When it comes to uninterruptible power supplies (UPSs), most of us picture a sleek unit tucked away in an environmentally controlled room sitting quietly, ready to protect against any power problems that may come their way. However, UPS units are also used in a variety of environmentally demanding situations — applications that often require them to operate in a wide temperature range. One example is protecting the unique and harsh demands of gas and oil fields.
Oil/gas is under great pressure at the well head. To control their flow, a motor-operated valve (MOV) is installed at the well head and controlled by a supervisory control and data acquisition (SCADA) system. Control of the MOV is critical, because it must always be secured in a closed position, should the SCADA system detect any abnormality at the well head. Typically, local electric utility power lines or on-site generators serve these sites. However, power outages are not uncommon, which could result in the MOV being left in an open state and the oil flow left uncontrolled. The ramifications of an uncontrolled MOV could range from a costly oil spill cleanup to total loss of the well. To protect against this problem, a UPS is typically incorporated into the SCADA system and MOV control.
In addition to MOV control, the SCADA system monitors the well head pressure, controls on-site ground water and oil separation systems, monitors various containment tank levels, and acquires critical data from various site sensors. It continuously communicates the site status in real time to a central monitoring location via a wireless transceiver. A reliable UPS is essential to keeping the SCADA system communicating during a power outage
Inside look at a UPS
The operational temperature specifications for the majority of UPS products on the market today are stated as 0°C to 40°C. These UPS products have been designed for use in an indoor, temperature-controlled environment. Furthermore, they have undergone safety agency testing over the manufacturer’s stated temperature range and have received an ETL or Underwriters Laboratories (UL) listing for use over the stated range. As part of the safety agency product evaluation, the temperature ratings of key electronic components, displays, plastics, circuit board materials, insulating materials, and batteries are also verified to remain within their temperature specifications, while the UPS is operating over the entire manufacturer-specified temperature range.
As most of us know, oil and gas well head sites are located throughout the world in some of the most severe operating environments — locations that feature a wide temperature range. Installing a commercial off-the-shelf type UPS in this type of location might not be a wise investment. A wide temperature range UPS (e.g., -30°C to +60°C) is more suitable for operation in an oil field. These units are built with high-temperature components and designed to operate in harsh environments. However, they must also be paired with robust batteries with a wide operating temperature range.
Batteries are a Weak Link
The majority of UPS products rated under 10kVA use valve-regulated sealed lead-acid (VRLA) batteries as their source of backup energy. The typical VRLA battery has manufacturer’s operational temperature specifications that are very similar to the standard UPS temperature rating. Let’s take a closer look at the operational temperature range of a battery used in a UPS having a 0°C to 40°C rating:
- Discharge mode (-15°C to +50°C)
- Recharge mode (-15°C to +40°C)
- While being stored (-15°C to +40°C)
- Battery plastic material (60°C max)
For applications that are within the battery’s operational temperature range (click here to see Fig. 1), note the effects of elevated temperatures on both the battery’s service life and self-discharge rates. Battery manufacturers typically state their expected battery life at 25°C, which is the average home, office, or computer room temperature. Assuming the battery spends its entire life in the 25°C environment, the expected service life of this type of battery is 5 yr. However, if the battery spends its life in a 50°C environment, the service life is reduced to less than 1 yr. As you can see, heat greatly reduces the life of most batteries due to the acceleration of the chemical actions inside the battery.
If stored without recharging, VRLA batteries self-discharge at a temperature specific rate over time. If allowed to discharge to a level below 60% depth of discharge, irreversible damage may occur inside the battery. The damage is caused by deposition of lead sulfate on the surface and in the pores of the active material of the batteries’ lead plates. The capacity retention chart (click here to see Fig. 2) shows the effects of the self-discharge rate at an ambient storage temperature. Assuming the battery was fully recharged when stored at 25°C, it is recommended that it be recharged every 12 months. If stored in a 40°C environment, the battery must be recharged every 5 months.
Failure to keep the UPS batteries properly charged is the top cause of premature battery failure. Often, a UPS is unplugged with its batteries in a discharged state for several months when not in use. The lack of charge and battery self-discharge, combined with any low level current demanded by the UPS, causes the batteries to become excessively discharged.
The number of discharge and recharge cycles will also affect the service life of the battery. A typical UPS, if allowed to run in battery mode until the automatic low battery cutoff kicks in, will discharge the battery to an 80% depth of discharge. As noted in Fig. 3 (click here to see Fig. 3), if the battery is operated in a 77°F environment, the battery should provide about 300 full discharge/recharge cycles before it requires replacement. In most UPS applications, a full discharge to low battery cutoff is a rare event; thus, cycle service life would never be an issue. However, should a UPS be used in a high temperature environment where it was discharged on a daily basis, then the batteries would have to be replaced every six months or less.
The Bottom Line
Never use a UPS that is not rated for the temperature environment of its intended installation. If you’re designing an installation or installing a UPS in an extreme operating temperature environment, then select a UPS that has been designed to provide reliable operation over the required temperature range.
Stout is the vice president of engineering for Falcon Electric, Inc., located in Irwindale, Calif. He can be reached at: [email protected].