Power losses in static UPSs have gradually decreased over the past 20 years. Early thyristor based on-line systems (referred to as double-conversion or IEC ‘VFI’) with transformers at the input and output had full-load operating efficiency of 83-85 percent. Now transistor (IGBT) based line-interactive (VI) transformer-less machines achieve 97.5-98 percent.
More efficient, less heat
Energy efficiency has increased by 15 percent while cooling demands have fallen. Reliability has increased, with a module MTBF rising from less than 25,000h to more than 150,000h. Output voltage waveform distortion has gone down from five percent to one percent. Noise is down from 95dBA to 70dBA, and the physical footprint is down by a staggering 90 percent.
Even double-conversion (VFI) has reached 96.8 percent efficiency, and the cost per kW capacity has fallen to its lowest level ever.
This is good for the purchaser, but the only way to make a profit from UPS is to provide after-sales-services. By 2008, where was there left to go?
In Europe, all UPSs had already become transformer-less, thanks to Europe’s four-wire distribution, while in North America, transformer-less UPSs are still a minor novelty and often regarded as somewhat exotic. At the same time, vendors like APC adopted line-interactive topology (IEC ‘VI’) which saved energy, albeit without any frequency protection – not technically ‘on-line’, although advertised as such, but working well enough in stable grids.
The principle of eco-mode is simple: when the utility is stable, the UPS switches itself into bypass mode and the losses reduce. If the utility deviates, the load is switched, which represents a risk, but the return can cover the cost of the UPS in two years
‘Eco-mode’, introduced by Invertomatic in Switzerland in the 1990s but dropped due to lack of sales, was resurrected (along with ‘modular’ UPS, which overcame partial load problems endemic in most data centers). 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 (needing far lower power than a flywheel) 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 4ms 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 percent efficiency for more than 95 percent of the year on stable grids, with the bonus of excellent low-load efficiency.
Some unscrupulous salesmen 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’ which operate at 2ms instead of 4ms, and some that monitor the load distortion and make decisions about the grid, but the basic concept remains – if the utility is stable you save energy.
Rewards usually come with risk and eco-mode is no different. 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, which may be small but the user must balance it with a return - which can be high enough to cover the entire UPS cost in less than two years.
As energy costs rise and the concept is proven, eco-mode is being accepted. 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.
Enter silicon carbide
The risk, whether real or perceived, will remain and limit the adoption of eco-mode but a development emerging from 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 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 percent module efficiency.
Silicon carbide - or carborundum - can increase double-conversion efficiency to 99 percent, and end the worry about eco-mode
However, a change from silicon to silicon carbide (better known as carborundum or occurring in nature as the extremely rare mineral moissanite) can increase UPS module efficiency to 99 percent in double-conversion.
Synthetic silicon carbide powder has been mass-produced since 1893 for use as an abrasive, for instance in 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 percent loss?
This article appeared in the Power Supplement distributed with the Feb/March 2017 issue of DCD magazine