by May 11, 2002 0 comments



Think of a UPS, and an image of a small box under a table providing backup power to a single or bunch of PCs comes to mind. This can have a rating of a mere 500 VA to a few kVA. A UPS system, however, isn’t just that little box. On the contrary, it can provide backup to hundreds or even thousands of PCs, with ratings from a few hundred kVA to a few MVA. Such systems are used by organizations to provide backup power to a variety of load, like the IT infrastructure, telecom equipment, sensitive medical equipment in hospitals and even heavy machinery load. Knowing how to select a good UPS, therefore, is important. 

Likely load? 
The first step to choosing a large UPS system is to understand the nature of your electrical load and its dynamic response. Electro-mechanical load like motors used in industrial applications need significantly more power to turn on initially. When they’re switched on or off, they generate significant EMI (Electro-magnetic Interference) on the line, and can also draw current twice their rated value. Account for this when you plan the capacity of your UPS. 

A large UPS system can provide backup to hundreds of PCs and servers. They can also provide power to telecom or sensitive medical equipment as well as heavy machineryNon-linear load like computers, hubs and switches used in an IT infrastructure, on the other hand, can draw up to five times the current during the first half cycle. How sensitive the load is to power problems is another issue. If you have sensitive telecom equipment that would take days to reconfigure if it reboots due to a power glitch, you need a UPS that will not let this glitch pass through. Similarly, if you need backup for medical equipment, then the UPS must conform to stringent emission norms. There are various international EMI standards, laid out by IEC (International Electrotechnical Commission), which UPS manufacturers must follow.
Standards’ stuff 

The main UPS standards for EMC include the following. 

IEC 62040-2: UPS Part-2 – EMC requirements, test methods, and performance levels 

IEC 62040-3: UPS Part-3 – Method of specifying the performance and test requirements

To meet these standards defined by IEC, UPSs are typically subjected to the following tests.

  • CE (Conducted Emission): Frequency 0.15—30 MHz
  • RE (Radiated Emission): Frequency 30—1000 MHz
  • I/P current Harmonics: As per IEC 61000 — 3 — 2 
  • Radiated Immunity: As per IEC 61000 — 4 — 3 
  • Electrical Fast Transient: As per IEC 61000 — 4 — 4 
  • Low Frequency signal test: 140 Hz to 360 Hz at amplitude 10V
  • Electrostatic Discharge: Contact/Air discharge

Where do you spend?
Besides the cost of the UPS, you must account for several other major cost heads. The first, of course, is the one-time implementation cost, which consists of the UPS system and the battery bank. As both these take up a lot of floor space, you have to consider the room cost. If you’re providing backup power to equipment that must be up 24 hours, you might have to put two UPS systems in parallel for redundancy. If it’s highly sensitive equipment, you might have to go for an online UPS system conforming to all EMI standards, which costs much more than a line-interactive one. Plus, you’ll need a generator backup for prolonged power cuts, because batteries shouldn’t be stretched for this. 

Large UPS systems require huge battery racks like this one to provide the backup powerYou must also take into account the running costs, which can be quite significant. Large UPS systems require cooling, as higher temperatures can reduce the life of the batteries. Therefore, you have to take into account the electricity bill for the air conditioning. Then there’s the cost of batteries, as the average life of the SMF (Sealed Maintenance Free) batteries used by most UPS systems is three years. The cost of replacing these batteries can run into a few lakh rupees for very large UPS systems.

Finally, there’s the cost of support and AMC, which depends upon how important it is for your systems to be back on after a failure. 

An important feature for a large UPS system is management and reporting capability. For instance, batteries are an important part of a UPS system, so the UPS should be able to analyze the batteries’ condition and their ability to handle the load. If this ability goes down, then it should be able to send an alarm to the concerned person. Most UPSs come with SNMP (Simple Network Management Protocol) support, which allows them to be integrated with a network-management system. 

A UPS should also be able to trip itself and remain undamaged during a short circuit. If it’s a 3-phase UPS, which is usually the case in ratings above 30-40 kVA, then it should be able to trip only the phase having the short. Another important factor for a 3-phase UPS is called load unbalancing, in which theoretically a UPS takes 100% load from one phase and 0% from another. So check for the UPS’s ability to handle full load unbalance for a non-linear type of load. In a 3-phase system, each phase delivers a third of the total power. So check the degree of power that a phase can deliver beyond a third of the power. This should be at least 25% more.

Other features to consider are the kind of over temperature and overload protection. High temperatures can reduce battery life, and a UPS should be able to handle some amount of overload. 

The Pillar batteryless UPS stores energy in a flywheel instead of batteriesClaimed efficiency 
Efficiency is another important factor in a large UPS system. In simple terms, efficiency is how much power a UPS is able to deliver to the load, and how much is being lost. So if a 100 VA UPS has an efficiency of .95, then the UPS is delivering 95 VA, and the remaining 5 VA is being lost. Efficiency is important as a higher efficiency increases the life of the UPS. It’s taken into account under two scenarios. First, when the UPS is operating from battery, the efficiency of the inverter determines the battery life. Second, when the UPS is delivering power from the mains, called line-to-line efficiency, power is lost as heat inside the UPS, and this affects your electricity bill. 

Many factors determine UPS efficiency, including the quality of transformers it uses, and the circuit design. Efficiency can only be measured in a lab environment under stringent test conditions. So ask the UPS vendor for the claimed efficiency of his UPS, perhaps in the form of a test report. Finally, check the MTBF (Mean Time Between Failure), or the life of the product. 

We would like to thank the following for providing valuable inputs for this article. Dr AK Agarwala, IDDC Labs, IIT, Delhi; Anil Munjal, PCI; Sushil Virmani, Powerware International; PM Bhardwaj, 
Instrumentation India 

A Batteryless UPS

The front panel of a large UPS systemA UPS without batteries? How’s that possible? Here’s how. There are two types of UPS systems, static and rotary. In a static UPS, all the backup energy is stored in batteries, while in a rotary system, all energy is stored as rotational energy in a device called the flywheel. Pillar of Germany manufactures rotary UPS systems by the same name. These are sold in India by
PCI. 

The Pillar UPS stores energy in a flywheel it calls the POWERBRIDGE. During normal operation, the load is supplied power through a choke, which keeps the output voltage constant over a wide input range. During a mains failure, the flywheel’s rotational energy is used to power a motor/generator, which in turn supplies continuous power to the load. The device works such that the power supply to the load remains uninterrupted and clean. The motor/generator can be coupled with a diesel generator to provide backup power for prolonged power cuts. 

Compared to a static UPS system, this one saves on the cost of batteries. Plus, it doesn’t require cooling, as in the static UPS systems, thereby reducing the electricity bill. Finally, it takes lesser floor space; the POWERBRIDGE itself takes only about 1 cubic meter space. Though the UPS is available in capacities ranging from 150kVA to 1600
kVA, ratings above 1000 kVA are claimed to be more cost effective. Such high ratings are, of course, meant to provide backup power to heavy critical load in industrial applications, telecom or call centers. 

The UPS has a claimed MTBF of 600,000 hours, which is quite high. The cost of a 1 MegaWatt batteryless UPS is Rs 1.5
crore. 

Contact: PCI, Delhi. Tel: 011-5762562/3/4. 
E-mail: primegroup@vsnl.com

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