An uninterruptible power system (UPS) is the central component of any well-designed power protection architecture, as it ensures that the power business-critical IT resources rely on is both dependable and clean. The UPS has two main functions:
- To provide power conditioning and backup power when utility power fails, either long enough for critical equipment to shut down gracefully so that no data is lost, or long enough to keep required loads operational until a secondary AC source, like a generator, comes online.
- To condition incoming power so that all-too-common sags, surges, spikes and outages don’t damage sensitive electronic equipment.
UPS systems differ in their topology and functionality but can be subdivided into double-conversion, line-interactive and standby designs, though these terms have been used inconsistently and manufacturers implement them differently. In addition, UPS technologies are further subdivided into either ‘Static’ or ‘Rotary’ systems, the key differentiator being how the load is fed i.e. via an inverter in a static system and via a rotating machine in a rotary system.
Voltage and frequency independent (VFI)
UPS systems are called dual or double conversion because incoming AC is rectified to DC to keep batteries charged and drive the inverter. The inverter re-creates steady AC power to run the IT equipment. VFI systems are generally considered the most robust form of UPS.
Voltage independent (VI)
Voltage independent or true line interactive UPS systems have a controlled output voltage, but the same output frequency as the input. Utility power feeds directly to the output and IT equipment, and the rectifier keeps the batteries charged. The inverter is paralleled with the output, compensating for voltage dips and acting as an active filter for voltage spikes and harmonics. Rectifier and inverter losses only occur when incoming power fluctuates. Flywheels and motor/generator sets also qualify as VI.
Voltage and frequency dependent (VFD)
Voltage and frequency dependent (VFD) or standby UPS, is operationally similar to VI and is sometimes mistakenly called line interactive. In VFD systems the inverter is turned off, so it can take several milliseconds to start producing power after being activated. The bypass is normally engaged, just as with VI, so equipment operates directly from the utility or generator. Since the inverter isn’t working until power fails, there is no voltage control or power consumed, enabling efficiencies as high as 99%. Power failure or voltage outside of range opens the bypass switch, disengaging input from output; the inverter starts operating from the batteries. The rectifier is only large enough to keep the batteries charged.
Whatever the UPS application, there are several factors that need to be considered for any system design.
UPS System Design Considerations
Basic functionality (e.g. static, rotary, double conversion etc…)
Energy storage and autonomy (e.g. battery, kinetic energy, diesel engine)
Performance characteristics (e.g. reliability, fault clearing, harmonics, power factor)
Robustness, life expectancy
System sizes & modularity (e.g. kVA ratings, overall footprint, power density)
Cost (TCO) and efficiency considerations
With over 30 years’ experience designing bespoke, resilient systems and value engineered options, Business Critical Engineering Limited can offer you independent expert advice to suit your specific requirements.
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