Troubleshooting Industrial Power Quality Problems

Jul 1, 1998 12:00 PM, By Ken Michaels, F.D. Hayes Electric Co.

The right approach and practical test methods will ensure success time and again.

Most power quality consultants are successful because they get a fresh look at their customers' power systems. Well, many of us have the responsibility of troubleshooting our own power quality problems affecting industrial equipment. However, when troubleshooting power systems, don't overlook the obvious. Try not to dismiss things you are accustomed to seeing everyday. This can help you save time and money. Before soliciting the services of consultants and equipment manufacturers, follow these basic rules of thumb and practical test procedures:

EMI/RFI Sources. Look for sources of electromagnetic and radio frequency interference within industrial control cabinets. For example, it is not uncommon to see autotransformers and isolation transformers in the same control cabinet as a programmable logic controller (PLC). The electromagnetic compatibility between the transformers and PLCs may be compromised if the two components are close to each other. It may be practical to use a gaussmeter to measure the field strength between the two and determine if the tolerances of the PLC have been exceeded.

Electrostatic discharge. Don't discount electrostatic discharge (ESD) as a cause of equipment problems. This is especially true in areas where plastic extrusion or molding is part of a production process. ESD voltages in excess of 25V can degrade integrated circuits. Human contact is not a good indicator of an electrostatic discharge condition because electrostatic charges must exceed 4000V in order for the human body to have the sensation of a static shock. Therefore, you should use an electrostatic voltmeter to measure the electrostatic charge.

Harmonics. Make a comparative measurement between average-responding rms and true rms ammeters before authorizing a costly harmonic analysis. Both ammeters can be clamped around the same circuit conductor to make two important determinations. First, verify the ampacity rating of the circuit conductor. Then divide the average-responding rms value by the true rms value to get a ballpark value of the total harmonic distortion (THD). A ratio of 1 indicates there is little or no harmonic distortion. Any ratio less than .75 indicates moderate to substantial distortion and requires further analysis. Keep in mind this only indicates the level of harmonic current, not the actual THD.

Other measurements. Transverse-mode noise, measured between circuit conductors and common-mode noise, and between circuit conductors and ground, can adversely affect process control equipment. Therefore, you may have to use an oscilloscope with a line decoupler to make electrical noise measurements. You should make two-point bonding measurements (using an earth ground resistance tester) between different grounding references within the industrial facility. Make these measurements to locate missing or loose bonding connections along the grounding electrode system.

When attempting to capture AC voltage quality disturbances via power line monitoring, connect the monitor directly to the affected equipment. In most cases, plant maintenance personnel will connect a power line monitor at the electrical service entrance to capture utility-generated disturbances. This may be a misapplication of the power line monitor because the interaction of on-site equipment causes most voltage quality disturbances. For this reason, begin power line monitoring at the equipment location. Once the disturbance waveform is captured, you can backtrack from there to find the source. Monitoring at the equipment location can also give a better indication of the equipment's response to the disturbance. This will help in the selection of power conditioning equipment needed to mitigate the disturbance.

Finally, check all conductor terminations. Loose connections can show up as voltage quality problems and worsen the effects of electrical noise, electrostatic shock, harmonics, and other voltage quality events. Let these basic rules and practical test procedures help you troubleshoot more effectively.