Evaluation criteria for UPS selection

While selecting an UPS for a requirement, different manufacturers have different specifications, which makes the comparison confusing. The comparison and selection should be based on the below technical parameters and the relevance for the particular requirement.

The 3 main components of the UPS are rectifier, inverter and battery. The rectifier acts as a load to the electrical mains and the following characteristic of the rectifier will decide the type of upstream infrastructure required.

• Input current distortion (THDI) : Harmonics,
• Input power factor,
• Input start-up current,
• Number of wires (3ph or 3ph+Neutral),
• Efficiency (influences global efficiency),
• Maximum output power,
• Input voltage & frequency tolerances
• Generator compatibility

The inverter of the UPS System will be a source for the critical loads connected to the UPS and based on the need of the load the following parameters decides the quality & capacity of the UPS.

• Nominal apparent power (VA)
• Nominal active power (W)
• Capability to support load Power factor (low power factor and Leading mainly)
• Inverter efficiency (influences global efficiency)
• Output voltage distortion (ThdV, with different load types)
• Max load current crest factor
• Overload, Inrush current and short-circuit capability,
• With or without galvanic isolation

The battery is the heart of the UPS system and the selection of battery is more important as it decides the duration of operation in the event of mains failure. The battery also plays an important role in deciding the Capex and Opex cost. The following points has to be evaluated to have the right battery configuration.

• KW considered for battery sizing
• End cell voltage
• Ageing factor
(incase of low backup time)
• No of battery bank in parallel
• Design life of the selected battery
• Required of charging current and its compatibility with UPS