Power Electronics

UPS Operation Must Be Consistent with its Environment

When used in a controlled temperature environment ranging from 0°C to 40°C (32°F to 104°F), any domestically available on-line UPS should meet this temperature requirement. Most have been tested and approved for operation over this temperature range by a safety agency such as Underwriters Laboratories. However, there is an ever-increasing requirement to use on-line UPS technology in much wider temperature environments. Many harsh power environments are located in remote outdoor locations (Fig. 1), where power sensitive equipment and the on-line UPS must be installed inside buildings without any climate control systems, or in protective NEMA-rated enclosures. For example, the outdoor temperature in Phoenix, Arizona in the summer can be over 120°F, while the low temperature in Prudhoe Bay, Alaska in the winter can be below -22°F (30°C). Even though it is protected from the elements, attempting to use an off-the-shelf UPS in these extreme temperature environments is a poor decision that can result in a UPS failure.

Typically, a standard off-the-shelf on-line UPS specified for operation over a 0°C to 40°C temperature range is submitted by the manufacturer to a safety agency for an engineering evaluation. As part of the evaluation, a temperature profile is taken of highest heat generating components and heat sinks to assure they do not exceed their maximum temperature ratings when the UPS is operated at the manufacturer’s specified maximum temperature. The UPS can incorporate high power components like transformers and chokes that can overheat and not only cause the UPS to fail, but present a risk of an internal UPS fire. Therefore, the safety agency also reviews the internal components, circuit board, and plastic materials used in the UPS to ensure they are within their temperature ratings and limits.

Most on-line UPS manufacturers design them for operation in the standard 0°C to 40°C environment. But, installing the UPS in buildings without temperature control in the summer in Phoenix would be using the UPS outside safety agency listing status. In many cases, the UPS is designed for use in the stated temperature environment, so internal components near their temperature limits could exceed their maximum temperature ratings. This could result in the UPS having a reduced reliability and life span, or an outright failure. At the higher temperatures of Phoenix, plastics used in the UPS construction or for battery manufacture can become deformed or cracked.

To meet demands of an elevated temperature environment, a UPS should be designed to meet that requirement. Batteries are the first concern. Typical UPS batteries will have their service life dramatically shortened when operated in a high temperature environment above 40°C. Further, the UPS battery charger should have temperature compensation to prevent the batteries from being damaged due to overcharging when their temperature is over 40°C. Next, the batteries used must have an operational temperature rating that meets or exceeds both the low and high temperature limits of their rating.

Standard UPS batteries are typically not rated for temperatures above 40° to 50°C. Furthermore, per the battery manufacturer’s specification, at 50°C, the battery service life might be reduced from five years to a few months. Fig. 2 shows the battery service life versus temperature for a typical VRLA (valve-regulated lead-acid) battery.

Temperatures below 0°C present their own set of unique problems. Due to the electro/chemical design of most VRLA batteries, temperatures below -20°C can limit the batteries ability to deliver sufficient current to power the UPS (Fig. 3). As a consequence, when operated at 0⁰C, battery runtime can be reduced to less than 50 percent of its typical value. Plus, below -40°C the electrolyte found inside electrolytic capacitors used in the UPS can greatly lower the capacitors’ capacitance or even freeze, causing capacitor over-pressure safety vents to rupture. This can cause the internal electrolytic capacitors to slowly dry out over the following months, resulting in a UPS failure.

In addition, if not rated for less than -40°C, some integrated circuits and optical isolators can malfunction, causing the UPS to go to an alarm condition until warmed up. Again, a full UPS failure can result. At this low temperature, batteries can freeze internally. The plastics used in their external housing can become brittle and subject to cracking. As the battery electrolyte freezes, it expands the plastic case and can cause the batteries to leak acidic electrolyte inside the UPS when the ambient temperature raises enough to allow the batteries to thaw out. This often renders the UPS unusable, requiring it to be replaced. Therefore, a standard on-line UPS with an operational temperature rating of 0°C to 40°C should not be installed in protected outdoor locations having temperature extremes outside its rated limits. However, that is often attempted.

There is a large scale propagation of power sensitive microprocessor-based programmable logic controllers (PLCs), SCADA and telemetry systems, natural resource excavation and production, wind turbines, solar generation sites, communications repeater sites in remote locations, without temperature controlled environments. In these applications, there is a growing demand for UPS and battery systems with wide operational temperature specifications that meet the associated environmental conditions. Some UPS manufacturers are beginning to meet this demand by designing and offering UPS products that have wider operational temperature specifications, plus the associated safety agency testing and approvals.

