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.
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.
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.
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.
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.
