Anyone who has heard a refrigeration unit groan through a brownout or been frustrated by an inexplicable series of control system trips should have a healthy respect for the effects of power quality on equipment reliability and energy efficiency. Few doubt that sags and harmonics contribute to downtime, off-spec production and shortened equipment life.
But, like so many potential projects that improve uptime, product quality and life cycle cost, it’s hard to make a financial case for spending on power conditioning. These days, it’s much easier to gain support and financial backing for the rapid paybacks you can calculate from improving energy efficiency.
“Power-quality solutions have historically been sold as insurance policies to protect against the next damaging sag, surge or interruption,” says Daniel Carnovale, P.E., power quality solutions manager, Eaton (www.eaton.com). “Those benefits are hard to justify. By contrast, energy savings offers easy justification.”
The problem is that some of the solution providers significantly overstate the savings and customers are deceived. Carnovale says, “While these solutions provide excellent protection, they often provide very little, if any, energy savings.”
Common sense as well as the Electric Power Research Institute (EPRI) point out that you can only save energy that is wasted. The losses in a typical plant power system range from 1% to 4%, so eliminating them would, at best, reduce your bill by 1% to 4%.
But these days, even a couple percent of energy savings can add up to a worthy return on investment, and paying attention to power quality in the form of power factor and peak demand can reduce some industrial power bills as much as 30%.
Sources of savings
Sags, surges, interruptions and harmonics wreak havoc on equipment. It’s easy to believe they contribute to inefficiencies, and they do. But how much?
You might find low-hanging fruit in your existing power quality or uninterruptible power supply (UPS) equipment. Strides have been made in the efficiency of the rectifier stages that condition incoming power and connect capacitors or batteries to support sags and outages. Just a few years ago, “Typical losses were 10% to 15%, and worse at lighter loads,” says Ed Spears, product marketing manager, critical power solutions, Eaton. “Now we are in the range of 92% to 98%, even 99% efficient. This from better semiconductor technology but also topology. We can operate in modes where we can allow more straight utility power to come through.”
From there, savings directly from reducing inefficiencies drop off rapidly. For example, in motors, “A 3% voltage imbalance can lead to a 25% to 30% current imbalance, and increase motor losses by 10% to 20%. It also raises motor operating temperatures,” says Mike Melfi, manager, advanced variable-speed motor technologies, Baldor (www.baldor.com). But the increase in loss might not add up to much. “Altogether, a 95% efficient motor might be reduced to 94%,” Melfi says.
Figure 1. Harmonics are multiples of the fundamental frequency (60 Hz in North America), and are produced by nonlinear loads.
Harmonics are multiples of the fundamental frequency (60 Hz in North America), and are produced by nonlinear loads (Figure 1). “Unwanted harmonics equal heat, which equals loss,” says John Perry, product marketing manager, power factor correction products, Eaton. “Wiring sizes, conduits and other components can be minimized by minimizing harmonics.”
Ironically, some energy-saving technologies such as electronic lighting ballasts and variable-speed drives (VSD) can add harmonics, as can computer power supplies. These cause inefficiencies and shorten the lives of motors, transformers and other connected equipment. Even when the phase loads are balanced, “The third harmonics can add up in the neutral conductor,” says Rich Vesel, product manager, electrical balance-of-plant equipment, ABB. (www.ABB.com). “At 60 Hz, the neutral currents of a balanced three-phase system cancel, but the third harmonics don’t cancel — they add. An undersized neutral can run hot and might even become a fire hazard. Balance the phases, oversize the neutral line, and use harmonic filtering to minimize the effects.”
Furthermore, some of the ways plants deal with harmonics add to energy inefficiencies. “People used to throw in K-rated transformers all over the place because harmonics were eating up their standard transformers, but they would lose 2% to 4% efficiency,” says Carnovale. “Newer versions are much more efficient, and they handle the harmonics — they can absorb them like a capacitor set. You can use them in pairs to cancel out harmonics.”
The upshot is, while cleaning up power might pay off in higher system efficiency, that’s not where you’ll find 25% savings. “Companies want to install a black box and save a lot of energy, but unless they have a power factor situation and the utility is imposing a penalty, they’re not going to save 20% to 30%,” Carnovale says. “If they replace an inefficient UPS, they might pick up 10%. For harmonics, it’s going to be more like 2% to 4%.”
Understand power factor
Finding the proverbial 20% to 30% reduction in electricity bills doesn’t require an electrical engineering degree, though you might need to consult with a few to obtain your goal. So it’s worth having at least a rudimentary understanding of active power (also called true power), reactive power, apparent power and power factor.