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Building Operating Management

ABCs of UPS Systems

To find the right system, start with the basics: UPS system options and facility power requirements

By James Piper   Power & Communication

Facility executives today face increased demands for a clean, stable supply of electrical power. That’s especially true in mission critical facilities like data centers. But building electrical systems are anything but free of disturbances. Voltage spikes, brownouts, electrical noise, distortion of the sine wave and blackouts are all common occurrences due to a wide range of faults both inside and outside of the facility.

But it’s not just data centers that require clean power. A variety of equipment and circumstances — from computers and communication systems to test equipment and manufacturing processes — require the electrical supply to be free of disturbances if the equipment is to operate reliably and without interruption.

Moreover, many of today’s building systems are also susceptible to power problems. Life safety, temperature control and building automation systems all require a clean, stable supply of electrical power if they are to operate properly.

There are also power requirements for a variety of building systems during power outages. Emergency lighting systems, building security systems, life-support and critical medical equipment — all will require reliable backup power in the event of a loss of utility power.

UPS systems offer a reliable solution to a wide range of power-related problems. The systems can provide continuous power to critical loads in the event of an interruption or a total loss of utility power. Additionally, the systems can filter out many of the common electrical system disturbances that interfere with the operation of sensitive electronic equipment.

There are four basic designs for UPS systems: standby off-line, standby ferroresonant, on-line and rotary. Each system design can be configured to match the specific requirements of the application.

Design Choices

In standby off-line systems, electrical loads are fed directly by utility power. Utility power also feeds a battery charger connected to a bank of batteries and a power inverter. If utility power is interrupted, the load is automatically switched to the inverter. Transfer typically takes place in 1.5 to 4.0 milliseconds, a fraction of a single cycle of alternating current.

Standby off-line systems, the least expensive UPS option, have limitations. Because the system is off-line, it doesn’t filter utility power. Connected loads are not protected from transients, noise, voltage spikes or other disturbances.

Battery capacity in the systems is limited, making the systems better suited for small loads, typically 1.5 kVa or less. The limited battery capacity also provides power for only five to 15 minutes for most loads and systems. Standby off-line systems are best suited for use with individual critical loads, such as a desktop computer, file server, telecommunications panel or life safety control panel.

While the off-line systems do transfer power rapidly, there still is a momentary loss of power that can disrupt the operation of some sensitive equipment.

Standby ferroresonant systems are similar to standby off-line systems, only with the addition of a transformer at the load connection. Electrical loads are fed by utility power through the transformer. The UPS system charger, batteries, and inverter remain off-line until the utility power is interrupted. As with standby off-line systems, there is an outage that lasts a few milliseconds. The ferroresonant transformer, however, has sufficient energy stored in it so that it can provide a ride-through capability, eliminating any power interruption. This means that standby ferroresonant systems can be used with power sensitive loads. In addition, the transformer provides some level of voltage regulation, helping to filter momentary spikes and drops in voltage.

On-line UPS systems offer the ultimate level of protection for sensitive and critical loads. Unlike standby systems, power flows continuously through the system’s charger and inverter. Should utility power be interrupted, batteries continue to feed the power inverter. The process of converting the incoming power to direct current and back to alternating current eliminates practically all power disturbances.

On-line UPS systems are good for use with loads that are sensitive to power transients, voltage variations, and other common electrical disturbances. Since the system is operating at all times, there is no power interruption to the connected loads.

On-line UPS systems can be sized to match practically any electrical load. Run times can be extended by adding a standby generator to keep the system’s batteries charged.

Rotary UPS units are available in several different configurations. In its simplest form, an electric motor driven by utility power turns a flywheel and electric generator. Building loads are connected to the output of the generator. In the event of a power outage, the flywheel acts as an energy storage device, powering the generator for anywhere from a few seconds to a few minutes.

Another rotary design includes a rectifier, storage batteries, inverter and static switch. When utility power is lost, the combination of the flywheel and the battery power the generator. Carryover time depends primarily on the sizing of the batteries. Other rotary designs use engine generators to power the system in the event of a loss of utility power for an extended period of time.

Rotary UPS systems offer excellent protection against momentary power outages, voltage dips, voltage spikes and other power disturbances. With no large battery banks, the associated battery maintenance costs are largely eliminated. Its generator design offers excellent protection from in-rush currents and short circuit protection. Their primary disadvantage is their higher first cost.

