Every year, industries around the country lose between $13 billion and $26 billion in downtime due to problems with the quality of electric power delivered to factories and other industrial facilities, according to Jane Clemmensen, a well-known power quality consultant now working at Frost and Sullivan. If these numbers sound alarming, consider a Sandia National Laboratories study from July 1998 that estimated voltage sags alone cost industry $114 billion.

The economic havoc caused by voltage sags results from rising levels of automation and computerization in manufacturing facilities. Just a decade ago, it hardly mattered if the voltage sagged, or even if it ran consistently low. Back then, the result would be an overheated motor at worst. Today, a voltage sag or dropout lasting just milliseconds can bring an automated factory floor to a halt.

The average number of voltage sag incidents at U.S. factories runs at 45 per year, and 20% of companies are finding themselves significantly affected by power quality problems, according to a study released by the Electric Power Research Institute (EPRI). The potential for power quality-related economic losses will only get worse as more loads are controlled electronically instead of electromechanically-with the proportion of electronically-controlled loads rising to an estimated 60% by the year 2000.

What's at Fault?

Voltage sags are brief (less than 60-cycle) decreases in voltage that are measured in two ways: the magnitude of voltage drop measured as a percent of normal voltage, and the duration of the event, measured in cycles.

Most voltage sags are caused by operation of the protective devices on a utility system to clear a remote fault in the power system outside factory walls. These usually take the form of single line-to-ground faults resulting from lightning strikes, wind, or ice conditions. Accidents with vehicles, bird, animal and tree contact, planned electrical switching, and contaminated insulators can also cause faults. While utility companies seek to reduce the number of faults through good design and maintenance of the power delivery grid, faults cannot be entirely eliminated.

Most sags last less than a second, reducing voltage to 50-70% of normal. A sag of just two or three cycles, however, even at a small percentage below normal voltage, can affect electronic equipment and computerized controls that run today's manufacturing processes.

Common factory floor equipment such as motor contactors, adjustable speed drive controls, computers, programmable logic controllers, and robotics are all susceptible to sag-induced shutdowns or damage.

The Information Technology Industry Council (ITI) has developed a set of voluntary standards covering several kinds of power quality parameters. Theoretically, digitally controlled equipment should be able to ride out events falling within these tolerances. Unfortunately, equipment owners cannot depend on their power supply being this clean under all circumstances. A whole host of events, such as lightning strikes or on/off cycling of motors, can push power beyond these limits. Nor can equipment owners assume that their process equipment adheres to these standards.

At whatever levels equipment trips, the result of the downtime in a plant is the same: lost productivity, idled workers, damaged in-process material, extra clean-up costs, restarting difficulties, delivery delays, and-ultimately-upset customers.

External Solutions

DVR block diagram

Although most voltage sags emanate from outside factory walls, plant managers and engineers have several options to reduce power quality impacts on plant productivity.

The first line of defense is to work with local electricity providers to improve the power supply by first looking at ways to reduce faults, such as changing the protective scheme, implementing aggressive tree trimming programs, or cleaning insulators. Then power consumers and providers can consider upgrading the infrastructure delivering power to a factory. Options here include moving feeds underground, putting in a second feed, establishing transmission service at the site, and migrating the site to an express feeder. All of these cost money, which a utility may or may not want to subsidize in order to keep an important industrial customer satisfied.

The total operational costs of some upgrades are hard to calculate or involve hidden costs. For example, setting up a medium-voltage static transfer switch to solve problems requires two completely independent power sources and may involve extra costs required to maintain reserve capacity on the second source of power.

Some solutions carry technical risks. A tap-changing transformer may cut out 10% of sags by raising overall voltage a few percent, but may increase the probability of line swells and excess steady-state voltage conditions which can stress equipment and reduce its useful life.

Inside Jobs

Inside the factory, operators can make equipment less susceptible to disruption by desensitizing the load, changing settings on drives, or increasing capacitance to create carry-through time. This, however, may reduce output from the factory if the machines run at a slower rate.

Installing power conditioners is another solution for individual loads. Constant voltage transformers work well for constant- or low-power loads, and they will handle many sags. One can also replace overly sensitive process equipment with equipment designed to withstand higher power variances.

Another option is to install an online uninterruptible power supply (UPS) system. UPS systems require a lot of real estate, are expensive to maintain, add to a factory's overall power consumption, and must be sized to match 100% of the load. As an alternative, operators can install individual UPS systems for high-priority pieces of equipment inside the facility, but a proliferation of small UPS units could create other problems such as harmonics, increasing the overall risk of power quality failures.

New Technology: Series Compensation Devices

DVR mounted inside a trailer.

With the exception of changing from overhead to underground utility feeds into a facility, most of the approaches involve accessories or upgrades to individual pieces of equipment. This can be a good approach if one or two individual loads are the only ones needing protection. However, if multiple loads or a large percentage of the total plant load is involved, this can get very expensive and inflexible, especially as factory floors evolve over time. It would be better if operators could manage power quality on a whole-plant basis, without resorting to devices attached to individual pieces of equipment.

