How to Select A Single Phase UPS System

In a server room or data centre environment, single phase UPS systems can play a critical role keeping file servers and IT networks running during a power outage. Even when the mains power supply is present a single-phase UPS can provide protection from mains borne power problems.

Single Phase UPS System Checklist

To make sure you install the right single-phase UPS for your application use the following guide and let us know if you come across other aspects that you feel are important to consider.

UPS Topology

BS EN 62040-3 characterises three types of UPS topology including standby, line interactive and on-line UPS. For critical single-phase applications like server protection then the most appropriate formats are line interactive and on-line.

Line interactive UPS are classed in BS EN 62040-3 as Voltage Independent (VI). When mains power is present, a line interactive UPS will provide some protection from mains power problems e.g. brownouts, sags, surges, spikes and electrical noise using its built-in automatic voltage stabiliser. The inverter is powered up but does not support the server load until the input mains power supply fluctuates widely or there is a complete power outage. Then the inverter switches in to the line and provides power to the server loads using a battery set. Line interactive UPS can therefore provide a momentary break in electrical power when transferring to and from battery power, but this is generally well within the internal capacitance (energy reserve) of the switch mode power supply within the servers themselves.

On-line UPS are classed as Voltage and Frequency Independent (VFI). This is because the inverter is always connected to the load during normal running and provides a digitally generated sinewave output. When the mains power supply fails, the inverter simply relies on the DC supply from the battery set to power the inverter. On-line UPS also feature an additional safety feature compared to line interactive UPS in the form of a built-in automatic bypass. If the inverter output starts to collapse (during an overload or UPS internal fault) the load is automatically and safely transferred to the mains power supply (if present).

Overall on-line UPS provide superior and break-free power protection for server type loads and can have longer runtime battery packs installed. Line interactive topology UPS typically max out at 2kVA with on-line UPS capable of providing up to 20kVA single phase output available. Some on-line UPS can also be installed in a parallel redundant (N+X) configuration.

UPS Form Factor

The question of UPS form factor or format is important. If you are using 19inch rack mount server racks to house your servers, you may also want to use a similar format UPS system. Most UPS manufacturers offer tower, rack mount and dual format UPS units, with a Dual format capable of being installed as a tower or rack mount UPS.

If the tower format is chosen this will invariable take up a lot more U height within the server cabinet. The UPS will have to sit on the floor of the cabinet or a special shelf. If there are battery extension cabinets, to give extra runtime, then these will also have to be installed on the shelf or even outside or alongside the server cabinet.

A 19 inch rack mount UPS system, as the description implies, can be installed in a suitable server cabinet. The UPS may be installed on a shelf or if the weight is generally below 30Kg on slide out shelves. The advantage of this type of arrangement is that a rack mount UPS will more efficiently fill the space available as a horizontally mounted unit that will probably only take up from 1 to 4U in height. Any battery extension packs installed may also be rack mountable as may a UPS maintenance bypass switch if installed.

UPS Battery Technology

Traditionally the online choice for a UPS battery was Valve Regulated Lead Acid (VRLA) and the batteries were known as VRLA maintenance free lead acid batteries. Lead acid was the most appropriate battery technology for UPS systems as this type of battery is ideal for standby power and infrequent charge/discharge cycles. As lead acid batteries dominate the standby power world, the manufacturing volumes give rise to scale economies and the batteries are widely available.

Within recent years, Lithium-ion single phase UPS systems have become available, offering more advantages over those UPS fitted with lead acid batteries. The Lithium-ion (Li-ion) battery technology has emerged as a UPS battery contender thanks to the need for more cyclic batteries in the energy storage, electric vehicle and mobile phone industries.

There are several types of lithium-ion battery available and the areas where they stand out over lead acid batteries include:

  • Longer working life of up to 10-15 years compared to 3-7 for lead acid batteries. Lead acid batteries can be 5 or 10-year design life batteries with replacement expected around years 3-4 and 7-8 respectively.
  • Temperature performance in that lead acid batteries require a 20-25˚C ambient. For every 1degree rise above 30˚C battery design life halves. Lithium-ion batteries can work up to 40˚C without degradation as can the electronics on a UPS allowing server room and data centre operators to increase the ambient temperatures within their facilities and lower their cooling costs. For more information on data centre environments we recommend ASHRAE: https://www.ashrae.org/about/news/2016/data-center-standard-published-by-ashrae
  • Smaller size and weight due to the increased power densities over lead acid batteries leading to more compact designs and easier installation.
  • Lower maintenance and improved Total Cost of Ownership (TCO) due to the longer operating life and even with a 30% premium for Li-ion technology over traditional lead acid UPS batteries.
  • Improved operational performance and more suited to multiple and faster charge/discharge cycles. Lead acid batteries typically have a design life of 300-400 complete charge/discharge cycles and can take up to 24 hours to recharge to 80% capacity. Any quicker than this can degrade the battery. Lithium-ion UPS batteries are far more suited to energy storage type applications with multiple power outages over a short time frame, requiring a fast ability to recover to full charge capacity.

Other aspects to consider for any server room or datacentre application include:

  • Electrical works
  • Maintenance bypass arrangements
  • Load and PDU connections
  • Network monitoring over TCP/IP and SNMP or Modbus
  • Maintenance and inspection arrangements

These are just some of the essential questions that must be answered to select the most appropriate single-phase UPS system for a serve room or datacentre application. For help and advice please contact our power consultants who can offer advice and support or visit your installation for a comprehensive UPS site survey.

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