Page 40 - Energize July 2021

TECHNICAL

High efficiency large UPS systems:

Risks and benefits

by Mike Rycroft, Now Media

Data centre server farms and industrial processes require an uninterrupted supply of electricity to
function. The growing financial and environmental costs associated with the energy use of data centres
and industrial applications have given rise to a drive to improve the efficiency of the power supply. All

uninterrupted power is provided through uninterruptible power supply (UPS) systems, and the efficiency of
the UPS affects the overall efficiency of the plant.

he function of the UPS is to ensure a continuous high- No-load losses
quality supply of electricity to the load, by bridging At zero load, all the input power is used by the UPS, hence the
Tthe start-up time of standby plant and protecting the name “no-load” losses. These losses are independent of load
load from power dips and surges on the supply. Conventional and are attributed to powering such things as transformers,
3-phase UPS systems can achieve up to 96% efficiency in capacitors, logic boards and communication cards. No-load
normal operational mode but have been subject to lower losses can represent over 40% of the total UPS loss (Schneider).
reliability and power quality when operated in higher efficiency
modes. Developments have led to new operational modes Operational losses
which can increase efficiency while maintaining the reliability As the UPS load increases, a larger amount of power must be
and quality of power supply. UPS efficiency gains of a few handled by a numerous number of components in its path.
percentage points can lead to significant energy savings with Switching losses from transistors and conduction losses of
consequent financial gains. semiconductors and rectifiers vary in proportion to the load
and therefore contribute to proportional losses.
Efficiency and load level
2
Efficiency is the ratio of the power delivered to the load, to the I R losses
power supplied to the UPS. The difference between the two As load increases, the electrical current running through its
is due to losses within the UPS. Increasing efficiency involves components increases, and causes resistive losses in the UPS
reducing losses. In redundant N+1 and 2N systems, UPSs are dependant on the square of the current, which is dissipated
lightly loaded, typically operating in the region between 20% as heat. Square-law losses become significant (1% to 4%) at
and 40% of their maximum load (Eaton). The efficiency curve of higher UPS loads (Schneider).
a modern UPS is shown in Figure 1. Figure 2 illustrates how the different losses contribute to
overall loss.

Figure 1: Typical UPS efficiency curve (APC). Figure 2: Power loss graph (Schneider).

energize | July 2021 | 38