Power factor will be low due to both distortion and phase displacement unless corrective measures are applied. Power factor correction must be carefully designed and implemented to prevent dangerous resonances at the high voltages employed. The harmonics produced by nonlinear loads may interfere with or damage other equipment on the same line. High voltage harmonics may even radiate from the power lines sufficiently to interfere with independent electronics or violate FCC regulations. The unsteady current drawn by arc furnaces contributes to sags and swells.
Metals is an energy intensive industry even after the initial refining. Factories will have rollers, presses, stampers, machine tools, and other motor driven machinery. Variable speed drives are nonlinear loads that draw harmonic currents. These currents will increase heat loss in motors and transformers. Motors have a low power factor, and are impacted by sags, swells, phase imbalance, and other power quality events. Power quality can cause drive failure or overload relay tripping, causing costly unplanned downtime. Mechanical overloads may be assumed when overload relays trip, and valuable labor hours lost troubleshooting them. Electronics such as PLCs that run alongside the lines are also susceptible to damage from poor power quality.
Welders, plasma cutters, and electric discharge machining are widely used in the metals industry. Arcing loads can create radio frequency interference. Arcs are also nonlinear loads. Welders have a low and variable power factor, and a low demand factor, both of which contribute to a large total cost per kilowatt hour consumed. Welders’ intermittent high load can cause sags, and their inductive kickback can cause swells. Newer inverter welders reduce some of these concerns, but are power electronics and thus susceptible to damage from poor power quality much like VFDs. Metals plants are likely to employ electroplating, anodizing, and other lower power electrolytic processes. The DC for these baths is commonly produced from the same 480V distribution network used to operate other machinery. The harmonics drawn by these baths will affect all other devices on the bus, and will increase heat losses in the step down transformer. Phosphating and powder coating are other protective processes that do not require DC, but do require heat. Electric heaters are a clean load, but nonetheless costly to operate due to their high energy consumption. Heaters operated intermittently will have a high demand charge. Some temperature controllers will produce harmonics or interharmonics.
Metals plants, particularly those producing specialty alloys, are also likely to use computer numerical control (CNC) machining. CNC machines are susceptible to damage and errors from voltage fluctuations, otherwise known as sags and swells. The control electronics are vulnerable to erratic operation and premature failure if the voltage waveform is distorted due to nonlinear (harmonic) loads. Machining errors will cause costly scrap and rework losses if power quality is not corrected.
A comprehensive assessment and thorough power quality monitoring are the first steps to correcting power quality. Denison’s professional services are ready to find root causes and recommend solutions.
