Clean Power

Keep it free of surges, sags, and hidden harmonics.

When Mark Robinson talks about cleanliness, he means something entirely different than most of his fellow workers do at Minneapolis-based General Mills Inc. As the electrical and controls engineer at the food producer’s plant in Cedar Rapids, Iowa, his job is to worry about the cleanliness of the facility’s electrical power. When “dirty” power polluted with momentary sags and interruptions finds its way into the automation, it can turn the electronics off and shut the plant down without warning.

Although recovery for some processes is as simple as turning the machinery back on, it is much more complex for the production of some cereals. Recovery there entails both cleaning the equipment and leading the process through a start-up phase until it reaches steady state. Not only do these electrical failures waste material and cost the company in lost production, but they also can damage expensive electronics.

These sags, as well as surges, can come unexpectedly from a variety of sources, such as failing capacitors and damage caused by storms. “Most of these blips are of short duration because relays in the system usually detect and isolate faults within five to 10 cycles,” notes Robinson. “They are long enough to shut the entire line down, though.”

Such shutdowns tend to occur more frequently these days because electronics have become more sensitive to changes in power quality. “Voltage sags and brief interruptions are by far the most expensive power problem for most industrial automation users in the developed world,” reports Dr. Bill Brumsickle, vice president of engineering at SoftSwitching Technologies Corp., of Middleton, Wis.  “But damaging voltage deviations do not leave evidence behind them—often, the lights will not even blink.”

Even though people may not see any evidence of sags as they occur, the equipment can feel them acutely. The symptoms are seemingly random stoppages in production and premature failures in the electronics. These expenses have been escalating to the point where a growing number of facilities are recognizing power quality as a problem and are investing in technology that can “cleanse” their electricity.

At the General Mills plant in Cedar Rapids, the initial cleansing process included installing capacitors and replacing components that would go offline easily during a sag. With this level of protection, “we could handle a sag down to about 80 percent without any processes going down in the plant, as long as it didn’t last too long,” says Robinson.

Yet, there continued to be about eight sags a year that were significant enough to shut down portions of the plant and wreak havoc in cereal production. Because a large number of these sags occurred during storms, the initial plan for dealing with them was to do what the plant was already doing for its critical utilities. It called for installing an expensive set of diesel generators and switching all aspects of cereal production onto them whenever the radar in the boiler room detected an approaching storm.

Robinson put the plan on hold, however, when he discovered a cheaper and more effective technology, SoftSwitching’s DySC Dynamic Voltage Sag Corrector. During a short voltage sag in three-phase power, the device uses internal inverter and cross-coupler technology to borrow power from the unaffected phases to boost voltage in the affected one. It also can draw from capacitors whenever a sag affects all three phases, thereby freeing the user from dependence upon batteries.

Cheaper, yet effective

According to its manufacturer, the sag corrector is also efficient, consuming less than 1 percent of the energy in the application, which makes it 99 percent efficient. “To reduce susceptibility to common voltage sags, some facilities have ‘tapped up’ their internal distribution voltages by as much as 5 percent, which increases system losses by as much as 10 percent,” says Brumsickle. “In such cases, installing a DySC and returning to nominal voltage levels can reduce energy usage by over 5 percent.”

General Mills installed the sag correctors at Cedar Rapids. Rather than backing up the entire cereal plant with large-capacity units, much like was proposed in the diesel-generation plan, Robinson opted instead for a less-costly solution. He put only the control systems on 13 smaller correctors, one for each unit operation. During a power sag, the correctors regenerate the wave to the sensors, instrumentation, input/output (I/O), probes and the other components in the control system. The exception is the programmable logic controllers (PLCs), which had already been connected earlier to an uninterruptible power supply (UPS).

Because motors are not connected to the correctors either, they start to coast whenever power falls below the level necessary to sustain them. To prevent the variable-frequency drives (VFDs) from turning off, Robinson programmed them to regenerate—that is, to redirect the energy from the coasting motor to the DC bus, which keeps the VFD alive until the power is restored. “We don’t have to survive very long, only seven to twelve cycles,” he says. “So our VFDs have been able to ride through these outages, as long as we keep the control power active.” He did not bother with systems such as the packaging line because they are easy to restart and come to steady state immediately.

Since Robinson had the sag correctors installed, cereal production has been able to ride through brownouts and brief power interruptions. Besides saving the wasted material and time, the technology not only reduces wear and tear on the electronics and machinery, but also extends its protection beyond just the bad weather that the diesel-generation plan was designed to guard against.

