What Is An Uninterruptible Power Supply?

On-line UPS Systems

A relatively straightforward question always deserves a straightforward answer. Perhaps it’s just not as clear in the UPS systems industry.

In general terms, an uninterruptible power supply is a device that stores energy and release it to provide a source of power when the mains power supply fails or fluctuates. So it’s a battery or at least a battery management system? Well batteries form a part of a UPS system but the two other primary components needed for an uninterruptible power supply are:

  • Battery Charger: a method to charge the battery. This may be what you would commonly know as a battery charger in a very simple design or be part of the UPS Rectifier assembly. The rectifier converts the mains AC sine wave into the DC levels required to charge the battery and drive an inverter.
  • Inverter: the inverter takes an DC power source (battery or Rectifier output) and converts this to an AC supply to power the connected loads.

Not all uninterruptible power supplies are however ‘uninterruptible’. Actually you may consider some to be ‘Interruptible Power Supplies’. Two types of design fall into this category:

  • Standby UPS: the most basic type of UPS system in which the mains power supply is routed through a filtering circuit to ‘clamp’ spikes and electrical noise. When mains power is present the UPS charges its battery via a battery charging circuit. The system may have an automatic voltage stabiliser (AVS) or automatic voltage regulator (AVR) assembly. When the mains power fails, an inverter switches on and powers the load from the battery set. The inverter may take up to 10mS to power up and this should be sufficient time to ensure a ‘no-break’ supply. This is dependent on the electrical capacitance of the Switch Mode Power Supply (SMPS) within the computer, PC, server or other type of network peripheral. The inverter output waveform will typically be a square-wave or step-wave.
  • Line Interactive UPS: in this type of system the inverter is line interactive. This means that during normal mains present operation, the battery is charged by a battery charger and the mains power is fed through an automatic voltage stabiliser (AVS) or automatic voltage regulator (AVR) assembly. An AVS/AVR has a wide input voltage window and will step-up or down the incoming mains voltage. This is known as a ‘Buck/Boost’ feature. The output of the AVS/AVR may be fed through an EMI (electro-magnetic interference) filtering circuit and a relatively clean and stable supply is provided to the load. The mains frequency is unaffected. Only the voltage is corrected. When the mains power fails or fluctuates outside the operational range of the AVS/AVR an inverter takes over supplying the load with power from the battery set. The inverter is turn-on period can lead to a small ‘blip’ in supply of around 4mS, a period well within the electrical storage capacitance of a modern Switch Mode Power Supply (SMPS) – the power supply within your computer or server. This type of UPS system is therefore marginally ‘interruptible’ but nothing that should disturb a modern PC or server. The inverter output waveform will generally be either a square-wave or sine wave. The inverter may be powered but idle when mains power is present so that the reaction time is far faster than that of a standby power system.

For a UPS system to be truly ‘uninterruptible’ it must provide a no-break electrical supply and the only type of UPS that actually does this is an ‘on-line UPS system’.

Online Uninterruptible Power Supplies

Online systems have a battery, DC rectifier/converter and inverter assembly. The key points are:

  • Inverter: the inverter is constantly powered and supplying the load with an AC power source. The outputwave is a digitally generated sine wave that is normally ‘purer’ than you would see from a normal wall socket.
  • Rectifier: at the front end a DC rectifier/converter takes the AC input mains power and converts this into two levels of DC. One to power a charging circuit and the other to power the inverter.
  • Automatic Bypass: the output will be connected through an automatic bypass switch. This may be static or relay-based. The static bypass is break-free where as a relay-based an introduce a level of interruption but as with a line interactive UPS design the interruption is minimal.

The on-line UPS topology is not just the only true form of uninterruptible power supply. Thanks to its built-in arrangement, the UPS is fault-tolerant. If the inverter develops a fault condition or is overloaded the UPS transfers the load to the bypass supply and this prevents interruption of supply to the load. It is therefore the type most recommended for data centres, comms rooms and server power protection.

The three types of UPS system vary in price and the size range available. Standby UPS systems are generally available up to 1kVA and offer the lowest price of the three topologies. Line Interactive are the next level of ‘uninterruptible power supply’ and you can find examples up to around 2kVA. On-line systems are the most expensive but provide true uninterruptible output power and their size range can run from 500VA single phase to over 1MVA three phase.

In the small-end sub 2kVA UPS manufacturers tend to be a bit ‘sketchy’ when describing their standby and line interactive UPS systems. At EcoPowerSupplies you can rest assured that we will help you select the right topology for your application. For more information visit: www.EcoPowerSupplies.com or call us on 0800 612 7388.

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This entry was posted in UPS Systems