During an outage, a UPS system ensures that critical operations continue uninterrupted in an office by providing interim power until it switches to a generator or other power source. UPS systems can also alert staff of potential problems and automatically shut down equipment to minimise disruptions. Hence, managers need to properly gauge the size of the UPS systems required, based on the load it can support. But determining the proper UPS system size as per the load requirements can be challenging as the specifications of these devices are not always clearly stated. For instance, voltage information on equipment nameplates is not always accurate for sizing purposes and power factors can be confusing.
By Srabani Sen
Monday, April 09, 2012: What specifications should one look for?
Determining the size of a UPS system for an office is not rocket science, but one needs to ensure that it satisfies safety criteria and electromagnetic compatibility standards, while offering intelligent monitoring and network management functions. However, one needs to consider certain parameters that are listed below:
Load requirement: One needs to choose an appropriate UPS system that can support the
required load, both VA and W—you need to add a 25 per cent buffer capacity. Usually, a 20 per cent higher capacity than actually required is recommended to make sure the UPS system is not subjected to 100 per cent load under any circumstance. This precaution increases the life of the UPS system and also allows the end users to increase their load in future, up to the buffer capacity taken.
As a normal practice, a UPS system should never be loaded beyond 80 per cent of its capacity. An ideal system should also have a similar or higher power factor than the load. This will ensure that you do not have to further oversize the UPS. Too much oversizing of the UPS leads to reduction in efficiency, thus leading to wastage of power.
For very critical applications, one should also consider redundancy of UPS systems. Redundancy can be achieved by having more than one UPS system so that if one fails, the other takes over its load and that load gets uninterrupted power supply. The normal practice is to have two similar sized UPS systems instead of one (1+1). In this case, the redundancy is achieved at the cost of efficiency, because when both the UPS systems are working, they are loaded to less than 40 per cent of their capacity. To overcome
this problem, modular UPS systems have been designed, which use much smaller modules (10 kVA to 50 kVA) to make up larger UPS systems. Modular systems are also generally designed to accept one more module than is required for their rated capacity, making them inherently N+1, which is capable of operating at much higher efficiency levels and at a lower cost. The other benefit of a modular design is its ability to grow in capacity as per one’s needs.
It is also very important to understand runtime to make sure the battery bank is designed to get maximum output/backup support. Sometimes, the end users’ backup requirements might be just one hour; whereas they land up investing on a higher rated battery for longer periods of backup, which also takes up more office space.
One needs to calculate the volt ampere (VA) and watt (W) rating (maximum apparent power in VA and maximum actual
power in W [VA x power factor]). First of all, one needs to list down the entire critical load to be supported by the UPS system (computers, printers, lights, fans, etc). Thereafter, calculate the VA and W rating of each product and calculate the actual load to be supported.
UPS sizing can be determined if one knows the wattage, volt ampere (VA) or power factor (PF) of the load—any two is sufficient (VA x PF = W). Calculate the VA and W requirement of all the equipment to be connected to the UPS system since it has to cater to both the VA as well as W of the load. UPS systems are normally of 0.7, 0.8 or 0.9 PF, that is, a 10 kVA UPS may be designed to provide 7 KW, 8 KW or 9 KW.
It is important to understand that in UPS systems, there are three specifications for PF—input PF, rated PF and load PF. Since the UPS system gets incorporated between the main supply and critical loads, the UPS acts as a source to the critical load and at the same time is a load on your main supply. The input PF determines how the power is being drawn from the main supply and should be close to unity. Currently, UPS systems are available with active power factor correction (APFC) technology, which helps to improve the input power factor up to 0.99. The rated PF determines the capacity of the UPS in kVA/kW. For example, a 100 kVA UPS system rated at 0.8PF will deliver 80kW. The output PF of the UPS system determines the suitability of different PF loads that can be connected to the UPS system without de-rating the capacity. As an example, a UPS system with output PF of 0.6 lag to 0.9
lead will support almost all types of loads without degrading the performance. The wider the output PF range, the better the UPS system.
Input power factor correction (PFC) is another important feature for a UPS system. General online UPS systems with SCR models utilise the input voltage phase at various angles according to the requirements of load and charging, but a user has to pay for the full wave. A PFC model, on the other hand, uses digital signal processing PFC technology and utilises the full wave while drawing only the required current in a similar phase angle, thereby consuming less power and obviously slashing the electricity bill.
A normal SCR based 5 kVA online UPS system consumes 28 Amp AC current when running at 80 per cent load, while PFC online UPS systems take 17 Amp AC current for similar loads, thus saving 11 Amp of electricity consumption. This saving works out to roughly 2 kVAH per hour, and if the unit runs 10 hours a day, this becomes 20 units a day; which is roughly 500 units a month and 6000 units a year. Going by the minimum commercial rate of Rs 5 per unit, the customer can save Rs 30,000 a year, an amount that is almost the entire cost of the 5 kVA PFC online UPS.
