FACILITIES power+cooling
The transformerless UPS’s free-running full- wave bridge rectifier also allows a reduction of total input current harmonic distortion (THDi) in a three-phase UPS system from 30% to less than 3%. This limits harmonic emissions and makes a considerable contribution to meeting the Energy Network Association’s recommendations for electromagnetic compatibility between an electricity supply system and the equipment it feeds.
The opportunity to reduce operating costs can be demonstrated by an example. Consider a 100 kVA load, supplied by a parallel redundant UPS system comprising two 120 kVA modules. Fully redundant power protection is being provided as each module can completely support the load one fails.
However the arrangement also means that during normal operation the modules share the load equally and each runs at just 42%
of its rated capacity. This takes them to a relatively disadvantageous point in their efficiency curve. Fig. 3 below compares the implications of this for transformerless and transformer-based UPS implementations.
Note that the transformerless implementation runs at 96% efficiency, while its transformer- based equivalent achieves just 91%. As a result, the transformerless UPS’s energy bill – including both direct losses and cooling costs - is less than half of the transformer- based UPS’s. Over five years, this could produce over £24,000 total energy cost saving.
Modular UPS systems and scalability
As mentioned earlier, eliminating the transformer allows considerable savings in size and weight as well as energy. As we shall see, this has profound implications
for UPS topology and scalability. Because the newer design actually eliminates the need for a 12-pulse rectifier as well as the transformer, the 120 kVA UPS of our example above weighs little more than a quarter of its transformer-based equivalent; 310 kg compared with 1200 kg. Meanwhile, its footprint reduces from 1.32 m2 to 0.64 m2.
Clearly these physical reductions are in themselves welcome in space-constrained environments. Yet they have ushered in a more profound development; the concept of UPSs as rack-mounted modules rather than free-standing floor units.
This means that a single vertical cabinet could be installed into an ICT area and populated with one plug-in rack-mounting UPS module of an appropriate rating. A second module could be added either to achieve N+1 redundancy or to increase total UPS capacity. Later, if the demand for
Fig. 1: Transformerless UPS block diagram 42
www.dcsuk.info I May 2014
Fig.2: UPS transfer efficiency curve
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