UPS systems - A question of UPS architecture: static or rotary?

Typography
Articles

When deciding whether to deploy static or rotary Uninterruptible Power Supplies (UPS), there is no easy way of weighing up comparative advantages and disadvantages. UPS systems vary greatly in physical size, weight, form factor, capacity, supported input power source, technological design, and cost.  APC by Schneider Electric makes a comparison of 3-phase static and rotary UPSs to support a data centre 

The Market
Compared to the massive global market share which static UPS systems enjoy, rotary UPS systems occupy only a small niche. According to IMS Research, only 4.3% of projected worldwide UPS revenues in 2008 will be rotary UPS systems, while the remaining 95.7% consists of static UPS.

They are also a niche within the data centre market where static UPS predominate at power levels of 500 kW and below, with the segment between 20 kW to 200 kW almost exclusively static.

Rotary and flywheel UPS systems begin to appear in use in the 200 kW to 500 kW range, for niche applications such as military and industrial. For mega data centres (< 100,000 square feet) where 500 kW to multiple megawatt UPSs are required, both architectures are present.

The Technologies
A UPS system is defined as static because, throughout its power path, it has no moving parts. The rectifier inside of the static UPS system converts the incoming utility AC current to DC, and the inverter converts DC back to clean sine-wave AC to supply the load. Regardless of the details of the internal topology, at some point there is a place where DC current interfaces with the ‘energy storage' medium - most commonly batteries, in which case it charges the batteries and receives power from the batteries when the utility power supply is distorted or fails.

In data centre applications, a 3-phase static UPS typically has a battery runtime of 5 to 30 minutes. Runtime is dictated by the size and criticality of the load together with available battery capacity. Static UPS battery systems are generally sized to allow enough time, during an outage or disturbance, to support the load while the power source shifts from utility to a standby generator. Should the generator power fail to come online, the UPS is configured with enough battery runtime and technological intelligence to allow for an orderly shutdown of the load.

The rotary UPS system is so-called because rotating components (such as a motor-generator) within the it are used to transfer power to the load. The true definition of a rotary UPS system is one whose output sine wave is the result of rotating generation.

Why the choice?
Rotary technology has been utilised for many years and came into prominence at a time when loads would commonly exhibit a low power factor and high harmonics. At first synchronous condensers which over time began to incorporate motor generators, inverters and rectifiers. Batteries or flywheels were then added for energy storage and the modern rotary UPS system was born. Ironically, the original reasons rotary UPS systems came into being, do not exist for data centre managers exist since most IT equipment is power factor corrected.

Characteristics of rotary and static UPS sytems

  • Investment

Rotary UPS systems are a fixed investment, usually oversized to accommodate future, unknown load requirements. In addition to being neither modular nor scalable (as with some modern static UPS), the upfront costs may be as much as 40% more than a comparative static system.

Auxiliary equipment costs for rotary UPS systems may also be higher since they require an external bypass switch together with special ventilation equipment to purge fumes from working areas. In the case of diesel rotary UPS, the construction of an additional building may be required to house the unit.

  • Maintenance

For a given level of availability, mechanical equipment such a rotary UPS system requires a maintenance regime incorporating weekly, monthly, annual and five-yearly checks. By comparison and depending on environment, most static UPS systems usually require only one annual maintenance visit.

While in general electronic equipment such as static UPS has a more extended useful working life than mechanical rotary UPS, they do require an investment in maintenance of, for example., batteries, in addition to occasional replacement of cards and circuit boards.

 

  • Environmental Impact
Static UPS systems tend to be installed in a building or data centre whereas rotary UPS tend to be outside in a specially built enclosure. Because they often rely on flywheels as their source of energy storage (providing only up to 10 seconds of back up), they may also be noisier as diesel generators are activated during any power ‘situation'. Batteries and flywheels both support the load until back up mains flows, however, by virtue of their greater runtime, battery supported systems may not require generator power unless the outage is extensive (most tend to be of very short duration).
  • Reliability
Both systems are quite reliable and an analysis of the MTBF of major components does not reveal any great advantage either way. However, the weak link is diesel generation which, according to the IEEE (standard 493) experience quite high failure rates (one start in 74). This may present an unacceptable risk to data centre operators.
  • Efficiency
Static UPS topologies run more efficiently than their rotary counterparts over the entire normal operating range with a very significant advantage below 50% load. Rotary UPS systems also appear to sustain higher fixed losses such as .that utilized to preheat the engine coolant and lubrication systems, to power the controls, flywheels, and pony motors associated with the rotary UPS at zero load, and the frictional and windage losses. These standby losses represent the amount of energy required to keep a motor running or to keep a flywheel spinning.
  • Architecture
Rotary UPS systems lend themselves to centralised architecture, whereas static UPS have the flexibility to also deploy as distributed UPS solutions. The advantage with rotary is that the all aspects of power backup can integrated into a single solution. While this may be attractive from a management perspective, it does present the potential of a single point of failure scenario.

Conclusion
There is a broad range of applications for static UPS systems and certainly they are the solution of choice for data centres where there is a trend towards to modular, pre-engineered solutions for all aspects of physical infrastructure including power protection, distribution and cooling - coupled with a need for high availability and high efficiency solutions. Rotary UPS systems are perhaps more suited to environments characterised by multiple short inrushes of power, for example satellite and broadcast stations. Some rotary UPS systems are used in high security installations to prevent electrical eavesdropping, as a cost effective alternative to tempest filters.

For more details, please visit www.apc.com/gb and download a copy of APC white paper #92 "A Comparison of Static and Rotary UPS".