Heeding the advice of a local consultant, plant managers at an automotive glass plant in rural western Kentucky installed uninterruptible power supplies (UPSs) on every programmable logic controller. Up to that point, voltage sags caused by remote faults on the electrical power system were producing an extraordinary number of outages, which cost the company hundreds of thousands of dollars in scrapped product. Unfortunately, investing substantial capital in power conditioning equipment didn't resolve the problem, leaving the plant managers more frustrated than ever.
This scenario demonstrates the importance of understanding process controls before attempting to resolve related problems. In an effort to discover the true source of the outages, the glass plant's managers asked EPRI PEAC engineers to conduct a power quality audit, identify weak links in the plant's processes, and propose cost-effective solutions. These engineers have investigated dozens of similar facilities that use gas furnaces in conjunction with logic controllers, flame eyes, control relays, and motorized blowers. While there are many different variations on these process controls, the components are generally alike, regardless of the application.
The Weakest Links
The first critical process line at the plant is the main furnace, which melts the primary glass (see the figure). The main furnace must stay on during this entire cycle because the glass needs to maintain a precise temperature. If a furnace flameout occurs, it disrupts the process long enough to render the cooled glass unusable. Before the plant managers contacted EPRI PEAC, voltage variations were causing more than 20 flameouts per year.
The second critical process factor is the high-intensity discharge (HID), high-pressure sodium lighting that illuminates the windowless facility. Anytime the voltage dropped below 80% of system nominal, these lights would extinguish. Everyone was left in the dark for three minutes before the lights would restrike.
After discussions with the plant's electrical personnel and reviewing the one-line circuit diagrams, the EPRI PEAC engineers knew that assessing just a few of the glass lines and motor control centers would suffice because nearly all of the process control devices and electrical configurations were identical from one line to the next. Therefore, during the site visit, the investigators examined the main furnace, furnace control room, furnace motor control center, primary headlamp glass lines, specialty glass furnace control cabinet, and specialty glass furnace motor control center. Each of these areas had a mix of power quality concerns, which are summarized in the table.
Once the investigators knew which components were the most sensitive, they were ready to make the appropriate recommendations to improve the power quality performance of each component. The following is a brief overview of the weak links and the recommended solutions.
The plant's programmable controllers had previously been identified as a weak link because they were continually tripping out and loosing their programming and control over the process. If the controllers shut down, the rest of the process would go down by default. The local consultant originally had the right idea installing UPSs on these controllers. Unfortunately, it solved only one of the plant's problems and did nothing to reduce the number of costly process shutdowns. The good news turned out to be that the installed UPSs were only loaded to 20% of their rating, which means there was an opportunity to condition some of the other critical load with no added cost.
Furnace temperature controls
These controls consist of a variety of flame detectors and some temperature sensors. Other EPRI PEAC investigations revealed weaknesses in these types of detectors when they were tested with a portable sag generator. The engineers found that voltage sags caused the flame detectors to erroneously tell the controller a flameout had occurred. The temperature sensors also would send inaccurate information to the controllers, resulting in process shutdowns. The simple solution, therefore, was to add these components to the existing UPS protection.
Gas flow to the furnaces
Investigators found that 120VAC solenoids controlled the valves that enable gas to flow to the furnace. Numerous other site tests have indicated that these solenoids are sensitive to voltage sags. In the brand used at this site, the manufacturer offers a PQ time delay retrofit that keeps the valve engaged through momentary voltage sags (if they last less than a couple of seconds). With this retrofit installed, the investigators were confident they eliminated one of the plant's primary weak links.
Motorized blowers, rollers, and support elements
After looking through the motor control centers and an assortment of control cabinets, the investigators identified four different styles of relays, contactors, and motor starters that were sensitive to voltage sags. They selected six properly sized constant-voltage transformers as well as a few coil hold-in-devices. This equipment enables the motor control elements to withstand voltage sags but still shut down if someone presses one of the emergency stop buttons throughout the facility.
HID, high-pressure sodium lighting
The plant manager didn't want the expense of changing the ballasts to more PQ-friendly types or installing the fast restrike circuits that would make the lighting systems immune to voltage sags and interruptions. Instead, the company opted to replace 10% of the HID lighting with previously purchased quartz lights. The quartz lights are not susceptible to the dropout and restrike problems associated with high-pressure sodium and metal-halide types of industrial lighting.
Approximately one year after EPRI PEAC's audit, the plant manager reported that the power conditioning recommendations were working well. The facility had experienced only one process stoppage, which correlated with a transformer failure that caused a half-day power outage at the facility. As you may expect, the plant manager was pleased. The additional power conditioning hardware solved the problem and cost the plant less than $10,000. Not bad for a problem that had once cost it approximately $1 million each year.
Doug Dorr is the business development manager for EPRI PEAC in Knoxville, Tenn. You can reach him at firstname.lastname@example.org.