“The idea that all power disturbances come from the utility is about as outdated as the belief that one large uninterruptible power supply (UPS) in the basement of your building will solve all power-related problems,” said Wes Tazzia, of Controlled Power Co.
“Every time a copy machine is used in your office or an air compressor is activated in your plant, power disturbances may be induced. In the past, when companies depended on large mainframes as their only source of computing power, a single point of clean power was adequate.
But this is just not true in today’s highly networked environments.”
In networked computer systems, several PC clients are tied to one or several servers distributed throughout office buildings or plants. Since malfunctioning of a server can be catastrophic, its power source must be protected, so it is usually fed from an uninterruptible clean source.
Individual PCs may also require individual protection. This is especially true if they are used in critical applications or noisy environments, such as when they function as controllers of ATE or complex instrumentation setups.
“As businesses migrate to client/server implementations, UPS are considered ‘must have’ items to protect data integrity,” said Ron Seredian of Clary Corp. “Because of this evolution, the UPS industry is growing 20% per year, with most growth experienced in the lower power end; that is, for UPS of less than 5 kVA.”
Disturbances–Types and Cures
Common types of power disturbances and their causes are listed in Table 1. Power analyzers can help you determine which of these may be most prevalent at your facility.1,2
If noise, transients, spikes or surges are the main threats, filters and surge suppressors will alleviate these problems. Fluctuating voltage levels can be rendered harmless through voltage regulators. But to combat power interruptions, you need a UPS.
Three basic UPS architectures are used today: standby, on-line and line-interactive. The standby UPS feeds the load from utility power until an outage is detected. Upon detection, the load is switched to a local battery and AC converter combination. On-line UPS convert AC line power to DC to keep a battery charged and convert DC battery power back to AC. The load is continuously fed from the constant battery power, regardless of utility-line conditions.
The line-interactive technique combines the advantages of standby and on-line techniques and has recently gained popularity. Many implementations are used today.
Line-Interactive Technology
All line-interactive UPS include intelligent line-monitoring and control circuits and an energy storage source that is continuously ready to supply power to the load. For short interruptions, capacitive or ferroresonant magnetic storage may suffice. To withstand longer outages, batteries are used.
A block diagram of a typical line-interactive UPS is shown in Figure 1. Normally, power is provided through the switch and regulator to the load. Voltage regulation is commonly achieved by electronically switching step-up/step-down transformer taps or by a ferroresonant transformer. Filters may be included at the input or output.
When a line power failure is detected, the switch opens and power is provided from the battery through the inverter to the load. Since the battery and inverter are constantly connected to the load, no power interruptions occur.
“Line-interactive topology is more efficient, minimizes heat and stress on batteries and electrical components, and is a dual power source,” said Joe Granda, Product Manager at American Power Conversion. “All three of these advantages increase the reliability of the UPS.”
Similar to standby UPS, line-interactive technology is highly efficient because power is normally supplied directly from the line. Heat and stresses are minimized because the line-interactive UPS supplies only incremental power, compensating for line voltage sags via the step-up transformer. Like the on-line UPS, it also provides continuous power.
However, the line-interactive UPS does not provide the complete isolation between the line and load that is afforded by the on-line UPS. “Noise is one of the biggest problems in data communication since it corrupts packet transmissions, leading to resends, which result in diminished performance,” said Mr. Seredian. “For these most sensitive applications, I believe there is now a shift back to on-line UPS, particularly those that offer true isolation.”
Shut-Down/Status Reporting
Most UPS include an RS-232 port that enables transmission of internally sensed status information to a local PC or a central location. Most UPS suppliers also provide monitor software to aid in early fault detection. The monitoring facilities of some UPS conform to the Simple Network Management Protocol, which is widely used for network administration.
“Network managers always want to know about any power problems before their end users are affected,” said Mr. Granda. “They also want to protect data by gracefully performing an unattended shutdown when necessary. As a result, management software associated with UPS products has become a necessary ingredient for managers.
“In fact, the rate of software purchases per UPS has been continually increasing,” Mr. Granda continued. “With the upper tier software packages, basic shutdown features are augmented by sophisticated diagnostics and UPS control.
“To be most useful, such software products should provide a secure graphical interface that can be accessed from any workstation on the network. This allows administrators to remotely manage the power to multiple servers from a central location,” he said.
Trends
“UPS manufacturers are facing increased demand for smart units that communicate interactively on LANs,” observed Ted Gladis, Marketing Assistant at Superior Electric. “Passive UPS technology is definitely on the way out.”
As a result, more UPS will include microcontrollers and microprocessors to provide the monitoring and control features being demanded. Companion software packages, available in multiple versions to run on popular operating systems, will interface with installed networks. But the greatest emphasis will be on increased reliability.
Since the backup power provided by any UPS is furnished from internal or external batteries, monitoring their usage and life expectancy is essential to providing the required reliability. Features enabling “hot swapping” of batteries will also be incorporated in more UPS.
Advancements in battery technology will affect UPS product life cycles, emphasized Mr. Gladis. “As battery manufacturers achieve better performance in a more compact design, UPS manufacturers must alter their existing products accordingly.”
A sampling of today’s surge suppressors, regulators and UPS and their key attributes is provided in the Power Conditioning Equipment chart. For detailed information on these products, please circle the corresponding numbers on the reader service card in this issue.
References
1. Jacob, G.W., “Analyze Your Source and Load Before You Buy Conditioning Equipment,” EE-Evaluation Engineering, December 1994, pp. 71-75.
2. Power Measurement Handbook, Valhalla Scientific Corp.
3. The Power Protection Handbook, American Power Conversion.
Table 1
Noise and Short-Term Transients
Conducted EMI and RFI emanating from electronic equipment, especially units containing switching power supplies; short spikes and impulses generated when electric/electronic equipment is switched on or off.
Spikes and Surges
Induced when switching on and off heavy electric equipment which represents inductive loads, such as motors or arc welders; also caused by lightning.
Undervoltage/Overvoltage
Momentary sags caused by excessive startup currents of electric machinery or capacitive loads. Long-term undervoltage and brownouts are usually due to planned voltage reduction by utilities or faulty power-distribution systems. Overvoltage is usually due to utility regulator problems.
Power Interruption
Temporary overloads causing circuit breakers to trip and shut off power. Prolonged power outages are due to power utility outages caused by equipment malfunctions or acts of nature.
Copyright 1995 Nelson Publishing Inc.
December 1995