• Reddit
• Digg
• Facebook
• MySpace
• del.icio.us
• StumbleUpon
• Technorati
• Furl
• Newsvine
• Google Bookmarks

You may also be interested in...

Photo: @iStockphoto.com/dlerick

On November 7, 2002, torrential rains moved through the San Joaquin Valley, CA, causing multiple plant shutdowns due to voltage sags at Del Monte’s fruit packing plants in Modesto, Kingsburg, and Hanford. Fortunately, the rains occurred after the summer production months of July, August, and September, when the plants operate 24 hours per day, seven days a week. The shutdowns led Mark Stephens, manager of industrial power quality systems at the Electric Power Research Institute (EPRI), to work extensively with Del Monte and other food processors in California presenting ways to make food processing systems more robust and to ride through such storms.

Alex McEachern, owner of Power Standards Lab and an acknowledged international expert on power quality, says the facts of nature and man won’t allow the elimination of voltage sags of less than a second. “We can’t get rid of birds, snakes, squirrels, tree trimmers, and lightning,” he says. 

As a result, he says, there will always be power quality problems. The solutions advocated by both men are based on economics—the smaller and cheaper, the better.

McEachern points out that 95% of electrical power goes to lights, heaters, and motors. The 5% that goes into other equipment is the economically important stuff. For example, in a Big Box store, 1% of the electricity goes to operate cash registers. If electricity is lost for 10 minutes, losing lights and air conditioning is manageable, but doing without cash registers adds up to the loss of big bucks.

Stephens, who is also the author of many EPRI power quality reports over the past 10 years, has become an advocate of smaller solutions at the control systems level, to make systems more robust and able to ride through power quality problems. McEachern agrees with Stephens. As the designated US expert in front of the International Electrotechnical Commission on power quality measures and voltage sag immunity, McEachern treats power quality as a compatibility problem, while others may see power as the problem to be fixed.

Developments in Technology
Uninterruptible power supplies (UPS) have been the traditional method for coping with poor power quality. Flywheel technology is now replacing lead acid batteries in UPSs. However, as McEachern says, flywheels, although much less expensive than batteries and virtually maintenance-free, are still expensive solutions.

EPRI’s research and case study investigations over the past decade have identified voltage sags and interruptions, transients (both capacitor-switching and lightning-induced), and harmonic distortion to be the power quality phenomena that have the highest importance to end users. The large majority of power quality problems are related to these three items.

Due to the high occurrence rate and the general high cost and complexity of typical solutions, says EPRI, short-term voltage variations are one of the most, if not the most, important category of power quality phenomena for end users. It reports that 96% of all voltage sags and outages last 10 seconds or less. Therefore, “protection of end-use equipment from voltage sag phenomena is the most important power quality consideration.”

Stephens says earlier generations used relay-based control systems to power computer-controlled process lines. With the transition to programmable logic controllers (PLCs), which have become the backbone of industrial automation, manufacturers have found PLCs are susceptible to voltage sags, interruptions, and transients. More recently, manufacturers have made improvements in equipment—especially variable frequency (adjustable speed) drives—to make systems more robust.

“We are moving away from making power better and toward making equipment more tolerant of disturbances,” says McEachern.

On average, once a month there will be voltage sag. Yet, sensitive equipment may not be tough enough to handle the available incoming power. All equipment should be designed to handle those sags, he argues.

To assist equipment manufacturers, the International Electrotechnical Commission (IEC) has established tolerance level specifications, IEC 61000-4-34, to identify normal voltage sag, adds McEachern. 

EPRI documented Stephens’ study of the Del Monte fruit production facilities and the proposed solutions, in “Assessing Power Quality Impacts and Solutions for the California Food-Processing Industry.” It was funded in part by the California Energy Commission’s Public Interest Energy Research Fund (PIER).

This work with Del Monte led the Northwest Food Processors Association to ask EPRI to develop food-processing guidelines for power quality. More recently, Stephens and an engineering team has been working on a new IEEE (Institute of Electrical and Electronics Engineers Inc.) standard, P1668, “Recommended Practice for Voltage Sag and Interruption Ride-through Testing for End-use Electrical Equipment less than 1,000 Volts.”

This standard is aimed at providing a general voltage sag standard that can be used throughout different industries. “We hope this standard will be more cross-cutting throughout industry and provide a means for more complete testing of equipment to voltage sags,” says Stephens.

EPRI was also instrumental in the earlier development of industry-specific standards, such as SEMI F47, which lays out specifications for semiconductor processing equipment voltage sag immunity. It defines the voltage sag tolerance expected from semiconductor tooling equipment. SEMI (Semiconductor Equipment and Materials International) is the global industry association serving the advanced manufacturing supply chain through development of international standards and other activities.

Small Is Beautiful ...and Cheap
In an industrial process facility, components within the control panel “should be able to survive minor voltage sags without causing the system to trip offline,” using robust relays, contactors, and power supplies, according to EPRI. If the control for the process machinery consists of mainly PLCs, relays, contactors, and drives, the use of a control level power conditioner or 24-V-direct current (DC) power scheme for the control can be very effective, says Stephens.