New technology, in the form of the series compensation device (SCD), has emerged with the potential to address power quality issues on a facility-wide basis at lower cost and with higher reliability than traditional approaches. SCDs represent an active approach to factory-wide power conditioning. SCDs monitor power flowing into a facility for sags, swells, and other disturbances and correct variances by injecting a voltage of compensating amplitude, frequency, and phase angle into a line within four milliseconds of the onset of a disturbance.

SCDs typically reside between (series connection) the utility primary distribution feeder and an industrial facility or between the power source and the load to be protected, monitoring and correcting power on a factory-wide basis. In case of a sag, the SCD inserts the appropriate voltage waveform into the main feeder voltage before it goes into the factory.

Mounted in trailers or skids, SCD units range in size from two to 10 megavolt-amperes (MVA) inverter ratings to protect 2 to 40-plus MVA feeder loads. Smaller, platform-mounted units provide protection in the 300 to 750 kilovolt-ampere (kVA) range. Low-voltage compensation devices are also available in 100-500 kVA range for application within a facility to protect a process line or individual device.

A typical medium-voltage SCD consists of four major subsystems: power line monitoring and control; power electronic converters; energy supply system; and the series injection transformer. The computer-controlled monitoring system observes power as it comes into a factory and sends commands to the other subsystems to inject compensating voltages into the power line to correct faults.

The three, independent single-phase, pulse-width-modulated inverters are fed by the energy storage system, interfacing via a direct current (DC)-to-DC boost converter. The boost converter regulates the voltage across the DC link capacitor that serves as a common voltage source for the voltage source inverters. The compensating voltage synthesized by the inverters follows commands sent to it by the SCD's digital control system. The compensating voltage is then injected into the distribution system by a set of three single-phase series injection transformers.

The SCD uses reactive power to restore voltage sag, unless real power is exchanged at the SCD alternating current terminals-in which case, the SCD taps into its energy storage system. The rating of the SCD inverters becomes the limiting factor for normal load current seen in the primary windings and reflected in the secondary windings of the series insertion transformer. For line currents exceeding the inverter rating, a by-pass scheme protects the power electronics.

Costs and Benefits

SCDs combine sophisticated digital monitoring and control electronics with a reserve power source to inject a sagged power line with compensating voltages that correct for power quality anomalies.

Medium-voltage SCD equipment costs approximately $200-300 per kVA of coincident load served, well under the price of an equivalent UPS-based solution. Because the SCD's capacity does not need to be as large the load it protects, since it is just providing what is missing in the waveform, it typically requires less than half of an equivalent UPS' nominal power rating in most applications.

Reducing power-quality-related shutdowns is particularly important in high-value-added industries such as semiconductor, pharmaceutical, chemical, petrochemical, textile, paper, and food processing. In the semiconductor industry, for example, where production lines consist of very expensive and intricate equipment, typically operate 24 hours a day, and manufacture goods literally worth more than their weight in gold, a process interruption can cost between $250,000 and $1 million per incident. In a wafer fab, a three-cycle disturbance can result in a plant shutdown followed by a three-day restarting process. Other industries have found SCD systems well worth their cost in maintaining optimum production levels without interruption from power quality faults.

Since SCD systems sit between the power grid and a manufacturing facility, they offer a variety of ownership and operation options. Options include systems completely owned and located at the plant they serve, utility-owned SCD systems installed at the behest of electric power customers, or systems leased from and maintained by third parties. The wholly-owned model works best for companies that insist on total control of power going into their buildings or those buying electricity from companies unwilling to offer SCD-monitored power as a value-added service. The utility-owned model offers power users the advantages of not having to tie up capital in an SCD system and to place responsibility for power quality maintenance on the utility, which will usually sell the quality-guaranteed power at a premium price. The third-party model outsources power quality maintenance, reducing capital costs to both the utility and power customer.

With deregulation of the utility industry, power quality will undoubtedly emerge as a competitive differentiator among electric service providers focusing on the industrial market. Not only will specialist, industrial customer-focused electric companies emerge in deregulated markets, but one can imagine companies offering various grades of power quality to customers. It won't be too much longer when "fill it with premium" has the same meaning at the gas station as it does when ordering electric power from an outside supplier.

SCD Applications

Many industries already use one or more SCDs (series compensation devices) to protect their loads. A semiconductor manufacturer in Chandler, AZ, working through the Salt River Project, installed a pair of 6 mega-volt ampere (MVA) SCDs protecting a 45 MVA load on a 12.47 kilovolt (KV) line. In the first nine months of operation, the SCDs corrected more than 20 sags that would have impacted production.

A textile manufacturer, Orian Rugs in Anderson, SC, installed a 2 MVA SCD on a 12.47 kV system to protect a yarn extrusion process. In the next two years, the SCD corrected over 140 sags.

A paper mill in Scotland, Caledonian Paper, plc, installed a 4 MVA SCD to protect an 8 MVA paper machine load on an 11 kV distribution system. By correcting sags, the mill was not only able to avoid damage to production equipment, wasted material, and lost production, but also to run at full speed and maximize production output.

Bonlac Foods, a dairy in Stanhope, Australia, has seen a noticeable drop in the amount of outages in its dry-milk-processing plant since an SCD was installed on its 22 kV system in February 1997. In the first year, the SCD filtered out 33 power flicks that would otherwise have interrupted the plant. The following year, it dealt successfully with 27 such interruptions, saving Bonlac approximately $100,000 per month.