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The Different Types of UPS Systems

Uninterruptible Power Supplies are designed to provide an alternative source of standby or backup power when the mains power supply fails. This is generally derived from a sealed lead acid, maintenance free battery set and may occur with or without a break. In addition when mains power is present, uninterruptible power supplies can and do vary in the quality of their output supplies and this is often a characteristic of their design.

EN/IEC 62040-3 defines three types of static uninterruptible power supplies, each with varying degrees of electrical performance and therefore ability to protect critical loads. The three types include:

  • On-line UPS – which have a digitally derived output supply provided under normal circumstances by a constantly running inverter, with regulated output voltage and frequency tolerance. The inverter draws a DC supply from either a rectified mains power supply (AC-DC) or the battery set, and because the inverter is running continuously the load does not experience a break in supply. On-line UPS are also known as ‘Double-Conversion’ UPS because of the AC-DC and DC-AC operation. This type of UPS also includes an automatic bypass. A sensing circuit monitors the inverter output supply and automatically transfers the load to the mains power supply (if available) if the inverter output starts to collapse; if there is an overload or inverter output problem. Under EN/IEC 61000-2-2 On-line UPS are classified as Voltage and Frequency Independent (VFI)
  • Line Interactive UPS – have an inverter which is normally idle. The UPS is classed as Voltage Independent (VI). A built-in automatic voltage stabilizer responds to voltage changes in the mains power supply within a pre-defined input window and corrects for sags, surges and brownouts. EMI filters are typically used to suppress spikes and electrical noise. When the mains power supply fails, the inverter is connected to the load and is powered by the battery set. The transfer time is dependent upon where in the sinewave the break in supply occurs but is typically 2-4mS and therefore well inside the tolerances of a modern file server or PC with a switch mode power supply.
  • Standby UPS – are both voltage and frequency independent (VFD) according to EN/IEC 6100-2-2. In this type of UPS a basic filter clamps to reduce spikes and electrical noise to pre-set levels. The inverter is only activated when the mains power supply fails or fluctuates widely. A standby UPS may include a low specification automatic voltage stabiliser or rely purely on its inverter to correct for mains power supply voltage changes that occur during sag, surge and brownout conditions.

UPS System Selection Criteria

On-line UPS systems with their constantly running, regulated output inverter provide the best form of power protection for critical loads. The built-in automatic bypass system also provides a safe failure-to-mains should the UPS inverter fail or the system become overloaded. As the inverter is sized for continuous running, on-line UPS are also the most suited for long runtime applications. This can be achieved by installing additional battery extension packs.

On-line UPS typically run from 700VA to 800kVA and can be either transformerless (typically 700VA-200kVA) or transformer-based (10-800kVA). Transformerless UPS tend to be quieter in operation, more compact and efficient, and therefore more suited to small to mid-range datacenter type applications. Transformer-based UPS are more robust and suited industrial or large datacenter applications. Larger on-line UPS also tend to be capable of scalable operation and are therefore suited to parallel/redundant operation.

On-line UPS can also be used as frequency converters (50/60Hz and 60/50Hz). Some designs are available for 400Hz aircraft applications. On-line UPS are also capable of provide load protection from harmonics using additional filters if required.

Line Interactive UPS provide the next best level of protection. Their inverters generally have a sinewave output and a line interactive can provide reasonable protection from sags, surges, brownouts, spikes and electrical noise. Line Interactive UPS typically run up to 2kVA, with some of the larger sized models capable of extended runtime pack installation. If this is not the case the UPS has to be oversized for the load to achieve the runtime, if practically possible.

Standby UPS are typically only available in models up to or less than 1000VA (1kVA) in size. The offer very basic power protection and a limited runtime capability when the mains power supply fails.

Energy Efficiency

Efficiency also varies with UPS topology and its configuration. For example transformerless on-line UPS can achieve efficiencies of around 96% in on-line mode. Transformer-based UPS systems can achieve similar levels of efficiency using high efficiency transformers. Higher levels of 98-99% can be achieved using built-in eco-mode functionality which can mimic a line interactive UPS. Energy efficiency is an important measure within a datacenter application as the larger the UPS the greater the cost of electrical energy wasted and heat generated. In smaller systems, protecting rack cabinets, efficiency and heat output is also important but more so because of the confined rack space – and heat generated by the servers themselves.

Please call us on 0800 612 7388, complete an enquiry form form or email us for more information on the EcoPowerSupplies range of UPS systems.