How to avoid downgrading your UPS system availability when selecting a modular UPS It is widely acknowledged that in the vast majority of UPS applications a double conversion serial on line UPS is the best type to have. However, a single “double conversion” UPS has its limitations because, for example, they are manufactured with a finite maximum power rating and, as they are a single UPS, they contain single points of failure. Fig.1: Typical VFI “Double Conversion, serial online” UPS. These limitations can be overcome by connecting two or more single UPS in parallel -a practice known as “paralleling”. When single UPS are paralleled to increase system capacity such paralleling is known as “parallel capacity” and when they are paralleled to increase system availability it is known as “parallel redundancy”. Fig.2: Simple parallel UPS configuration Regardless of whether the UPS are being paralleled for “capacity” or “redundancy” there are two methods that can be used to parallel UPS: “Decentralised” parallel architecture and “centralised” parallel architecture (commonly referred to as DPA and CPA respectively). If an increase in the power rating, or “capacity”, of the UPS system is the sole reason for paralleling then a CPA system is ideal. This is because a CPA system is cheaper than a DPA system because some of the Newave Energy Holding SA . Via Luserte Sud 9 . CH – 6572 Quartino . T +41 (091) 850 29 29 . www.newavenergy.com system components are shared by the individual UPS within the system. Unfortunately such “centralised” systems make a compromise on the UPS system’s availability. Fig.3: Typical parallel UPS configuration using Centralised Parallel Architecture (CPA). Note that each UPS in this system has shared logic and static switch and therefore has multiple singe points of failure. If an increase in the availability of the UPS system is the overriding reason for paralleling then a DPA system MUST be selected. This is because DPA systems do not have the same “centralised” system components present in CPA systems and consequently do not have the same “single points of failure” present in CPS systems. DPA systems do, however, have the same capacity increase capabilities as CPA systems. Fig. 4: Typical parallel UPS configuration using Decentralised Parallel Architecture (DPA). Note that each UPS in this system is able to act entirely independent of the others and therefore has no single points of failure. Newave Energy Holding SA . Via Luserte Sud 9 . CH – 6572 Quartino . T +41 (091) 850 29 29 . www.newavenergy.com Driven by a strong increase in the number of “mission critical” loads over the past 10-15 years there has been a increasing trend for parallel redundant UPS to be installed. Nowadays the vast majority of medium & large 3-phase “free standing” UPS are sold and installed as part of parallel redundant systems and, almost without exception, all of these systems use DPA configuration. For many excellent reasons (which are the subject of another discussion) the fastest growing segment of the 3-phase UPS market is currently that of the 3-phase modular UPS. And because of these excellent reasons, customers are increasingly replacing their traditional free-standing parallel redundant “DPA” UPS systems with modular UPS systems. However, customers must take great care when selecting a modular UPS because not all modular UPS systems are the same. The majority of modular 3-phase UPS systems currently available use CPA technology, so, an unsuspecting customer could be investing in what they believe to be an upgrade of their UPS system when they are, if fact, investing in a downgrade of their UPS system. Such downgrading will always occur when a CPA system is used to replace a DPA system. Of course, the art of Marketing, and the skill of the salesperson, is to highlight the benefits of their products whilst glossing over (or completely ignoring) their down-sides so it is not always easy to identify whether a modular UPS employs DPA or CPA technology. Because of the inherent weaknesses of CPA technology no manufacturer will boast that their products are CPA so the customer needs to find out such things for themselves by asking the right questions. These questions are: • • • • • • Does each UPS module have its own, independent static switch? Does each UPS module have its own, independent logic control? Does each UPS module have its own, independent control panel? Does each UPS module have its own, independent battery? Does each UPS module have its own, independent rectifier? Does each UPS module have its own, independent inverter? If the answer to any of these questions is “no” then the system is CPA and a customer could be unknowingly investing in a lower level of UPS cover. Still confused about the difference between CPA and DPA? Consider this:When someone jumps out of a plane they typically carry two parachutes – the main chute and the reserve. Both of these parachutes will have their own independent rip-cord because if the rip-cord of the main chute fails and there is no separate rip-cord for the reserve chute the jumper is in a lot of trouble. The two parachutes with two rip-cords can be said to utilise use DPA technology and the two parachutes with only one rip-cord can be said to use CPA technology. Which parachute would you chose to jump out of a plane with? Newave Energy Holding SA . Via Luserte Sud 9 . CH – 6572 Quartino . T +41 (091) 850 29 29 . www.newavenergy.com