The products that will run on a UPS system must be efficient and reliable. This will help to reduce the electricity
consumed and thereby reduce the operating costs (OPEX). For centralised UPS configurations with higher ratings and for utility/facilities, it may be beneficial to run the UPS system on eco-mode, which further improves the overall efficiency by a few percentage points leading to significant savings. The compact size of the product ensures that it occupies minimum high rental floor space. If you are looking for a product with the capacity to be expanded in future as per the load requirements, then modular UPS systems are the best solution. These will gain popularity in the future because of the ease of scalability, maintenance and repairs, apart from offering the flexibility to build redundant configurations.
Assessing the requirements: If one is looking for a UPS system for individual users, small businesses, small servers, AV equipment, etc, then a line interactive UPS system with AVR (automatic voltage regulator) is ideal. If the requirement is for a medium sized business, a small data centre, or any other critical application requiring less than 20 kVA, then the best suited would be a single phase online UPS system, which would take care of all the nine types of power problems—power failure, power sags,
surge, under voltage, over voltage, line noise, frequency variations, switching transients and harmonic distortions. For large business applications, a three phase online UPS system would be required.
Safety margins should always be considered to avoid the UPS system running on 100 per cent load, which may reduce the life of the UPS. In general, a 20 per cent higher capacity is recommended. Future expansions are always a probability; hence it is advisable to select the UPS size keeping in mind the expansion plans for the next few years. A 20-25 per cent growth is usually considered.
Battery backup is also an important consideration. The cost of batteries is quite high—almost half the cost of the UPS
system in the case of larger versions, and this is a recurring expense. For smaller UPS systems, backup times go up to 2 hours, but for larger systems, it is important to consider alternate power backup sources due to the high cost of batteries.
Manageability: Power management software is required to ensure that all work in progress is saved and that sensitive electronic equipment is correctly shut down if the power outage exceeds the battery runtime of the UPS system. Without software, the UPS system simply runs until its batteries are depleted and then drops the load.
Operations and maintenance: To make an informed decision on service support, you must accurately assess your own technical and service capabilities. You should also look at the various UPS product designs to gauge how easy it is to swap out battery and power modules.
Other parameters to be considered prior to selecting a UPS system for an office are:
- The detailed monitoring of UPS displays through LED and LCD displays is a good option.
- Remote manageability and control of UPS systems through NMS/ BMS always helps.
- Online or offline topology
- Sine wave or square wave form
- Overload percentage option
- Crest factor
- Maintenance process and costs
- Overall dimensions and environmental factors
TOP TIPS FOR CHOOSING THE RIGHT UPS SYSTEM FOR YOUR OFFICE
- Check for the device’s compatibility when selecting
- Conduct a cost-benefit analysis by weighing the benefits of the best price against supplier reliability, product quality and technology.
- Warranty and post sale support is a vital element to consider when choosing the right UPS system.
- While deciding on the size of your UPS system, check for the compatibility of the battery bank also (regarding charging current and re-charging duration).
- Consider the floor space occupied by the UPS system.
- Customers should focus on the availability or mean time between failures (MTBF), total cost of ownership (TCO), MTTR, and the adaptability of the solution.
- They should look into the vendor’s credentials and strong after sales record/service capabilities.
- Understand your loads, applications and the criticality of the application. The products must have the capability to cope up with unforeseen environmental challenges.
- Understand the UPS algorithm and the life span of this technology. While buying a UPS, the main attraction should never be the lowest price. Since it is a long term investment, the total run years and the maintenance expenditure on the device should be looked into.
- Branded UPS systems should be preferred and after sales service contracts should be closely looked at.
MAKE THE BEST PICK
There are several different UPS topologies that provide varying degrees of protection. While all four of the most common UPS topologies outlined below meet the input voltage requirements for IT equipment, there are key differences in how the result is achieved, as well as the frequency and duration of the demand on the battery.
Standby UPS systems:
These smaller devices provide an economical source of protection from power outages, surges and sags. If you just need basic protection for small systems like desktops and cash registers, a small standby UPS system will be the ideal choice.
Line interactive UPS systems:
If you need voltage regulation and power protection for moderate loads (500 VA to 6 kVA), particularly for commercial or office applications, a line interactive UPS system is your best bet.
Double conversion online UPS systems:
These convert the raw incoming power into refined power through a process of double conversion. They change it from dirty AC to DC, clean it and then convert it back to AC. For very sensitive, expensive equipment that needs clean, true sine wave power, a double conversion UPS system would be a smart option.
Double conversion on-demand systems:
If you need to clean power for high density data centres, and if energy efficiency and a flexible configuration are top priorities, then double conversion on-demand UPS systems would be the only choice. It combines line interactive and online modes.
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