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On-line UPS Systems

On-line UPS provide the ultimate form of uninterruptible power with a digitally generated sinewave output supply, the ability provide back-up for long duration runtimes and a built-in automatic safe-failure-to mains static bypass arrangement. On line UPS can be single or three phase, with three phase systems generally capable of running in parallel/ redundant N+X configurations for added resilience and scalability.

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On-line Uninterruptible Power Supplies

On-line uninterruptible power supplies range from 500-700VA single phase up to larger three phase UPS systems from 10kVA to several hundred kVA systems that can be operated in parallel/redundant configurations to reach MVA-sized installations. On-line UPS systems include tower formats, 19inch rack mount, modular, transformerless and transformer-based systems.

An on-line UPS system is one of the three UPS topologies identified by IEC EN62040-3. An on-line UPS is Voltage and Frequency Independent (VFI) as it generates its own digital sinewave and is not reliant on the incoming mains power supply waveform. The UPS also features a static transfer switch (STS) and monitoring circuit. Should the output waveform of the UPS be seen to current limit and collapse due to an internal fault or output overload, the bypass automatically transfers the critical load(s) to the mains power supply. This may be direct or through EMI/RFI filters to maintain some form of spike and noise protection.

On-line UPS provide a pure sinewave output suitable for the most demanding critical IT and laboratory loads. The UPS is sometimes referred to as a double conversion UPS (AC-DC-AC) or triple conversion (AC-DC-DC-AC) UPS dependent upon its power electronic conversion stage arrangement.

In normal operation a rectifier (typically IGBT-based) converts the incoming AC waveform into two levels of DC. The first charges the lead acid battery and the second powers the inverter. The Inverter ‘inverts’ the DC supply into a suitable AC supply to power the load.

As the inverter section in an on-line UPS is set for continuous running, this means that it is ideal for both short and long duration runtimes. For long back-up times additional UPS batteries are connected to the UPS system. In a server room or datacentre environment the ideal battery backup time is 10-30minutes if there is a local standby power generator.

Valve Regulated Lead Acid (VRLA) UPS Batteries

The most commonly used battery for an on-line uninterruptible power supply is a valve regulated lead acid (VRLA) battery which is also referred to as a maintenance-free lead acids battery. There are two design life types available. The 5-year design life will typically require replacement around years 3-4 of operation and the 10-year design life VRLA battery around years 7-8.

The actual replacement time is dependent upon several factors and the charge/discharge cycles the UPS is battery is put through. The most important life-limiting factor is the ambient temperature range the UPS battery is operated within. This should be 20-25˚C. If the temperature is above this the battery life will degrade and for every 1˚C rise above 30˚C the battery design life halves.

For on-line UPS the battery set may sit inside the UPS cabinet or be supplied in an external cabinet or open or cladded rack arrangement. The UPS rectifier will generally provide enough recharge capacity to reach 80% within 24hours and then trickle charge to 100%.

In a typical on-line UPS, the lead acid battery is being used in standby mode, to provide intermittent battery power when the mains power supply fails or fluctuates.

Lithium-ion UPS Batteries

For sites that require off-grid power from an on-line UPS and continuous running or a faster recharge and cycle time, then lithium-ion batteries are a solution. Lithium batteries have a far more rapid charge/discharge cycling ability than lead acid but require a more sophisticated battery charging management circuit.

In the single-phase on-line UPS market, there are some small UPS systems with lithium batteries available. These tend to be for special applications including CCTV, EPOS, 5G and Edge computing.

Some modular UPS systems can accommodate lead acid or lithium batteries via firmware changes and alternate electronic connections. For most online UPS systems, they can only be configured and supplied to work with one or the other battery type.

In terms of three phase Monoblock on-line UPS, lithium batteries are offered as a separate self-contained system cabinet that can be installed alongside a commercial or industrial UPS system.

Lithium UPS batteries can also have a longer working life (10-15 years) and are less sensitive to higher ambient environments. There use within server room and datacentre applications could allow for a higher ambient and less demand on local cooling to improve energy efficiency.

UPS Warranty Periods

The warranty period for an on-line uninterruptible power is 24-36 months (2-3) with longer options up to 5 years (60 months) which may or may not include the battery set.

UPS Trade-in Service

For site retrofits and upgrades, Server Room Environments provides a complete trade-in service including a rebate that can be used to off-set the price of a new installation and disposal according to the relevant WEEE/RoHS regulations via a certified waste management centre.

For more information on our on-line UPS systems please contact our project team.