UPS topology and power quality for mission-critical systems

Maximising power quality in today’s hybrid electrical networks is typically a balancing act between many different factors, all of which must be considered.

One of the most overlooked factors in maximising the quality of power supplied into businesses today is the link between power protection and quality - more specifically, the type of uninterruptible power supply (UPS) deployed to protect sensitive and valuable electrical and electronic equipment.

Simply speaking, during short-term outages the UPS will provide backup power to mask the outage. During long-term outages the UPS will ensure the availability of power for a sufficient period of time to allow systems to shut down properly. In situations where a generator is available, a UPS will ensure the protected loads continue to receive a stable and regulated power supply throughout the outage period.

But in addition to providing backup power, a UPS system will also perform an equally valuable function by protecting equipment from fluctuations in power quality.

How a UPS handles incoming power - the UPS topology - determines how effective the UPS can be in shielding sensitive electronics from power fluctuations. It also determines how frequently the UPS has to go to the battery to protect the load, which ultimately has an impact on the lifetime of the battery and its performance.

Topology and UPS performance

In 1999, the IEC released a series of standards* to ensure consistency in how the performance of a UPS is identified. The standards defined three categories of UPS types: voltage and frequency dependent (VFD), voltage independent (VI) and voltage and frequency independent (VFI). UPS manufacturers then responded to these categories by creating passive standby, line interactive and double conversion (or true online) topologies, respectively.

However, there are significant differences in performance between each of these three topologies.

The passive standby topology provides limited, if any, power conditioning capabilities and is generally used only in non-critical applications, typically to support powered desktop devices.

The line interactive topology, on the other hand, does provide some degree of power conditioning and consequently has been promoted as a solution for mission-critical applications by some UPS manufacturers. However, the difference between a line interactive and double conversion system is as significant as the difference between a passive standby and line interactive system.

The double conversion system provides significantly greater protection than the other two topologies. Some of its advantages over them include:

• The critical load is completely independent of the incoming AC input power.
• The critical load is always being supplied by the output inverter, which is always being supplied from the internal DC bus. When input power fails, there is no transitional sag in the output voltage because the inverter is already operating from DC.
• Fluctuations in input voltage and frequency do not affect the load, as the rectifier converts AC power into DC power to feed the DC bus. This gives the double conversion UPS the ability to operate indefinitely at full load and permits designs that even recharge its batteries with input voltages of 15% below nominal. It may also continue to operate, without discharging the batteries, through voltage sags of 20% below nominal. Likewise, if the input frequency is fluctuating in and out of load specification, the rectifier will continue to make DC power and the output inverter will continue to make 50/60 Hz power without using the battery.
• The double conversion UPS permits dual input. This means that it has separate inputs for both the rectifier and the bypass circuits. Single input models are available if required, but dual input UPS products are incrementally more fault tolerant.
• A true double conversion UPS can be used in a dual bus power system, where the UPS will sync to the designated reference source in all operating modes: on utility, on batteries or while on backup generator.
• A fault on the input line causes the UPS to go to battery power, but the UPS rectifier will not allow power from the DC bus to flow upstream.
• The double conversion UPS eliminates problems with generator compatibility that can occur with line interactive systems.

The bottom line

When supporting power from a generator, a line interactive system may switch to the battery every time other loads on the generator are started as this causes the generator’s output frequency to vary, which these systems may sense as an interruption. This can significantly shorten the battery life and/or its autonomy.

Load steps on a line interactive UPS being supplied from a generator may result in sagging of both the generator voltage and frequency, which would then force the UPS into its battery mode of operation.

Soon thereafter, when the line interactive UPS senses stable generator output, it transfers back to the generator supply and the generator output may dip again, which would create an unstable situation.

Such characteristics of other topologies result in a weaker solution, particularly in times of extreme stress, such as during a power blackout. And these weaknesses are often transparent to the end user - until the UPS fails to perform as expected and equipment starts shutting down unexpectedly or is seriously damaged.

There is a place for other topologies - but they should be used with caution if their deployment is intended as a protection for mission-critical systems.

*Note: The IEC 62040-3: 1999 standard - ‘Uninterruptible power systems (UPS) - Part 3: Method of specifying the performance and test requirements’ - defined the three performance categories VFD, VI and VFI mentioned in the article above. This standard is currently under revision but it is worth noting that, despite the UPS technology advances that have been made since 1999, the three performance categories remain unaffected by the revision that is due to be released in 2010. UPS manufacturers are nevertheless challenged to design and market a VFI UPS that does not necessarily have to be of double conversion topology. So far, this has not been achieved.

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