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Major Uninterruptible Power Supply
Technologies or Topologies
Compiled by Robert W. Meister WA1MIK
Each of the various UPS types deal with multiple power problems. Here's a short summary of the design features and the power problems they handle. In the table, the last three columns indicate which UPS types can be effective in dealing with the particular issue.
Stand-by or Off-line UPS technology (S):
Stand-by or off-line designs are intended to provide a low-cost solution for power failures, power sags and power surges. Utility power is provided during normal operation. Small voltage and frequency changes are not regulated by these units and are passed through to the equipment. When voltage or frequency changes become too severe, the inverter in the UPS converts DC battery power to AC power to run the equipment. Most of these units provide modified square-wave power. The APC Back-UPS series are an example of this type of UPS.
Line-interactive UPS technology (L):
Line-interactive technology provides basic power protection at mid-range prices. In addition to protection against power failures, power sags and power surges, these units protect against under-voltage and over-voltage. This design offers low-grade voltage regulation by "bumping" the utility voltage up or down before passing it through to your equipment. It does this by using an additional winding on the transformer that provides about 10-15% change. During the UPS voltage changes, the battery-powered inverter often powers the load, so the transformer wiring can be altered without upsetting things.
Although these units provide more voltage regulation than stand-by or off-line units, battery life is often sacrificed. The number of transfers to and from the battery by the line-interactive design exceeds that of the on-line units by as much as 10 to 1 during brownouts and power surges. Some of these units provide true sine-wave power. The APC Smart-UPS series are an example of this type of UPS.
On-line UPS technology (O):
These units are designed to provide complete power protection. They protect against all types of power problems by continuously using the battery and inverter to create 100% new, clean, regulated sine-wave AC power for your systems. Equipment is isolated from all types of power problems when supplied by an on-line UPS.
On-line units isolate equipment from harmonic distortion and are strongly recommended for critical applications that must meet the 5% maximum harmonic requirement stated in many equipment manufacturer's specifications and installation guides. On-line units are often called Double-Conversion units because the incoming AC is converted to DC to charge the battery and run the inverter, which converts the DC back to AC to power the load.
While the inverter is in use all the time, the battery is kept fully charged whenever there is utility power present. These units use battery power less than any other UPS technology. However, they often contain fans that run 100% of the time, so they may not be suited for quiet office environments.
This document explains the various topologies and provides advantages and disadvantages.
Nine Major Power Problems Handled
By Uninterruptible Power Supplies
|#||Problem Category||Causes and Other Information||S||L||O|
|1||Power Failure - A Total Loss Of Utility Power||Caused by lightning strikes, downed power lines, grid over demands, natural disasters, accidents.||Y||Y||Y|
|2||Power Sags - Short Term Low Voltage||Triggered by the startup of large loads, utility switching, utility equipment failure, or power service that's too small for the demand. Power sags involve voltages 80 to 85 percent below normal for short periods of time (one or more cycles). Possible causes are heavy equipment being turned on, large electrical motors being started, and the switching of power mains (internal or utility). A power sag can have effects similar to those of a power surge.||Y||Y||Y|
|3||Power Surge - (Spike) Short Term High Voltage Above 110% of Nominal||Can be caused by a lightning strike and can send line voltages to levels in excess of 6000 volts. Can be triggered by a rapid reduction in power loads, heavy equipment being turned off, or by utility switching. The results can potentially damage hardware. High voltage spikes occur when there is a sudden, rapid voltage peak of up to 6,000 volts with a duration of 100mS to 1/2 cycle. These spikes are usually the result of nearby lightning strikes, but there can be other causes as well.||Y||Y||Y|
|4||Under-voltage - (Brownout) Reduced Line Voltage for Extended Periods of a Few Minutes to a Few Days||Can be caused by intentional utility voltage reduction to conserve power during peak demand periods, or other heavy loads that exceed supply capacity. A reduction in the mains voltage without a complete loss of power. A brownout is a steady lower voltage state. An example of a brownout happens during peak electrical demands in the summer, when utilities can't always meet the requirements and must lower the voltage to limit maximum power.||N||Y||Y|
|5||Over-voltage - Increased Line Voltage for Extended Periods of a Few Minutes to a Few Days||Over-voltage can be triggered by a rapid reduction in power loads, heavy equipment being turned off, or by utility switching. A power surge that takes place when the voltage is 110% above rated RMS voltage for one or more cycles. The most common cause is heavy electrical equipment being turned off.||N||Y||Y|
|6||Electrical Line Noise - High Frequency Waveform Caused by RFI or EMI Interference||Can be caused by either RFI or EMI interference generated by transmitters, welding devices, SCR driven printers, lightning etc. Random, sporadic, or multi-frequency electrical signals that become part of a transmission making the signal or information more difficult to identify. Electrical Line Noise is defined as Radio Frequency Interference (RFI) and Electromagnetic Interference (EMI) and causes undesirable effects in circuits of computer systems. Sources of the problem include electric motors, relays, motor control devices, broadcast transmissions, microwave radiation, and distant electrical storms.||N||N||Y|
|7||Frequency Variation - A Change in Frequency Stability||Resulting from generator or small co-generation sites being loaded and unloaded. A frequency variation involves a change in frequency of more than 3Hz from the normally stable utility frequency of 60Hz. This may be caused by erratic operation of emergency generators or unstable frequency power sources.||N||N||Y|
|8||Switching Transient - Instantaneous Under-voltage (Notch) in the Range of Nanoseconds||Normal duration is shorter than a spike and generally falls in the range of nanoseconds. Switching transients take place when there is a rapid voltage peak of up to 20,000 volts with a duration time of 10 to 100 microseconds. They are commonly caused by arcing faults and static discharge. In addition, major power system switching disturbances initiated by the utilities to correct line problems may happen several times a day.||N||N||Y|
|9||Harmonic Distortion - Distortion of the Normal Waveform generally Transmitted by Nonlinear Loads||Distortion of the normal line waveform, generally transmitted by non-linear loads. Switch-mode power supplies, variable-speed motors and drives, copiers and fax machines are examples of non-linear loads. The presence of harmonics that change the AC voltage waveform from a simple sinusoidal to complex waveform. Harmonic distortion can be generated by a load and fed back into the AC mains, causing power problems to other equipment on the circuit.||N||N||Y|
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This page written 08-May-07 and first posted 23-Dec-07
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.