You may already be using PM techniques on your motors and drives, but what about the power to your equipment?
When it comes to predictive maintenance (PdM) on electrical equipment, the numbers don't lie. According to insurance claims data in NFPA 70B-2006, “Recommended Practice for Electrical Equipment Maintenance,” roughly half of the cost associated with electrical failures could be prevented by regular maintenance. Another study published in IEEE 493-1997, “Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems” (The Gold Book), cites a similar scenario, stating that a poorly maintained system can attribute 49% of its failures to lack of maintenance.
To determine the actual cost of a failure, it helps to consider three key categories:
Lost income (gross margin) due to downtime;
Cost of labor to troubleshoot, patch, clean up, repair and restart; and
Cost of damaged equipment and materials, including repairs, replacements, and scrapped material.
Your PdM inspection route probably already includes any motors, generators, pumps, A/C units, fans, gearboxes, or chillers onsite. By adding basic power quality measurements to production equipment maintenance procedures, you can head off unexpected failures in both production equipment and your power system. Unlike a comprehensive electrical system survey, power quality predictive maintenance (PQ PdM) focuses on a small set of measurements (click here to see Table) that can predict power distribution or critical load failures. By checking the power quality at critical loads, you see the effect of the electrical system up to the load.
Voltage stability, harmonic distortion, and unbalance are good indicators of load and distribution system health and can be taken and recorded quickly with little incremental labor. Current measurements can identify changes in the way the load is drawing. All of these measurements can be taken without halting operations and generate numbers that can easily be entered into maintenance software and plotted over time. For each measurement point or piece of equipment, determine what limit should trigger corrective action. Limits should be set well below the point of failure. As time goes on, limits may be “tightened” or “loosened” by analyzing historical data. The appropriate limits depend somewhat on the ability of your loads to deal with power variation. But for most equipment, your maintenance team can devise a set of default “house limits” based on industry standards and experience.
The cost of 3-phase power analyzers is lower now than ever, and it should only take roughly 15 minutes to take the readings discussed in this article. (Storing voltage sag data will add more time because it requires picking up the data after a day or so.)
Good voltage level and stability are fundamental requirements for reliable equipment operation. Running loads at overly high or low voltages causes reliability problems and failures. Here are a few rules of thumb to remember.
Verify that line voltage is within 10% of the nameplate rating (Fig. 1). As connections in your system deteriorate, the rising impedance will cause “insulation resistance drops” in voltage.
Added loads, especially those with high inrush, will also cause voltage decline over time. The loads farthest from the service entrance or transformer will show the lowest voltage.
Neutral-to-ground (N-G) voltage tells you how heavily your system is loaded and helps you track harmonic current. N-G voltage higher than 3% should trigger further investigation.
Voltage sag count
Taking a single voltage reading tells only part of the story. How is the voltage changing during an hour? What about during a day? Sags, swells, and transients are short-term variations in voltage. The voltage sag (or dip) is the most common and troublesome variety. Sags indicate that a system is having trouble responding to load requirements, and significant sags can interrupt production.
Voltage sags can cause spurious resets on electronic equipment, such as computers or controllers, and a sag on one phase can cause the other two to overcompensate, potentially tripping the circuit.
Sags have several dimensions: depth, duration, and time of day. Utilities use a special index to track the number of sags that occur over a period of time. To gauge the depth of the sags, utilities count how often voltage drops below various thresholds. The longer and larger the voltage variations, the more likely equipment is to malfunction. For example, the Information Technology Industry Council (ITIC) curve specifies that 120V computer equipment should be able to run as long as voltage does not drop below 96V for more than 10 seconds or below 84V for more than 0.5 seconds.
Current measurements that trend upward are a key indicator of a problem or degradation in your load. While equipment is running, monitor phase, neutral, and ground current over time. Make sure none of the currents are increasing significantly. Also, verify that they're less than the nameplate rating. Finally, keep an eye out for high neutral current, which can indicate harmonics and unbalance.
In a 3-phase system, significant differences in phase voltage indicate a problem with the system or a defect in a load.
High voltage unbalance causes 3-phase loads to draw excessive current and causes motors to deliver lower torque.
Unbalance is tracked in percentages (Fig. 2). The negative sequence voltage (VNEG) and zero sequence voltage (VZERO) together identify any voltage asymmetry between phases.
Using a power quality analyzer to do the math, high percentages indicate high unbalance.
The Leonardo Power Quality Initiative standard EN 50160, “Voltage Characteristics in Public Distribution Systems,” requires VNEG to be less than 2%.
Voltage harmonic distortion
Harmonic distortion is a normal consequence of a power system supplying electronic loads, such as computers, business machines, electronic lighting ballasts, and control systems. Adding or removing loads from the system changes the amount of distortion, so it's a good idea to regularly check harmonics.
Harmonics cause heating and reduced life in motor windings and transformers, excessive neutral current, increased susceptibility to voltage sags, and reduced transformer efficiency. As current harmonics interact with impedance, they're converted into voltage harmonics. Total harmonic distortion (THD) is a sum of the contributions of all harmonics. By tracking voltage THD over time, you can determine if distortion is changing (Fig. 3).
For voltage harmonics, IEEE 519, “IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems,” recommends less than 5% THD.
Scheduled electrical preventive maintenance is an important part of total operating costs; however, the added reliability of equipment operation is worth the incremental cost. Adding PQ PdM to traditional maintenance procedures will enhance equipment operation.
Thompson is a power quality specialist with Fluke Corp. in Everett, Wash.