DCN December 2016 - Page 23

modular systems helps to illustrate the issue. For a baseline case, consider a single UPS with no redundancy, comprised of four 250kW modules inside a single frame. Then consider an Internally Modular arrangement at the same rating. This differs from the first case by having a fifth 250kW module for redundancy purposes. The UPS output bus and the battery or DC bus is common to all modules, thereby representing a single point of failure. In each of these cases there is a battery module with four paralleled strings so that failure in one does not cause failure in all. Finally, consider a Parallel Redundant N+1 configuration. In this case, three separate 500kW UPSs are connected in parallel with a common output bus. In this configuration each module has its own battery system, which provides an additional level of redundancy. Capital cost Capital expenditure is least, hardly surprisingly, for the baseline single UPS with no redundancy. Then comes the Internally Modular option, and finally, the most expensive configuration is the Parallel Redundant configuration. The Internally Modular option analysed is 26.9 per cent lower in terms of capital cost than Parallel Redundant, and the non-redundant option is 6.1 per cent lower than Internally Modular. Capital costs are made up of a combination of the UPS modules 23