The pros and cons of eco-mode UPS: what does the future hold?

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Ian Bitterlin discusses the potential risks and benefits of adopting eco-mode UPS technology, and shutterstock_99993146considers the future impact of silicon carbide IGBTs.

In the early days of thyristor-based series online (usually referred to as ‘double-conversion’ or, now, by the IEC Standard 602040-3, ‘VFI’, Voltage and Frequency Independent), with transformers and filters at the input and output, the highest full-load operating efficiency you could expect was 83-85%. Partial load efficiency was much lower, mainly due to the transformers.

This has gradually increased during the past 25 years mainly due to a move to transistors (IGBT) that enabled the removal of input filters, isolation transformers and output filters. Added to that shift in technology, many UPS are now based on line-interactive (‘VI’, voltage independent in the IEC Standard) topology transformer-less modules achieving 97.5-98% at full-load.

However, we have seen a huge improvement in many important areas in the same period:

  • Energy efficiency increased by 15%, with partial load improvements of >25% points
  • A 90-95% reduction in cooling capacity provision
  • Reliability increased by a factor of six, with a module MTBF rising from <25,000h to >150,000h
  • Output voltage waveform improved from >5% to 1% distortion from the ideal sinewave
  • Input current harmonics down from 33% to <2%, helping generators cope
  • Foot-print reduction by a staggering 90%
  • Noise reduction, from 95dB(A) to <70dB(A)

It is interesting to note that when I was selling hybrid rotary UPS 25 years ago, we had a product that was 92% efficient compared to 83% of the best static product but customers were not interested in energy losses, only reliability, so I never mentioned the feature. Today, even double-conversion (VFI) has reached 96.8% efficiency at full load and higher than 92% at 25% load but, if all that were not enough, the cost per kW capacity has steadily fallen to its lowest ever level.

By 2008, in Europe, all UPS had become transformer-less and, about the same time, a move to increased adoption of line-interactive topology (IEC ‘VI’) offered clients energy saving, albeit without any frequency protection.

However, while the technology was improving, the serious pursuit of energy saving took a route that had been tried before, albeit without success at the time. The idea of ‘eco-mode’, originally introduced in Switzerland in the early 1990s but dropped for lack of sales, was resurrected.

The principle of eco-mode is simple – when the utility is stable the UPS automatically switches itself into bypass mode and the losses reduce, especially effective in transformerless designs. The rectifier still float-charges the battery (only 10s of watts needed, unlike flywheels that need more) but the inverter is throttled to zero and, in the best designs, the cooling fan load is reduced.

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 less than four milliseconds and well within the (albeit 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 in stable grids for >95% of the year with the bonus of excellent low-load efficiency as well.

There are some unscrupulous salesmen who try to avoid admitting ‘bypass operation’ by talking about ‘low-power state’ for the inverter but, make no mistake, the UPS is in bypass with little or no power quality improvement and the critical load is fed by ‘raw mains’.

Now, there are some ‘advanced eco-modes’ around which operate faster, 2ms instead of 4ms, by using  digital signal processing as opposed to analogue measurement, and some that monitor the load distortion and make decisions about the grid being able to accept it. However, the basic concept remains – if the utility is stable you save energy.

But are there risks? Yes, of course there are. In this world ‘reward’ usually comes with ‘risk’ and eco-mode is no different. There are tangible risks to enabling eco-mode: every time the utility deviates the load is switched – which is 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 and battery cost) of less than two years.

But there are other risks, even barriers, with eco-mode enablement. The first is that the designer needs to pay extra attention to surge suppression fitting a graded system of surge protective devices (previously called transient voltage surge suppressors, or TVSS) to protect the critical load from very short duration, high energy, transient surges coming from the utility.

The second, more of a barrier, is the problem of the distorted load current being drawn by the critical load. In the pursuit of high energy efficiency, the switched-mode power supplies in the load have been optimised for high utilisation but, generally and most often, loads are run at partial IT load and the effect is two-fold:

  • The load power factor become leading (capacitive), often reaching 0.90-0.92. When the UPS is in circuit this will be masked from the utility and the generators, but when in eco-mode the unwanted leading power factor will be reflected into the system. Technically, it is extremely risky to run eco-mode when on generator supply, although practically there has usually been a mains-failure that has disabled eco-mode just prior to the generators starting.
  • At partial load the switched-mode power supplies draw increasing levels of harmonic distorted (as high as 35% total) current that will distort the utility voltage, sometimes beyond the 8% limit set down in the EN standard. So, enabling eco-mode should be contemplated carefully and the risks compared to the rewards in lower energy bills.

Today, the result 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 although there are a few high reliability dual-bus facilities that are hedging-their-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.

One future development (which has already started in Japan) could make the energy advantage of eco-mode so marginal that the risk will not be attractive.

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) 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 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 – eg 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? Maybe the hyper-scale facilities but surely not enterprise or co-location operators.

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