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See also: "Power factor

A problem in the combination of a double-conversion UPS and a generator is the voltage distortion created by the UPS. The input of a double-conversion UPS is essentially a big rectifier. The current drawn by the UPS is non-sinusoidal. This can cause the voltage from the AC mains or a generator to also become non-sinusoidal. The voltage distortion then can cause problems in all electrical equipment connected to that power source, including the UPS itself. It will also cause more power to be lost in the wiring supplying power to the UPS due to the spikes in current flow. This level of "noise" is measured as a percentage of ""total harmonic distortion of the current" (THDI). Classic UPS rectifiers have a THDI level of around 25%–30%. To reduce voltage distortion, this requires heavier mains wiring or generators more than twice as large as the UPS.

There are several solutions to reduce the THDI in a double-conversion UPS:

Passive power-factor correction[edit]

Passive PFC

Classic solutions such as passive filters reduce THDI to 5%–10% at full load. They are reliable, but big and only work at full load, and present their own problems when used in tandem with generators.

Active power-factor correction[edit]

Active PFC

An alternative solution is an active filter. Through the use of such a device, THDI can drop to 5% over the full power range. The newest technology in double-conversion UPS units is a rectifier that does not use classic rectifier components (thyristors and diodes) but uses high-frequency components instead. A double-conversion UPS with an "insulated-gate bipolar transistor rectifier and inductor can have a THDI as small as 2%. This completely eliminates the need to oversize the generator (and transformers), without additional filters, investment cost, losses, or space.


"Power management (PM) requires

  1. The UPS to report its status to the computer it powers via a communications link such as a "serial port, "Ethernet and "Simple Network Management Protocol, GSM/"GPRS or "USB
  2. A subsystem in the "OS that processes the reports and generates notifications, PM events, or commands an ordered shut down.[15] Some UPS manufacturers publish their communication protocols, but other manufacturers (such as "APC) use "proprietary protocols.

The basic computer-to-UPS control methods are intended for one-to-one signaling from a single source to a single target. For example, a single UPS may connect to a single computer to provide status information about the UPS, and allow the computer to control the UPS. Similarly, the USB protocol is also intended to connect a single computer to multiple peripheral devices.

In some situations it is useful for a single large UPS to be able to communicate with several protected devices. For traditional serial or USB control, a signal replication device may be used, which for example allows one UPS to connect to five computers using serial or USB connections.[16] However, the splitting is typically only one direction from UPS to the devices to provide status information. Return control signals may only be permitted from one of the protected systems to the UPS.[17]

As Ethernet has increased in common use since the 1990s, control signals are now commonly sent between a single UPS and multiple computers using standard Ethernet data communication methods such as "TCP/IP.[18] The status and control information is typically encrypted so that for example an outside hacker can not gain control of the UPS and command it to shut down.[19]

Distribution of UPS status and control data requires that all intermediary devices such as Ethernet switches or serial multiplexers be powered by one or more UPS systems, in order for the UPS alerts to reach the target systems during a "power outage. To avoid the dependency on Ethernet infrastructure, the UPSs can be connected directly to main control server by using GSM/GPRS channel also. The SMS or GPRS data packets sent from UPSs trigger software to shut down the PCs to reduce the load.


Battery cabinet

The run-time for a battery-operated UPS depends on the type and size of batteries and rate of discharge, and the efficiency of the inverter. The total capacity of a "lead–acid battery is a function of the rate at which it is discharged, which is described as "Peukert's law.

Manufacturers supply run-time rating in minutes for packaged UPS systems. Larger systems (such as for data centers) require detailed calculation of the load, inverter efficiency, and battery characteristics to ensure the required endurance is attained.[20]

Common battery characteristics and load testing[edit]

When a lead–acid battery is charged or discharged, this initially affects only the reacting chemicals, which are at the interface between the electrodes and the electrolyte. With time, the charge stored in the chemicals at the interface, often called "interface charge", spreads by "diffusion of these chemicals throughout the volume of the active material.

