Maintaining UPS efficiency at low loads


Modern transformerless UPSs offer significant energy efficiency improvements over their earlier transformer-based counterparts, with consistently high efficiency levels over most of the load spectrum. Even so, if their load is allowed to drop below about 25 percent of their capacity, efficiency starts to reduce sharply.

Solving the problem

However, the problem can be solved by modular UPSs that support a feature called Xtra VFI – a smart way to minimise losses and improve efficiency when running in double conversion mode. Xtra VFI automatically reduces the number of active modules if the load drops; this decreases UPS capacity and keeps the modules that remain active more fully-loaded. Any modules that have been switched to standby can instantly and seamlessly resume active mode if the load increases.

Fig.1 shows how Xtra VFI improves UPS efficiency at low load.

Above: Fig 1

Xtra VFI is essentially scaling the UPS to the load, automatically, continuously, and in real time. In calculating the module count for maximum efficiency, the system allows for desired redundancy levels. These, together with the highest expected load step, can be configured by the user to guarantee the best possible protection level. If an electricity mains failure or alarm occurs, Xtra VFI is automatically deactivated, with all modules switched to active status. 

Xtra VFI application example

Kohler Uninterruptible Power’s PowerWAVE 9500DPA is a modular UPS that provides Xtra VFI capability. The UPS could support, for example, an 800 kW load with N+2 redundancy by using two 500 kW frames. Each would be fully populated with five 100 kW modules, giving a maximum total capacity of 1000 kW – enough for the 800 kW load with N+2 redundancy.

PowerWAVE 9500DPA

If the load drops to 200 kW, six of the modules will be switched to standby mode. The four remaining on line are sufficient to support the reduced load, while maintaining N+2 redundancy.


The PowerWAVE 9500DPA’s transformerless modular design means that it can be rightsized to the maximum expected normal load. However, even in the most carefully managed data centre environments, loads can vary unexpectedly for operational or business reasons. The Xtra VFI mode ensures that the UPS can accommodate these changes without loss of efficiency. As a result, the PowerWAVE 9500DPA will consistently operate at up to 96 percent efficiency – or ≥ 99 percent in eco-mode.




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