Stephens and his colleagues at EPRI have become proponents of utilizing 24-V DC to power PLCs or automated controller power supply and input/output (I/O) control power on new designs to make them more robust. Robustness is tied back to the DC signal.

“Most of the problems I see [can be fixed] within control systems,” he says.

As he explains, a 24-V DC power is installed as a main source for cabinet controls instead of a control power transformer. If the I/O control voltage is already DC, only the alternating current (AC) input power supply module needs to be replaced with the comparable DC input power supply module.

“You don’t need power conditioning on a motor for short events,” continues Stephens. “Up to 85% of events have a 15% drop in voltage for one cycle and this will shut down a PLC. The further you go away from the control cabinet the more expensive it becomes to solve power quality issues; it’s a matter of economics.”

Energy savings might also be available, depending on the efficiency of the DC power supply, notes Stephens. By lowering the amount of current, demand could be reduced and power factor improved. If AC control circuits are used, highly efficient power conditioner options are available, such as units like a dynamic sag corrector that use series injection technology. When an event occurs, the power conditioner will draw more current and inject the missing voltage back into the line.

Where a control level solution is not feasible because sensitive three-phase loads cannot be easily mitigated at that level, he recommends the use of large three-phase power conditioners such as a flywheel or series injection technology.

Stephens also identified another issue—the AC ice cube relay. It is often used to interface between PLCs and variable frequency drives or motor starters. It can also be used in the “machine on” or “emergency off” circuits. With sensitivities typically in the range of 70% of nominal voltage sag, these relays present a challenge to a control system that is trying to ride through voltage sags. For this reason, circuits with AC “ice cube” relays need power conditioning to keep them from causing the controls to be affected during voltage sag events.

Other Solutions
The EPRI food processing study contained additional troubleshooting recommendations of interest to a wider audience. 

• Avoid mismatched control power voltages. If the actual control system nominal voltage is lower than the expected nominal input voltage, for example, if a 230-V AC input power supply is connected to a 208-V AC source, the entire control system will be more susceptible to voltage sags.
• Most electronic devices come with power supplies rated at 100 V to 240 V, to cover standard voltages worldwide. In the US, if the current is at 120 V, equipment will continue to operate with backup unless the voltage sag drops 60 V. If equipment is connected to 208 V or 240 V, it will have more margin to be protected from sags.
• Use a robust DC power supply scheme to ensure that the 24-V DC power source remains steady during typical voltage sag events. Unregulated DC power supplies have the worst ride-through, while universal input switch-mode DC power supplies are the most immune over the range of possible voltage sag scenarios.
• Properly maintain the controller’s battery. Many PLCs utilize lithium-ion batteries to maintain its control programs and non-volatile memory data in the event of a power loss or voltage sag-induced shutdown. Battery death can cause loss of the PLC program and extended downtime due to the need to locate the latest back up, reload, and restart the process. Alternatively, utilize a state-machine programming method or non-volatile PLC memory.
• Consider the power source for analog input signals to ensure that the source is stable throughout normal voltage sag events.
• Utilize only compatible power conditioners. For control level power conditioning, use battery-less power conditioners if possible rather than the standard small battery-based UPS. 

There are many alternatives available now for providing power conditioning without the use of a battery, including constant voltage transformers, dip proofing inverters, dynamic sag corrector (a series injection device), or voltage dip compensator. Stephens has observed many small battery-based systems abandoned in place in control cabinetry. Once the battery dies, the manufacturer will bypass the UPS and get the line up-and-running again. Therefore, battery-less power conditioners offer the freedom from battery replacement.

If a UPS is used, avoid those with square-wave outputs, unless the controller manufacturer can assure that the system power supply and I/O cards are compatible. A line-interactive UPS that produces a true sine-wave output has effectively mitigated voltage sags in tests, says EPRI.

Photo: EPRI

EPRI's Mark Stephens conducts a power quality audit on a typical control cabinet in a glass bottle manufacturing facility, while Baskar Vairamohan (behind him) takes notes.

EPRI says adjustable speed drives have a variety of setup parameters that can be changed, that will influence the ability of the unit to ride through voltage sags. These parameters include flying restart, kinetic buffering, and adjustment of the DC bus trip level.

Transients
Transient over-voltages are usually caused by switching operations or lightning strikes to electric facilities and can have significant potential to damage electric power equipment or disrupt operation. Off-the-shelf and inexpensive transient voltage surge suppressor products are available to the user or original equipment manufacturer.

Low-frequency oscillatory transients are more difficult to treat. They are associated with the energizing of shunt capacitor banks on a utility’s distribution system. Because they contain substantial energy, their effects can be felt quite far electrically from the point of origin.

