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UPS and Energy Efficient Power Protection Explained

Author : By Robin Koffler, General Manager of Riello UPS, UK

Uninterruptible power supplies (UPS) play two important roles in power protection: 1) providing a source of standby power in the event of a mains failure and 2) conditioning mains electricity to remove power problems such as sags, surges, brownouts and fluctuations that can damage equipment, cause computers to lock, fail or crash and lead to data loss or corruption.

There are different types of UPS for different types of power protection needs and UPS products are divided into categories based on which of the problems they address. In the first instance, the load has to be sized, then the UPS topology, range and particular model can be selected. Europe's largest UPS manufacturer, Riello UPS, would always recommend drawing up a Power Continuity Plan which will help provide a power protection system with the highest possible levels of reliability and resilience. The plan must allow for the continuous operation of critical data irrespective of the quality or availability of a local mains power supply.
A UPS sits between mains power supply and 'the load' (equipment and systems in a building or data centre that need a continuous source of electricity). The UPS conditions the electrical current, converts it from AC to DC, and cancels out potentially harmful power problems. In the case of a total mains failure, the UPS powers the load from its back-up battery until another energy source starts up or the mains is restored. 

Main types of UPS
There are three conventional types of uninterruptible power supply: 
* On-line (or Double Conversion)
* Line Interactive
* Offline (or Passive Standby)
An On-line or Double Conversion UPS is used in environments where equipment is very sensitive to power fluctuations. It maintains voltage and frequency variations within prescribed limits so that output is independent of fluctuations in supply. Alongside conditioning mains power to abolish problems, this is the only UPS solution that provides break-free supply to the plant or data centre during a power cut. On-line UPS from around 10kVA can be used in parallel configuration to achieve even higher operating resilience and system capacity levels.

A Line Interactive UPS uses built-in passive electronic regulation devices to stabilize and regulate voltage fluctuations. When mains supply is present, the output frequency of the UPS tracks the input of the mains. Line Interactive and On-Line UPS use tower or 'rackmount' case formats and sit next to their respective loads, typically in a data centre or computer room. The main advantage of the line interactive UPS is that the inverter/converter unit is always connected to the output, powering the equipment. This design allows for faster response to a power failure than a standby UPS. An Off-Line or Standby UPS is typically used to protect single PCs and protects against surges or mains failure. It is compact, designed for desktop or wall mounting and uses its own output to track voltage and frequency variations in the mains supply power so it is primed and ready to power the load for a short duration (10-15 minutes) from its built-in battery in the event of a power cut. Most power cuts last for less than a few seconds, which means the load is protected by this type of uninterruptible power supply. In the case of an extended power failure, this type of UPS is designed to initiate a controlled shutdown of equipment to negate potential damage but it is not designed to provide extended runtime.

Extended Uninterruptible Power Supply Benefits
Larger uninterruptible power supplies can work in parallel to improve system resilience, mean time between failure (MTBF) and availability. 
For a network-sized uninterruptible power supply, the installation of an external UPS maintenance bypass enables routine or emergency maintenance to be carried out (without supply disruption) during normal working-hours. 
Extended runtime (the time a load is running on back-up power) can be acquired using a combination of extra battery packs and an external standby power source, such as a diesel generator or fuel cell.

The transformer-based UPS system 
While many UPS are transformerless, organisations reliant on larger data centre and voice networks, industrial processes, security or hospital applications, may require transformer-based uninterruptible power supply. This gives greater robustness, albeit with slightly lower efficiency and greater heat and noise generation. As to the role each type of uninterruptible power supply plays in power protection implementations, choice is down to load category, load size and the level of resilience required and the need for redundancy. UPS systems are grouped by manufacturers like Riello UPS into specific applications such as: IT, network, industrial, enterprise and rackmount.

IT and Computing 
IT typically covers smaller applications (300VA - 3kVA); information and communications technology (ICT) including home PCs, small office/home office and data and voice networks. 
UPS for this type of application include off-line, line interactive and on-line products up to 3kVA. On-line uninterruptible power supplies in this range are transformerless so as to achieve a small footprint, minimal weight and low noise and heat output. They are usually located in computer rooms, next to protected equipment. However, their reduced size means fewer output sockets, so in highly populated environments a greater number may be required, which will have an impact on capital and operating costs.

Network Applications
Network applications are typically 3kVA to 80kVA. Corporate data and voice networks such as those run by ISPs (internet service providers) and telecommunication companies fall into this category. UPS for this type of application are transformerless, which, again, minimises size and weight. They act as a centralised power source and are hardwired at both input and output due to the high levels of power required. The UPS may also require connection to a three-phase incomer. The loads themselves are more likely to require three-phase and dedicated power distribution switchgear. 

Rackmount Applications
Rackmount applications start from around 700VA all the way up to 30kVA. Rack cabinets have become a common installation format for multiple server projects to reduce space and cable runs in computer rooms. Manufacturers of uninterruptible power supplies have responded by creating rackmount UPS formats from standard product ranges. UPS can be installed in racks that are strong enough to take their weight. A typical rack cabinet has a loading of 150-300kg. In this instance, the UPS often sits next to battery packs (part of the power protection installation) and protected loads. Sometimes UPS and batteries will have their own, dedicated rack. They can also offer parallel architectures and so provide additional resilience and a scalable solution for fast growing sites with space restrictions.

Battery Sets, Flywheels and Fuel Cells
Battery performance is one of the most central features within a Power Continuity Plan. In a UPS the battery set provides a source of power when the mains supply fails. UPS up to around 30kVA have capacity for internal battery sets. Where this is not the case, or additional runtime is required, external battery extension cabinets or rackmount trays are used. Flywheels and fuel cells are promising, emergent technologies but batteries still remain the most widely used solution for UPS and many other systems. 

Flywheels provide a source of short duration DC power and can be used with large applications such as UPS from 60kVA upwards. Flywheels can be used in place of a battery set to provide the 10-45 second ride-through time required for the automatic starting of a standby generator, or simply to reduce the initial discharge of a battery set. Flywheels have clear advantages as they have a lifespan of up to 20 years (they do have a higher capital cost but this is offset against battery replacements and maintenance charges that you get with batteries). Flywheels are also compact is size, they are quicker to charge than a battery set (3-5 minutes) and their operation is less affected by ambient temperatures.

Fuel cells
Fuel cells are increasing in popularity across a number of applications due to their high efficiency and low environmental impact in terms of zero exhaust fumes. A fuel cell depends on the chemical reaction which occurs when hydrogen and oxygen combine to produce water. A fuel cell harnesses this reaction and will generate electrical energy, in addition to water and heat, as long as a fuel (hydrogen gas) is present. Fuel cells have potential applications to UPS such as becoming an alternative source of AC power to the mains (this configuration still requires a battery set). The fuel cell is powered by natural gas or hydrogen and is used where normal mains supply is unreliable and there is a suitable supply of natural gas. Another application for UPS is that fuel cells can support an existing battery set. The UPS functions as a normal UPS with the fuel cell providing an additional source of dc energy. The advantages are that here the fuel cell is more efficient and allows for a much smaller battery set to be installed. UPS is a complex subject but if it is addressed properly using the products and services of a professional manufacturer, expensive pitfalls can be avoided.

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