If a battery has been completely discharged (e.g. the car lights were left on overnight) and next is given a fast charge for only a few minutes, then during the short charging time it develops only a charge near the interface. The battery voltage may rise to be close to the charger voltage so that the charging current decreases significantly. After a few hours this interface charge will spread to the volume of the electrode and electrolyte, leading to an interface charge so low that it may be insufficient to start a car.[21]

Due to the interface charge, brief UPS self-test functions lasting only a few seconds may not accurately reflect the true runtime capacity of a UPS, and instead an extended recalibration or rundown test that deeply discharges the battery is needed.[22]

The deep discharge testing is itself damaging to batteries due to the chemicals in the discharged battery starting to "crystallize into highly stable molecular shapes that will not re-dissolve when the battery is recharged, permanently reducing charge capacity. In lead acid batteries this is known as "sulfation but also affects other types such as "nickel cadmium batteries and "lithium batteries.[23] Therefore, it is commonly recommended that rundown tests be performed infrequently, such as every six months to a year.[24][25]

Testing of strings of batteries/cells[edit]

Multi-"kilowatt commercial UPS systems with large and easily accessible battery banks are capable of isolating and testing individual cells within a battery string, which consists of either combined-cell battery units (such as 12-V lead acid batteries) or individual chemical cells wired in series. Isolating a single cell and installing a jumper in place of it allows the one battery to be discharge-tested, while the rest of the battery string remains charged and available to provide protection.[26]

It is also possible to measure the electrical characteristics of individual cells in a battery string, using intermediate sensor wires that are installed at every cell-to-cell junction, and monitored both individually and collectively. Battery strings may also be wired as series-parallel, for example two sets of 20 cells. In such a situation it is also necessary to monitor current flow between parallel strings, as current may circulate between the strings to balance out the effects of weak cells, dead cells with high resistance, or shorted cells. For example, stronger strings can discharge through weaker strings until voltage imbalances are equalized, and this must be factored into the individual inter-cell measurements within each string.[27]

Series-parallel battery interactions[edit]

Battery strings wired in "series-parallel can develop unusual failure modes due to interactions between the multiple parallel strings. Defective batteries in one string can adversely affect the operation and lifespan of good or new batteries in other strings. These issues also apply to other situations where series-parallel strings are used, not just in UPS systems but also in "electric vehicle applications.[28]

Consider a series-parallel battery arrangement with all good cells, and one becomes shorted or dead:

The only way to prevent these subtle series-parallel string interactions is by not using parallel strings at all and using separate charge controllers and inverters for individual series strings.

Series new/old battery interactions[edit]

Even just a single string of batteries wired in series can have adverse interactions if new batteries are mixed with old batteries. Older batteries tend to have reduced storage capacity, and so will both discharge faster than new batteries and also charge to their maximum capacity more rapidly than new batteries.

As a mixed string of new and old batteries is depleted, the string voltage will drop, and when the old batteries are exhausted the new batteries still have charge available. The newer cells may continue to discharge through the rest of the string, but due to the low voltage this energy flow may not be useful, and may be wasted in the old cells as resistance heating.

For cells that are supposed to operate within a specific discharge window, new cells with more capacity may cause the old cells in the series string to continue to discharge beyond the safe bottom limit of the discharge window, damaging the old cells.

When recharged, the old cells recharge more rapidly, leading to a rapid rise of voltage to near the fully charged state, but before the new cells with more capacity have fully recharged. The charge controller detects the high voltage of a nearly fully charged string and reduces current flow. The new cells with more capacity now charge very slowly, so slowly that the chemicals may begin to crystallize before reaching the fully charged state, reducing new cell capacity over several charge/discharge cycles until their capacity more closely matches the old cells in the series string.

For such reasons, some industrial UPS management systems recommend periodic replacement of entire battery arrays potentially using hundreds of expensive batteries, due to these damaging interactions between new batteries and old batteries, within and across series and parallel strings.[29]


See also[edit]