As the on-line UPS (Fig. 4) uses active electronics to continuously regenerate clean, new sinewave output power when being powered by the utility and battery power sources, it generates internal heat that must be properly managed. To address this requirement the UPS enclosure should be designed to ensure that internal batteries are thermally isolated from the internal heat generating elements of the UPS electronics. The UPS component selection must be made to allow as much thermal margin as possible when the UPS is operated at its upper temperature rating. The on-line UPS electrical operating efficiency must be increased as much as possible to reduce the amount of internal heat generated due to electrical losses. Finally, the internal forced air-cooling and laminar airflow through cooling heat sinks and across heat generating components are essential to removing internal UPS heat. For more on on-line UPSs, see the box “Why choose an on-line UPS?”

In most UPS units, the raw batteries are installed directly into the units. This subjects the batteries to the internal heat generated by the internal UPS electronics. As on-line topology UPS units have active electronics that are regenerating new sinewave output power while in both utility and battery modes, the heat being generated inside the UPS can become elevated from 15°C to 25°C above the ambient temperature. This can result in a reduction of battery service life.

Insulated Case

One approach to meeting operation at temperature extremes is to install the battery and other temperature-sensitive components in an insulated case, like the one shown in Fig. 5. This UPS has the raw batteries installed inside a thick walled plastic enclosure, in addition to having an internal battery space that is closed off to the internal heat generation electronics. Additional cooling fans pull cool ambient air through the battery enclosure to cool off the plastic box and reduce the battery temperature generated by any internal battery heating. Special wide-temperature-range batteries have been incorporated into the design having an operational temperature range of -60°C to + 80°C.

This UPS is cooled by high capacity cooling fans that draw air in the front of the UPS and out the rear panel. Further, the fans are speed controlled by the internal microprocessor that senses the UPS’ internal temperature and adjusts cooling fan speed accordingly. The UPS has been designed to maximize a laminar airflow path through internal heat sinks and across selected heat generating components. Also, higher temperature grade components have been incorporated into the UPS design.

To support operation at lower temperatures, heating elements and temperature control are employed. Designing for the colder temperatures can be a simple matter of adding a pad style heating element with a temperature controlled switch and adding it to the inside of the NEMA enclosure housing the UPS, or locating it inside the UPS and battery systems. Both methods are effective solutions.

In applications like steel mills, foundries and near high temperature furnaces, the ambient temperature may be so high even the ratings of a wide-temperature-range UPS will be exceeded. For these applications, a far costlier approach is used that utilizes a double walled NEMA 4X enclosure, with an internal to external heat exchanger or door-mounted air conditioner unit.


Why Choose An On-Line UPS?

Double Conversion On-line UPS technology, through its continuous regeneration of new AC power, provides the highest level of power conditioning and protection. On-line UPS technology is ideal for use as a combination high performance power conditioner battery backup system.

The IEEE defines UPS products in three distinct categories, off-line, line-Interactive and on-line. They provide three increasing levels of protection.

  1. The off-line UPS provides battery backup, basic surge protection and provides no voltage regulation when operating from utility power. They are so low-cost that they may not provide a true alternating current (AC) sinewave power when operating from battery. They should be avoided at all cost.
  2. The line-interactive UPS is similar to the off-line design, except it provides grossly regulated AC power when operating from the utility power. Voltage regulation is accomplished by electronically changing transformer taps whenever the utility power changes drastically. The UPS output can actually make a greater change in voltage that was made by the utility voltage.
  3. The on-line UPS maintains a tightly regulated AC voltage (±2% typical) to the critical equipment 100% of the time, whether being powered by the utility or from internal battery power. Inside the UPS the AC utility power is converted to DC power, removing all transients, voltage variations, frequency shifts and a new, regulated output AC is regenerated. The ±2% output voltage regulation is maintained over the entire wide input utility voltage range of the UPS (80Vac-135Vac typical). The same is true when operating from battery. The on-line UPS also protects against power line frequency shifts associated with generator sources, which the off-line and line Interactive designs offer no protection against. The on-line UPS unit’s tight output voltage and frequency regulation are essential for power sensitive equipment in remote locations which are subject to the most severe types of power related problems.

Application locations with the highest and lowest ambient temperatures have the most severe power problems, the types of problems that only an on-line UPS can mitigate while providing continuous, regulated, clean power to the connected equipment.

Related Articles:

Optimum Dead Time Selection in ZVS Topologies

UPS System for Offshore Platform Requires More Safeguards

Protecting the Protector: Adding Circuit Protection to UPSs

Selecting the Right UPS for the Job

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