is because the motor’s impedance looks somewhat like a short circuit when the rotor is stationary.
Once the rotor starts turning, the current reduces and eventually goes to a much lower, steady-state value. However, if during the operation, a load change on the motor causes it to come close to stalling or the locked rotor condition, then another sag can result for similar reasons.
Other types of loads have other distinct signatures in the voltage and current waveforms, when they turn on/off and/or when running steady state. Look for changes in harmonic currents, typical of rectified input, switching power supplies found in so-called electronic loads, such as laser printers, adjustable speed drivers, fax machines, etc. Based on the harmonic spectrum, the type of offending load can be further narrowed down.
If the load has a split personality, looking like an electronic load most of the time, and occasionally a highly resistive load, look for a load such as a laser printer or copy machine that periodically turns on a heating element within it. See how frequently the sag occurs and if the period between sags is a constant value. Also, see if the current waveform becomes highly distorted for short, periodic bursts, such as those with an electric arc welder.
There are plenty of clues in the signatures of the voltage and waveforms to help you find out “who done it.” If there aren’t permanently installed PQ monitors in the facility, it is a good idea to periodically monitor the different distribution circuits to see what the loads look like when things are running well. Since sags occur frequently, a key consideration is “how deep of a sag will result in misoperation or interruption of the process or equipment within the facility.”
This will make it easier to distinguish what went wrong when the plant tripped off line and the production manager is running around screaming at everyone to get the lines back up.
BINGHAM, manager of products and technology for Dranetz-BMI in Edison, N.J., can be reached at (732) 287-3680.