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Will Eco-Mode UPS Be ‘Finished Off’ by Carborundum?

The power losses in static-UPS have gradually decreased over the past 20 years or so. From the early days of thyristor based series on-line (usually referred to as double-conversion or, now, IEC ‘VFI’) with transformers at the input and output and full-load operating efficiency of 83-85% to transistor (IGBT) based line-interactive (VI) transformer-less machines achieving 97.5-98% we have seen a huge improvement;

  • Energy efficiency increased by 15% plus a drastic reduction in cooling capacity

  • Reliability increased, with a module MTBF rising from <25,000h to >150,000h

  • Output voltage waveform improved from >5% to 1% distortion from a sinewave

  • Input current harmonics down from 33% to <2%

  • Foot-print reduction by a staggering 90%

  • Noise, from 95dB(A) to <70dB(A)

Even double-conversion (VFI) has reached 96.8% efficiency but, if all that were not enough, the cost per kW capacity has steadily fallen to its lowest level ever. Good for the purchaser but, unfortunately, the only way to make a profit from UPS is to provide after-sales-services.

So, by 2008, where was there left to go? In Europe, all UPS’s had already become transformer-less, although, because of our 4-wire distribution, this was easier to introduce than it would have been in North America. In fact, transformer-less UPS in North America is still a minor novelty and often regarded as somewhat exotic. At the same time a move to increased adoption of line-interactive topology (IEC ‘VI’) offered clients energy saving albeit without any frequency protection from vendors like APC – not technically ‘on-line’, although advertised as such, but working well enough in stable grids. So, the idea of ‘eco-mode’, originally introduced by Invertomatic in Switzerland in the 1990s but soon dropped due to lack of sales, was resurrected along with ‘modular’ UPS. Modular UPS overcame partial load problems, which were endemic in most data centres, even today, but here we shall concentrate just on eco-mode.

The principle of eco-mode is simple – when the utility is stable the UPS switches itself into bypass mode and the losses reduce, especially in transformer-less designs. The rectifier still floats the battery (only 10’s of Watts needed, unlike flywheels that need more) but the inverter is throttled right back and, in the best designs, the cooling fans are dropped off. The automatic bypass (a thyristor switch) keeps the load on the utility until the utility shows the first sign of deviation – at which point the static switch transfers the load back to the inverter, all in under 4 milliseconds and within the (rather outdated) ITIC/CEBMA PQ Curve. The UPS then monitors the utility for stability and after a period, usually one hour, switches the load back to bypass. The advantages are clear; 99% efficiency and, in stable grids, for >95% of the year with the bonus of excellent low-load efficiency as well. There are some unscrupulous salesmen that mention ‘low-power state’ for the inverter but make no mistake, the UPS is in bypass with no power quality improvement and the critical load fed by ‘raw mains’. Now, there are some ‘advanced eco-modes’ around which operate faster, 2ms instead of 4ms, and some that monitor the load distortion and make decisions about the grid being able to accept it, but the basic concept remains – if the utility is stable you save energy.

But are there risks? In this world ‘reward’ usually comes with ‘risk’ and eco-mode is no different, so, yes there are tangible risks to enabling eco-mode. Every time the utility deviates the load is switched – the very opposite of the protection offered by ‘double-conversion’. This switching represents a risk to the load, albeit small and maybe even inconsequential, but the user must balance that risk with the reward. Make no mistake, the reward can be high - with a Return-on-Investment (covering the entire UPS cost) of <2 years.

The result now, in 2017, is a slowly growing acceptance of eco-mode and this will, no doubt, continue as energy costs rise and the concept is proved reliable. It is a fact that energy effectiveness is not always the most important metric that users aspire to but there are even a few high reliability dual-bus facilities that are hedging bets by enabling eco-mode in one bus and running VFI in the other, alternating each week. However, the risk, whether real or perceived, will remain and limit the adoption of eco-mode but one future development (which has already started in Japan) could negate any advantage of eco-mode.

Transistors are currently manufactured with layers of doped silicon. The best to date, for UPS, are of the Insulated Gate Bipolar (IGBT) type and these have become increasingly powerful and reliable. One drawback is that the faster you switch them (to achieve more precision) then the higher the losses. This is what mainly contributes to the upper limit of 96.8% module efficiency. However, a change from Silicon to Silicon Carbide (better known as Carborundum or occurring in nature as the extremely rare mineral Moissanite) in the manufacture of semiconductor switches like IGBTs holds the key to 99% UPS module efficiency in double-conversion. Synthetic silicon carbide powder has been mass-produced since 1893 for use as an abrasive e.g. silicon carbide paper for finishing metals.

Silicon Carbide IGBTs will initially cost more but the energy saving will rapidly be recovered – and all without switching the critical load to the raw utility and increasing risks of transfer. Hence, Silicon Carbide will spell the end of worrying about the enablement of eco-mode and possibly even kill off line-interactive (VI) UPS. Who will need to worry when you can get total protection of voltage and frequency protection with less than 1% losses?

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