KANSAS CITY, MISSOURI, US — In the article, “Energy cost management — looking at your energy bill” (World Grain November 2023, pages 50-57), many hidden charges in the electrical bill were identified that impact milling energy costs. In “Energy cost management — power factor” (World Grain March 2024, pages 58-62), the concept of power factor and its impact on energy cost was presented. This final installment of the energy cost management series will examine issues of power quality. The references identified at the end of this article are encouraged for further development and understanding of power quality. 

Power quality refers to how well the voltage, frequency and waveform fits expectations of a normal sinusoidal waveform. Divergence from expectation impacts both motors and sensitive electronic equipment used in milling operations. Unless identified and corrected, significant damage can result from poor power quality. 

Minor deviations within specifications often go unnoticed and have little long-term consequences on electric motors, for example. However, microprocessor-based equipment may be more sensitive to minor power quality issues. Disruptions including unexpected resets, electronic component failures, and random tripping of process control equipment may be caused by poor power quality.  

Power quality concerns and issues

Power quality is not a line item on the mill electrical bill, but poor power quality has hidden costs to the milling operation. Power quality issues and cost are not as easy to identify as power outages, for example, resulting in lost production affecting both fixed and variable manufacturing costs. The cost of poor-quality power or poor power quality events is hidden as the damage caused by overheating, which shortens equipment lifespan.

Power quality always has been a concern for electric motors, the workhorse of the milling industry. The growing presence of electronic sensors, instrumentation and controls has increased the complexity of electrical systems within the mill. The presence of highly sensitive electronic equipment in the mill is challenging and requires careful attention as these components are interconnected and their interaction must be managed for optimal efficiency and equipment life.  

Major power quality issues are well defined and graphically illustrated by Accuenergy and Helios Power Solutions (Hiliosps) websites. However, harmonic distortions are more complex in both definition and origin. All these issues are a cause for concern and require careful evaluation and consideration for optimal equipment performance and maximum equipment lifetime. 

The following is a summary of power quality issues.

• Voltage Sag/Dip: A sag or dip is a 10% to 90% decrease in voltage that lasts from a few milliseconds (half a cycle) up to 60 seconds. It can be caused by large motors starting up or shutting down, undersized circuits, short circuits or operations of surge suppressors in the power system. Microprocessor-based equipment (process controllers, computers, PLC, Variable Speed Drives) can malfunction due to voltage sags. Contactors or relays may trip, and machinery used in processing operations may lose efficiency or stop functioning. Dips can also cause momentary dimming of lights or Human Machine Interface (HMI) display issues.
• Voltage Swell: A voltage swell is the opposite of a sag/dip and is a short increase in the voltage above normal tolerances, typically at 110% or above. They can be caused by lightning, switching of lines or power factor correction capacitors, incorrectly sized power sources, startup or shutdown of large loads, or a fault on one phase of a three-phase system. Like sags, voltage swells can cause equipment shutdowns and failures. Variable frequency drives (VFDs) may stop functioning while lighting and computer displays may flicker. PCs, PLCs, and other microprocessor-based equipment may lose data or have data processing errors. 
• Transients: Also known as a spike and like a swell, a transient is a high-energy burst that lasts 1/60th of a second or less where the voltage is higher than expected. They range from a few volts to several thousand volts (some can be 6 kV or higher) and are caused by the sudden release of stored energy. Lightning or static discharge are common causes of transients, but they also can be the result of disconnecting heavy loads. Transients can be extremely damaging to any electronic equipment with a power supply, such as computers or instrumentation. Transients can destroy circuit boards and processors, resulting in memory loss and data corruption. We often use surge protection devices to protect computers from surges and transients induced by lightning  
• Harmonic Distortion: Harmonic distortion is a deviation from or deformity of a normal, sinusoidal waveform, or sine wave. Sometimes called “dirty” or “noisy” electricity, it can be introduced by any non-linear load within a facility, such as LED (Light Emitting Diode) lighting, DC (Direct Current) power supplies, VFDs (Variable Frequency Drives), welding machines, rectifiers, inverters, electronic starters, switching power supplies, discharge lamps. Unlike transients or sags, harmonics can remain at a steady state within the system. High harmonic distortion can cause multiple problems, including overheating of equipment like motors or wiring, reduced efficiency of machines, and nuisance tripping. Over time, harmonics can shorten the lifespan of equipment and reduce power capacity. Ultimately, the damage to equipment can be costly and cause unplanned shutdowns.
• Flicker Effect: The flicker effect is a flickering of lights induced by rapid variations in voltage. These voltage variations are caused by loads in which the power absorption varies very quickly: arc furnaces, welding machines, rolling machines, laser cutters. Flicker impacts workplace safety and productivity.

Credit: ©PG&E

Four-step solution 

The Power Quality Blog provides detailed background in the Industrial Power Quality Problems article by A. El Mofty K. Youssef, Alexandria Electricity Company, Alexandria, Egypt. They report four steps to solve power quality, including investigation, determination, analysis and prevention. Investigation of power quality issues begins with data logs of problem issues. In this instance, three separated logs of information are maintained and brought together for analysis and identification of problem areas. 

Their publication identifies standards applicable to power disturbances that may vary by location. Power quality delivered to individual components in your system is influenced by your distribution system, which has undergone constant changes. The results of these changes can influence system operation and equipment life.   

Pacific Gas and Electric Company (PG&E) in its referenced site shares a pdf file “Solving Power Problems for Your Equipment,” outlining a process for customers to answer the question: How can you tell if equipment failures are due to power-related problems? The fundamental step is maintaining a log of the problems encountered followed by visual inspection and taking measurements. The PG&E flowchart shows analyzing and solving power quality issues. A licensed electrician and power quality specialist is likely required to assist in identifying a power quality problem as well as implementing a solution. Following applicable electrical standards in your location such as the National Electric Code (NEC) in the United States is a critical requirement.

Jeff Gwirtz, a milling industry consultant, is president of JAG Services, Inc. He may be reached at jgwirtz@att.net.

Credit: ©SOSLAND PUBLISHING CO.