Of course, the system does not protect production against sags that last longer than two seconds or so. “If you lose all three phases for a couple seconds, you’re down,” notes Robinson. But because the Cedar Rapids plant only averages two of these extended sags per year, he says that it’s not cost effective to do anything about them.

Suppress harmful surges

As costly as they can be, sags are not the only power-quality problem plaguing users of automation. Power surges, the more famous relatives of sags, can be outright destructive. For this reason, General Mills installed surge suppression throughout its Cedar Rapids plant years ago. Surges have not been a problem there for quite some time, according to Robinson.

Such was not the case, however, at a 200-employee dairy farm in the central Midwest. Unfortunately, management there became aware of the vulnerability only when a lightning strike destroyed motor controls and sensors used by the pumps for treating the waste being generated by the dairy’s 30,000 cows. “The storm cost the dairy thousands of dollars in downtime, equipment repair and replacement,” says Rob Palomo, business development manager at SolaHD, a supplier of power quality-control devices based in Rosemont, Ill.

It became obvious to management that it needed to clean up its power if it was going to control the cost of complying with strict sanitation standards. The solution recommended by SolaHD and its local distributor was a mix of surge protectors for the service entrance transformer, two motor control centers, some PLCs and the 4-to-20-milliamp (mA) sensors for the flowmeters.

Specifying the right mix of devices for any application is the biggest challenge to protecting electronics against surges. “There are no ‘magic pills’ that can be applied in every situation,” says Suzette Albert, SolaHD’s power quality product manager. “The types of disturbances vary from facility to facility and different loads are susceptible to different disturbances.”

For this reason, she recommends looking at a wide variety of solutions and finding the application know-how to install the best one. Her rule of thumb is to use a combination of filters to guard against low-energy, low-frequency disruptions and transient-voltage surge suppression (TVSS) to prevent high-energy damage. Applying this rule faithfully will eliminate any surprises that may result from believing that you have only one of the two problems, but discovering later the hard way that both actually exist.

Albert also advises users to ensure that the solution is appropriate for industrial automation. “Many companies offer power quality solutions for offices and computer rooms,” she says. “However, these don’t meet the requirements out on a factory floor.”

A harmonious current

As troublesome as surges and sags can be, they are not the only problem affecting power quality these days. The harmonics generated by non-linear loads, such as VFDs, can be another headache. Unfortunately, too many engineers focus on the effect of harmonics on other electronic devices and tend to overlook the effect of the extra harmonic current that their electrical distribution systems need to carry.

“This current doesn’t really do any work,” explains Corey Morton, product manager, B&R Industrial Automation Corp., in Roswell, Ga. “There may be real costs resulting from this additional current, such as increased transformer temperatures (leading to decreased transformer life) or other inefficiencies in your distribution system. So, when you use a lot of VFDs, you really should look at the total harmonic content.”

Because the harmonics are so application dependent, many VFD vendors offer free tools to help engineers analyze the harmonic content at different points in their distribution systems and the effects of various mitigation methods. With most of these tools, you select the sizes of the inverters that you plan to use and enter the loads and duty cycles that you expect. The results of the subsequent analysis can help you to select the most cost-effective design for the application.

Although harmonic-mitigation technologies such as AC line reactors and DC chokes have been around for a number of years, the trend has been to deliver them in smaller packages. An example is Acoposmulti from B&R Automation. The technology relies on an active power supply module that can reduce fifth and seventh harmonics that a passive six-pulse rectifier can generate. The active power supply and accompanying filter technology result in very low harmonics and in a total power factor that is essentially 1.The active power supply can return kinetic braking energy to the AC mains, instead of wasting it by dissipating it as heat through resistors.

Although power regeneration reduces the amount of heat generated from dissipating braking energy across resistors, there are other losses associated with inverters that affect the size of the switching cabinet. To deal with this extra heat, B&R Automation can offer cold-plate cooling. The technique uses water to transport the heat generated by the inverters and power supply to the outside of the cabinet, where it can be dissipated into either the air or an existing liquid-cooling system.

“Liquid cooling is nice because it is an efficient way to remove the heat from the control room,” offers Morton. “Depending on the amount of heat and the size of the room, this could have a significant impact on the air-conditioning requirements.” It’s just another example of how the various technologies for keeping your electrical power free of surges, sags, and hidden harmonics can clean up your bottom line.

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