Harmonics
Harmonics, says EPRI, have increased significantly over the past two decades due to the increased use of non-linear loads, such as adjustable speed motor drives and switch-mode power supplies. McEachern went further: Over the past 70 years, loads like light dimmers and industrial heaters that draw wave forms have been added to the distribution system that was built for sinusoidal loads like heaters, lights, and motors.

And because harmonic distortion levels will begin to cause more problems for utilities, harmonics have received continuous attention from standards-making bodies and technical groups. IEEE has developed rules-of-thump to estimate limits on the percentage of total load represented by adjustable speed drives. Problems related to harmonics are usually confined to locations with inordinate amounts of nonlinear, harmonic current-producing loads, such as wastewater treatment plants where the entire load may be adjustable speed motor drives powering pumps. Or, they may occur where power factor correction capacitors on the end-user system, or at the utility distribution level create resonances that amplify the effects of nonlinear loads.

McEachern’s solution, again using the philosophy of finding cheaper solutions, is to install a larger transformer, rather than a harmonics filter. Usually the effects of harmonics at the customer level are seen in telephone interference and burned out transformers, he says.

Flywheel UPSs
Flywheel technology has really matured in the last few years, Stephens says. There are now a variety of manufacturers that offer flywheel UPS systems. Flywheels use motors to collect energy. Once utility power is disrupted, the momentum of the spinning flywheels keeps its internal “saved” power flowing to the equipment being served for 10 to 15 seconds, smoothing out the voltage sag. This gives a backup generator enough time to immediately take over from the flywheel if the interruption lasts longer than 15 seconds.

Matt Servis, director of engineering at Vermont television stations WFFF-TV and WVNY-TV, says when two solid state digital transmitters were installed, they decided to add a 65-kW flywheel UPS, manufactured by Active Power, because of the poor power quality in the area. The digital transmitters are driven by computers, he says, which are much more susceptible to dirty power than were the analog-driven older transmitters being replaced.

The transmitters are located on Vermont’s highest peak, Mount Mansfield, and are powered by a distribution line shared with a ski lift. When it operates, the line experiences voltage sags and spikes, and power drops out for two to three seconds. UPSs with battery backup were impractical, he says, because replacement batteries would have to be hauled up the mountain in a snow mobile if a battery went out between October and May. The lifetime of a battery is three years on average. Other solutions at the control panel level were not practical, says Servis, because the control system is an integral part of the transmitter, with power being fed through one transformer.

Since the flywheel was installed in October 2006, Servis reports it has discharged over 300 times, but the transmitter has experienced no outages. He is aware of problems now only when competitors go off the air. After it was installed, he experimented and pulled the power plug. The flywheel supplied power for several minutes—not seconds, and Servis reconnected the power before the flywheel ran out of power.

Jim Clishem, president and chief executive officer of Active Power, says his company’s flywheel UPSs offer particular advantages to healthcare and broadcast facilities where large amounts of power are required in short periods of time. In these situations, batteries deplete very quickly. The pulsation of current lends itself to flywheel operation, he comments. More flywheels, joined in multiples of units, can fit in smaller spaces than can batteries, Clishem says, by a factor of four.

Active Power flywheel UPSs cost between $55,000 and $60,000 for a 130-kV-Amps system, and $640,000 for a 2-MW system. Clishem reports maintenance costs for an Active Power system are one-third of a UPS system with battery backup, because of the need to replace batteries periodically.

It’s the Economics, Stupid!
To evaluate whether the solution is a small, inexpensive solution such as installing a 24-V DC power supply in control panels or a flywheel UPS, McEachern explains that the engineer needs to look first for free solutions, and then measure the cost of the events against the cost of other solutions.

He cites several examples: a company produces widgets at $1 million per hour. A sag shuts down the line for half an hour once a month. The manager’s solution is to pay six workers $500 at the end of the day to make up for the lost production. On the other hand, a large auto manufacturing plant operates by receiving its supplies on a just-in-time basis and receives bumper shipments four times a day. The bumper factory has power quality problems and the loss of tens of thousands of dollars at the bumper factory costs the auto plant millions of dollars. 

At a pharmaceutical plant McEachern recently visited, they were experiencing voltage sags once a month. After each incident, staff restarted machines in a standard routine. He sees this as a reasonable solution, as long as the steps to restart motors are written out to accommodate personnel changes.

In each case, McEachern asks managers what they are willing to pay to fix their problems. “Far more commonly, they are looking for common-sense, less expensive solutions,” he says.

The auto plant manager will be eager to seek a far more expensive solution than the widget company.

McEachern says a manager may be better off tolerating sags or adding a tiny UPS to serve only the control system, and not heaters, lights, and motors which are not sensitive to sags and similar interruptions lasting one or two seconds.

Another example: 1% of the power that runs a bank of elevators goes to the computer that controls the motor. Protection of the computer against a power quality problem is much less expensive than a large UPS to protect all the motors as well, given that motors are usually able to ride through voltage sags.

McEachern will always ask a client who has sought his analysis: “Is the solution less than the cost of the problem?”

His bottom-line message on power quality issues—Find the cheapest solutions!