Most ups vendors now offer transformerless systems, many up to 30 kVA output, a few up to 60 kVA and one up to 120 kVA. For many users, the size and cost benefits of the technology are encouraging paralleling for redundancy. For some, scaleability has proved to be the major attraction, removing the need to predict system growth by enabling simple, cost-effective expansion of the protection system.
Figure 1 shows a typical transformer-based, double-conversion, on-line ups. The transformer is used to step-up the voltage at the output of the inverter to a level compatible with the utility or generator supply voltage. A common misconception is that the transformer is also used to provide galvanic isolation, which is not the case.
Figure 2 shows a typical transformerless design, with the step-up occurring in the middle stage of the circuitry, via a dc-dc booster, enabling the inverter to supply the correct output voltage without the need for a transformer. The latter offers a number of benefits:
Paralleling for redundancy
In any single-module ups installation there are potential single points of failure. If a fault develops, the critical load may be exposed to raw mains until it is repaired. But two or more ups can be connected in parallel to provide a fault-tolerant system, where the fault stays within its appropriate subsystem boundaries and does not disable the entire system.
At the same time as the business criticality of many applications demands a parallel redundant ups system, the technology has been developed to dramatically reduce its footprint and cost. The challenge is to achieve parallel redundancy with the smallest footprint at the lowest possible capital and running costs.
For example, if a parallel redundant power protection system is supporting a 180 kVA load with two 200 kVA ups equally sharing the load, in normal operation each ups will be supplying only 45% of its rated output – a relatively disadvantageous point on its efficiency curve. If this load is supported by three 100 kVA ups in a parallel redundant arrangement, in normal operation each ups will deliver 60 kVA or 60% of its rated output – a relatively efficient point on the curve.
Table 1, over, compares the costs of these two alternatives, with the assumption that the 200 kVA ups is a transformer-based design, since this requirement is outside the range of transformerless units currently on the market.
Efficiency gains must be set against capital costs when deciding how many ups to parallel in a bid to reduce running costs, although ups costs can be offset against 12-pulse rectifier savings. Also, incremental costs for adding further ups in parallel to upgrade the protection system output will be the cost of a 200 kVA ups in the case of a transformer-based ups, and 100 kVA for a transformerless ups.
Space and weight arguments are equally persuasive.
This all looks to be good news for ups users, but it represents only the first phase in the revolution. Second-generation transformerless ups offer even more cost and flexibility gains.
ups in a rack
Taking the efficiency, size and weight gains of transformerless technology to their logical conclusion, the rack-mounted ups becomes a real possibility.
A three-phase, rack-mounted ups is now on the market, based on modules with up to 30 kVA output, bringing intriguing options for redundancy and upgradeability.
When assessing rack-mounted alternatives, it is important to bear in mind that the ups modules should be completely self-contained. In systems where there are common components such as a control module or static bypass, there may be a single point of failure that renders ups redundancy ineffective.
Rack-mounted ups offer numerous options for cost and space-efficient redundant parallel operation and/or upgradeability, with additional serviceability and manageability benefits.
In figure 3, three 30 kVA modules are used to support a 60 kVA load with n+1 redundancy. Alternatively, the user could employ two 30 kVA ups to support the 60 kVA load, and take advantage of the vertical scaleability of the system to cost-effectively upgrade to 90 kVA with no footprint penalty, by adding a further 30 kVA ups and battery bank. The user can also take advantage of the horizontal scaleability opportunities of the system by adding additional small-footprint ups and battery cabinets.
The sophistication that some large organisations are now bringing to their power protection resource using compact, rack-mounted ups is demonstrated in figures 4, 5 and 6. On the first floor (figure 4), a triple-module ups is configured with three 30 kVA ups modules supporting two server/comms racks with redundancy. On the ground floor, four 30 kVA modules support three server/comms racks with redundancy and two spare slots to allow for expansion.
The company decides to move one of the server/comms racks to the ground floor, overloading the existing power support system there (figure 5). It is a simple matter to shift one module and battery bank to the ground floor to restore the balance (figure 6). There are now spare slots on ground and first floors to allow for expansion of the protected system and further capacity can be added at any time by introducing another cabinet. The important point is that all of these changes can be made with minimum effort, low incremental cost and without using any more floor space.
As an added bonus, if one ups module should develop a fault, the user can easily move a module from another floor as an emergency measure pending service, albeit by temporarily sacrificing redundancy.
As modules can be swapped for service MTTR is reduced to a few minutes, and access is always via the front panel so that the use of floor space can be optimised.
While transformerless technology has been considered to have its limitations, the output of the modular, rack-mount format is only limited by the number of three-phase ups that can be paralleled. For some vendors, this is no greater than six. PowerWAVE three-phase technology allows Uninterruptible Power Supplies to extend this number indefinitely, bringing transformerless technology into the high-end ups market.
As transformerless technology has helped the ups to go modular, the combination of modular technology with advanced paralleling technology has extended the range of transformerless technology.
Peter Bentley is the sales director at Uninterruptible Power Supplies.
Source
Electrical and Mechanical Contractor
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