Networks Europe Sept-Oct 2015 | Page 49

is fed directly to the loads via the static switch. This mode is most often used if there is a problem with the rectifier or inverter, or if maintenance is being carried out on the UPS. A UPS with ESS has exactly the same four power function blocks. No new blocks that could increase complexity or reduce reliability are introduced. The ESS UPS makes better use of the blocks, however, by providing a fourth operating mode – ESS mode. In this mode, the static switch is closed and power is fed direct from the mains supply to the loads, just as it is in an ordinary UPS operating in bypass mode. The big difference, however, is that in ESS mode, the rectifier and inverter are held in a state of ‘extraordinary system preparedness’. This means that, if the mains power quality falters, the UPS can switch to full double-conversion operation in less than two milliseconds. This is so fast that the transition is invisible even to the most sensitive of IT equipment. ESS UPSs will normally operate in ESS mode almost all the time, switching to double-conversion mode only occasionally to deal with mains disturbances. In ESS mode, their efficiency is a truly impressive 99 per cent as the losses associated with the rectifier and inverter are eliminated. The potential for energy savings is huge and, as an added bonus, because ESS UPSs run cooler most of the time than conventional UPSs, they are more reliable and they have longer working lives. Testing While VMMS and ESS technologies offer effective ways of significantly increasing the efficiencies of UPSs during normal operation, there is another aspect of energy efficiency that needs to be considered. To ensure that the batteries used in UPSs are still capable of supporting the load for the required run time, they must be tested regularly. The most conclusive way of doing this is with a discharge test that, as the name suggests, involves almost fully discharging the batteries while their performance and capacity is monitored. www.netcommseurope.com The latest technologies make it possible to reach new energy efficiency levels. The traditional way of doing this is to discharge the batteries into a load bank – essentially a bank of resistors – which means all the energy the batteries have stored is converted to heat and wasted. In a big UPS installation this can be a significant amount of work – with a three MW installation recently tested, for example, the cost associated with annual battery testing was €50,000. Another advanced technology – Easy Capacity Test (ECT) – provides a solution for regular maintenance. With ECT, the UPS is temporarily reconfigured so that its own power modules can be used to feed the energy from the batteries back into the supply system. This eliminates the need for external load banks and all associated costs and, as a bonus, no waste heat is generated and the energy taken from the battery during the discharge test is put to good use. In redundant systems the testing can be performed concurrently