Data-centre architecture graphic

Uninterruptible power needs system-level choices

When the power fails, you want to be sure you have a backup plan. But choosing the right strategy is more involved than it looks.

From data centres to advanced manufacturing facilities, many installations need a reliable source of electricity, which means the use of an uninterruptible power supply (UPS). Although the UPS itself may only be able to sustain the installation’s electricity needs for a matter of minutes, that is long enough to begin a graceful shutdown to avoid damage to delicate equipment and loss of data or to allow for the start-up of an alternative generator. At first sight, the selection of a UPS looks simple. But the variables that control UPS selection are numerous, ranging from what needs to be protected, the efficiency of the system through to the type of connection to the grid itself, where you put the UPS and whether the equipment behind the UPS needs additional power conditioning.

The key advantage of double conversion over most competing UPS technologies is reaction time. All of the power supplied by the double-conversion UPS to the system is through a unit that takes energy directly from the battery. First the AC is rectified, so that it can be used to charge the battery cells, then it is passed as DC to an inverter that converts back to AC.

With a double-conversion UPS the power should never be interrupted if the mains supply experiences brief outages. The other main technologies, such as flywheels and diesel generators, take variable amounts of time to kick in, from a matter of milliseconds for the flywheel though to minutes for the generator. A further advantage of double conversion is its ability to present a load to the grid that is fully power-factor corrected even when equipment on the other side introduces unwanted harmonics into the AC supply.

However the security and convenience of double conversion can come at the cost of efficiency due to losses incurred by the two conversion steps that must be performed continuously. As those losses happen through the conversion of electricity to heat, they also add to the air-conditioning problems of installations such as data centres.

The line-interactive UPS avoids the double conversion step for most of the time, using the rectifier just to maintain a fully charged battery. Only when the power is interrupted does the rectifier kick in.

Many double-conversion systems now offer eco modes, which use an approach that looks broadly similar to line-interactive operation. But the line-interactive and eco modes can suffer from the problems of reaction time. For a power circuit to ramp up to the full output needed to maintain a large data centre, it can take time before the battery is supplying energy.

Faster start-up

The time issue has led UPS designers to look more closely at the circuitry inside their rectifiers and inverters to find ways to minimise the switchover time. One obstacle is the need to synchronise the inverter’s output with the AC cycle of the main supply.

Power FETs inside the inverter switch on and off at regular intervals determined partly by the AC cycle and partly by the downstream load’s requirements. The firmware that switches the bank of six power transistors through their gate terminals has to synchronise its commands with those factors. If started from scratch, this can take time.

Ed Spears, marketing programmes manager at Eaton, says one way around that problem is to keep the firmware and control electronics running: “The inverter controller generates gate commands continuously. It doesn’t fire the transistors, but it stays in sync.”

Delta conversion

A further variant of the battery-backup UPS is delta conversion, developed by American Power Conversion before it was bought by Schneider Electric. The idea behind delta conversion is that the entire power demand of the data centre is not sourced from the DC subsystem that feeds the battery. A small ‘delta converter’ charges the battery with the remainder of the energy passed through a transformer that separates the load from the mains input. By converting less energy to DC, the losses are correspondingly lower. The use of a transformer in the power path means that the delta conversion UPS has a greater level of isolation between the mains and load than the line-interactive architecture.

Flywheel systems

The momentum of a spinning wheel offers a completely batteryless alternative. The flywheel UPS has become a common method for providing data centres as well as medical and industrial installations with power while a diesel generator starts up.

Once spinning at full speed, the flywheel needs comparatively little energy to maintain its angular momentum. Its high mass means that it can store a significant amount of energy.

The capital cost of the flywheel UPS is typically high but the components need replacement less often than battery-based systems. Whereas batteries might need to be changed every five years to maintain a good charge density, the bearings on a flywheel UPS can remain in place for 10 or 15 years, which brings their cost of ownership down over its full lifecycle.

Leo Craig, general manager of Riello UK, says flywheel UPS has the drawback of offering comparatively brief autonomy times, up to around two minutes, which is meant to be long enough for a diesel generator to kick in. He adds: “It leaves little option if the generator does not start. On the other hand batteries offer longer autonomies of typically 20 to 30 minutes, thereby giving the opportunity for the critical load to be shut down in a controlled fashion in the event of the generator not starting.”

Studies performed close to a decade ago by organisations such as Lawrence Berkeley National Labs – as data-centre energy consumption climbed towards 10 per cent of the world’s demand for electricity – showed that the double-conversion strategy demonstrated much poorer efficiency in real-world situations than its headline numbers would suggest. The problem was that the effects of switching losses in both rectification and inversion become more significant as the current demand from the load dropped below 75 per cent.

Part-load efficiencies

Giovanni Zanei, product marketing director for power systems at Emerson Network Power EMEA, says: “Around five years ago, all UPS units were designed to operate at maximum efficiency at 100 per cent utilisation. This percentage has since changed due to the utilisation of data centres, which has now reduced to between 30 and 60 per cent.”

Utilisation levels some way below 75 per cent are typical for data centres because of the way they are configured for redundancy – at least two UPS systems are deployed side by side in case one fails when a power interruption occurs. The drop-off in efficiency with double-conversion UPS designs made them look to be poor choices compared to technologies such as flywheel, which have traditionally provided flatter efficiency curves. However, thanks to improvements in efficiency and eco-mode features things have now changed.

As a result, double-conversion UPS designs can achieve more than 95 per cent efficiency at 50 per cent capacity, and can now be more efficient at that level than a unit running flat out, removing many of the concerns over efficiency versus utilisation and changing the key choices.

“The choice between using a flywheel or double-conversion battery within a UPS system is dependent on the specific requirements and specifications of the data centre, including location, autonomy needs, environmental factors and costs,” says Zanei.

Gael Souchet, global product manager for Schneider Electric’s Galaxy VM lineup, adds: “Much has changed thanks to the technology evolution within power converters, and new ‘multi-level’ architectures. Components such as IGBTs have also enabled the technology to evolve further. Delta conversion may have been the best way to achieve high efficiency previously, but it is not the only topology used in 2015.”

High-voltage connections

Although double conversion has become more practical for many large systems, Souchet believes that there is still a place for delta conversion. “When connection to the electrical grid is made at a higher voltage, say 400kV and above, the availability runs near to, or at, 100 per cent, having never experienced any outage in Europe since its creation. This is now becoming more regular: we are seeing more cases where larger data centres require a higher level of energy or power to operate.”

As a result, Souchet says double conversion would typically be the preferred option for a system connected at 63kV or lower, but delta conversion is likely to make sense for a 400kV-connected system.

The battery technology itself is changing too, spurred on by the pace of development in consumer electronics. Traditionally, because space has not been a critical requirement in UPS installations, the battery-based systems have used valve-regulated lead-acid (VRLA) cells. Temperature is, however, an increasing concern, as that affects the amount of air conditioning required.

Craig claims: “There is an alternative technology now available, which uses lithium-ion batteries. Looking from an environmental point of view, lithium-ion can work in temperatures from 0 to 40°C without affecting battery life, hence removing the need to keep the battery room at 20 degrees as demanded by VRLA, and thereby offering a saving on cooling.”

The decision as to whether to install a UPS is in almost all cases a simple one: it’s a yes. But the choice as to which form of UPS is highly context-dependent and is changing all the time thanks to improvements in technology, both within the UPS and the loads themselves.

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