  1. ^ "Electricity storage: Location, location, location … and cost - Battery storage for transmission support in Alaska". eia.gov. Energy Information Administration (EIA). 2012. Retrieved July 23, 2012. 
  2. ^ E-book on choosing a UPS topology based on application type "Avoiding Trap Doors Associated with Purchasing a UPS System" (PDF). 
  3. ^ Solter, W. (2002), A new international UPS classification by IEC 62040-3, "doi:10.1109/INTLEC.2002.1048709 
  4. ^ Detailed explanation of UPS topologies "High-Availability Power Systems, Part I: UPS Internal Topology" (PDF). November 2000. 
  5. ^ "Hydrogen Fuel Cell UPS". 
  6. ^ "UPS On-Line Uninterruptible Power Supply Backup Power Source". Archived from the original on October 4, 2013. 
  7. ^ a b "Hybrid Rotary UPS" (PDF). Archived from the original (PDF) on December 4, 2014. 
  8. ^ a b http://h20000.www2.hp.com/bc/docs/support/SupportManual/c01173322/c01173322.pdf["dead link]
  9. ^ My Ton (Ecos Consulting), Brian Fortenbery (EPRI), William Tschudi (LNBL) (January 2007). "DC Power for Improved Data Center Efficiency" (PDF). Lawrence Berkeley National Laboratory. Archived from the original (PDF) on 2008-08-20. 
  10. ^ Active Power. "15 Seconds versus 15 Minutes: White Paper 107 Designing for High Availability" (PDF). 
  11. ^ Tripp Lite: UPS Buying Guide, http://www.tripplite.com/products/ups-Buying-Guide
  12. ^ Detailed explanation of optimized N+1 configurations"Balancing Scalability and Reliability in the Critical Power System: When Does N + 1 Become Too Many + 1?" (PDF). 
  13. ^ Detailed explanation of UPS redundancy options"High-Availability Power Systems, Part II: Redundancy Options" (PDF). 
  14. ^ Refer to safety standard IEC 60950-22 or a local derivative according to location e.g. EN 60950-22 (Europe); UL 60950-22 (USA)
  15. ^ Raymond, Eric Steven. UPS HOWTO, section 3.3. The Linux Documentation Project, 2003–2007.
  16. ^ Generex. "Multi-XS User Manual" (PDF). Multi-XS is an active RS232 data switch, designed to handle serial communications of one UPS with up to 5 / 10 computers 
  17. ^ APC AP9207 Share-UPS, User Manual, pp. 6–7, Port 1 is called the Advanced port because it supplies smart signaling, which provides the advanced capabilities available to a server running PowerChute plus software. The Advanced port provides full access to the Computer Interface port of the UPS. Ports 2–8 on the rear panel of Share-UPS are called Basic ports because they supply simple UPS signaling for On Battery and Low Battery conditions in the UPS. "Share-UPS User Manual" (PDF). Archived from the original (PDF) on April 24, 2012. Retrieved November 14, 2011. 
  18. ^ An example of an Ethernet UPS controller: Liebert IntelliSlot Web Card Communications Interface Card
  19. ^ APC Application Note #67 "APC Network Management Card Security Implementation" (PDF). Archived from the original (PDF) on April 24, 2012. Retrieved November 14, 2011. 
  20. ^ "How to calculate battery run-time". PowerStream Technologies. Retrieved 2010-04-26. 
  21. ^ Saslow, Wayne M. (2002). Electricity, Magnetism, and Light. Toronto: Thomson Learning. pp. 302–4. "ISBN "0-12-619455-6. 
  22. ^ Peter M. Curtis (2011). Maintaining Mission Critical Systems in a 24/7 Environment. Wiley. pp. 261–262. "ISBN "9781118041628. 
  23. ^ Michael F. Hordeski (2005). Emergency and backup power sources: preparing for blackouts and brownouts. The Fairmont Press, Inc. "ISBN "9780881734850. 
  24. ^ Leonardo Energy. "Maintenance Manager's Guide, Section 2.1". Retrieved August 1, 2012. ["dead link]
  25. ^ APC Inc. "Knowledgebase article: What is the expected life of my APC UPS battery?, Answer ID 8301". ["dead link]
  26. ^ "Maintaining and Testing Your UPS System to Ensure Continuous Power, Section: Maintaining a Battery Bank". The Data Center Journal. ["dead link]
  27. ^ BTECH Inc, BTECH's Focus – Predicting Battery Failure and Installation Manual, page 18, showing sensor wires for each cell/battery on a battery string, and also note that the current transducer sensors to detect cross-string series-parallel current recirculation.
  28. ^ mpoweruk.com, Battery and Energy Technologies, Cell Balancing, Woodbank Communications Ltd, Chester, UK.
  29. ^ datapowermonitoring.com, Battery Asset Management: VRLA ageing characteristics, Bart Cotton, founder and CEO, Data Power Monitoring Corporation, Batteries International, Jan 2005 Archived April 6, 2013, at the "Wayback Machine.

External links[edit]

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