Sunday, May 1, 2011

Standby or Offline UPS is one of the three main types of the static UPS. It is the simplest and least expensive uninterruptible power supply system. Actually, some don’t even consider it as a UPS, because of the presence of a momentary break during transfer or switching. Due to that, an offline UPS is sometimes described as a standby power supply (SPS). Nonetheless, the design employed by the standby UPS is the one commonly available at retail stores for protection of desktop and other small computers – it is usually available in sizes of up to 1.5 kVA only. 

Standby or Offline UPS Block Diagram
Standby or Offline UPS Block Diagram


Under normal operating conditions, the power for the sensitive load is supplied directly from the utility or mains supply and not by the inverter - the exact opposite operation when compared to an online UPS. The rectifier shall utilize the incoming supply to charge the battery, then, the battery and inverter are to be on “standby” until they are required to function. When the utility voltage supply falls below a standard set level, the standby UPS activates its DC-AC inverter circuitry through the automatic transfer switches. In this situation, the energy storage system or battery powers the inverter, which will then supply the sensitive load.

The transfer or switch time from the normal source to the battery-backed inverter is stated by most manufacturers to be around 1/4 cycle. In reality, it can take up to a little over 20 milliseconds, depending on the amount of time it takes the UPS to detect the interruption. Take note that the standby UPS transfer time is very important as most computers and other sensitive electronic devices would already start to malfunction in about 1/2 cycle or 8.33 milliseconds for 60 Hz (Refer to CBEMA and ITIC Curves). Thus, a transfer time of 1/4 cycle or 4.17 milliseconds would ensure continuity of operation for the critical load. Moreover, one must compare the UPS transfer time to the hold time of your power supply - which indicates the time a power supply can handle having no input before being interrupted. If the transfer time is longer than the hold time, then an offline UPS will not be able to protect the computers or other sensitive devices from interruptions, making it ineffective. Consequently, for a very vital function such as a data server, it is NOT generally applied.

Once successfully switched, the standby UPS is able to provide power for 15 to 20 minutes, depending on its capacity. That should be enough time for the computers to be properly shut down or an AC generator set to be brought online. Consequently, if the battery gets exhausted before backup power is connected, the inverter will be deenergized and the critical load interrupted.

Furthermore, a standby UPS does not typically provide any filtering as does an online type. Therefore, power conditioners may be installed in between the utility supply and the load. For example, using a harmonic filter and/or a surge suppressor in conjunction with the UPS. This  setup could also ensure that the transfer switches, as well as the sensitive loads, will not malfunction due to system disturbances during normal conditions.

Dugan, R., McGranaghan, M., Santoso, S., and Beaty, H.W. (2004). Electrical Power Systems Quality (2nd ed.). New York: McGraw-Hill.

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I am a Professional Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.