Identifying the Need

The first step in selecting a UPS system is identification of the need. Local and state requirements will identify some loads that are critical to the operation of the facility, even during a power outage. Examine each of those loads to determine which ones cannot tolerate even a momentary outage in power as well as those easily damaged by power disturbances. Loads that do not meet either requirement may be connected to a standby generator system instead.

Identify other loads that are important to the operation of the facility, and cannot be interrupted. Depending on the facility and its operations, these may include select lighting systems, building HVAC systems, utility systems, fire and life-safety systems, telecommunications equipment and data processing equipment. For each load, consider the impact that the loss of utility power would have on operations, including even momentary losses. Identify loads that are critical with respect to power outages or damage from power disturbances.

For each of those loads, determine what type of delay would result from even a momentary outage, and how that delay would affect operations. For example, some computer-based control systems may take as long as 10 minutes to reset after a temporary loss of power. If the length of the delay for a particular load is unacceptable, that load is a candidate for connection to a UPS.

Another factor to consider is the length of time that the load will operate on UPS-supplied power. Some loads, such as egress lighting, may be required for only 15 to 30 minutes. Other loads, such as computers, servers, telecommunication equipment, and the HVAC systems used to cool them, may require power until utility power is restored.

Off-line systems are generally used to power loads for only short periods of time. While increasing the number of batteries used can extend operating time, the systems are not designed to power loads indefinitely. On-line UPS systems, by contrast, can power loads indefinitely if a standby generator is used to maintain the battery charge.

Selection Factors

While the size and nature of the connected load will to a great extent determine the type and capacity of the system required, other factors should be evaluated before settling on a particular system.

  1. System Size. The connected load will determine minimum system capacity. That sizing should take into consideration not only the operating current requirements of the equipment, but also start-up and in-rush current requirements, particularly for rotating equipment, such as pumps and fans.

    No system should be designed just to meet minimum load requirements. A rule of thumb is that the system capacity should be two times the steady-state current requirements. While the additional capacity will result in increased installation and operating costs, the spare capacity will allow for inevitable load additions as requirements change.
  2. Battery Care. Batteries are the weak links in all UPS installations. In small, off-line systems, expect to replace batteries every two to three years, depending on system use. Since the capacity of these batteries is relatively small, maintenance and periodic replacement is not a major concern, as long as they are replaced on a regular basis.

    It is a different story for the batteries in on-line systems. On-line systems use a bank of batteries, typically lead-acid, that require proper siting, regular maintenance and periodic replacement.

    During the charging cycle, large battery banks give off heat. For example, a 10 kVa on-line system will generate approximately 6,000 Btu per hour. Allowing this heat to accumulate in the battery and UPS rooms will shorten the life of the batteries and other system components and increase maintenance requirements. In addition, lead-acid batteries give off hydrogen gas. To remove the heat and gas, battery and UPS rooms require a properly designed ventilation system.

    Battery banks also require ongoing maintenance. Water levels in lead-acid batteries should be monitored regularly. Battery connections should be checked for proper torque and cleaned of corrosion. Batteries must be periodically tested for capacity loss. Facility executives can expect to replace lead-acid and other battery types every five to seven years.
  3. Siting Concerns. Off-line systems tend to be small and therefore can be located practically anywhere. But the batteries, charger and inverter in on-line systems are large and heavy — factors that must be taken into consideration when siting them.

    On-line systems also tend to be noisy. It is not uncommon for a well-designed system to produce a noise level of 50 to 60 db at a distance of six feet. Steps should be taken to limit noise transmission from the UPS room to the surrounding areas in the building.
  4. Diagnostics and Communications. Most UPS systems come with a special diagnostics board that constantly monitors the operation of system components and provides an early warning of problems as they develop. Many systems also include a communications signal that can be used to warn facility executives of the need to shut down loads. This communications signal can also be used to direct automatic shut down of certain computer systems to prevent system damage or loss of data.

    Not all systems have these capabilities, however, and not all systems can communicate directly to computers and other connected building systems. If this option is important, facility executives should be certain that the UPS systems being considered will communicate with existing equipment.

UPS systems offer a way to improve the operating reliability of many critical building components. They can eliminate many of the problems brought on by disturbances in the utility-supplied power. They can allow for continued operation of these systems when utility power goes down. And as long as systems are properly matched to the building’s requirements, they can provide this level of reliability without breaking the facility’s budget.

James Piper, PhD, PE, is a writer and consultant who has more than 25 years of experience in facilities management. He is a contributing editor for Building Operating Management.

posted on 8/1/2007

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