Page 41 - Energize July 2021

TECHNICAL

Improving UPS efficiency
Other than technology improvements, there are two methods to
improve UPS efficiency: topology and modularity. Understanding
how these factors impact efficiency is essential to the selection of
an appropriate UPS system for a particular operation.
Topology describes the way in which components are
interconnected and can determine the mode of operation and
hence the operational losses. There are two common topologies in
use in the large UPS sector: line interactive and double conversion.
Figure 4: Double conversion UPS topology.

Line interactive (LI) UPS systems
LI systems consist of a single converter/inverter stage which load from disturbances and harmonics on the grid, as well
functions as both the battery charger and the inverter. Figure 3 as isolating the grid supply from harmonics and other quality
shows the basic topology. problems caused by the load.

The double conversion UPS uses two converter stages, the first
to convert from the incoming AC supply to DC and to charge
the battery, and the second to convert from DC to the AC that
is supplied to the load. Under normal conditions of a stable
supply, power flows through the two converters to the load.
When the supply is interrupted, the inverter will draw power
from the battery.
Disturbances on the incoming supply will be handled by the
incoming converter module and do not affect the supply to the
load. In addition, waveform distortion and power factor deviations
caused by the load are not reflected back to the supply.
Under normal conditions, with a stable supply, both converters
Figure 3: Line interactive UPS system. are in operation and carrying the load, and both will have losses
dependant on the load. Under supply failure conditions, only the
Under normal supply conditions with a stable AC supply, the load inverter operates, with load dependant losses. With return of
is fed directly from the supply and the battery is charged in parallel power, the inverter supplies the load, and the rectifier supplies the
with the supply via the converter/inverter (C/I). When the battery load plus battery recharge load, with higher losses. The average
is in a fully charged state, the load on the C/I will be very low, and loss will thus be almost equivalent to the steady state loss. The DC
losses will be close to no-load losses. When the supply fails, the system has the advantage of handling incoming disturbances as
C/I switches over to inverter mode and supplies the load for the well as disturbances from the load.
duration of the interruption. Losses under this state will be the full
load dependant losses. When the supply returns, the C/I switches Efficiency comparison
back to converter mode and recharges the battery. Losses during In normal operation the UPS function is only really required to be
recharge can approach full load levels. The LI can operate at a fully operational for the very short duration of power interruptions
very high long-term efficiency if supply interruptions are infrequent. or power disturbances, and this affects the difference in efficiency
The LI has the disadvantage of only responding to supply for the LI and DC topologies. The line interactive UPS operates
interruptions. Unless additional filters are fitted, quality problems under no-load conditions for most of the time and no-load losses
such as sags, surges, over-voltages and transients are all present predominate, allowing a high efficiency to be achieved. The double
in the output. In addition, harmonic distortion and interference conversion UPS version permanently carries the load, resulting in
caused by the load are fed back into the supply. lower efficiency.
The changeover from normal supply to UPS supply is not A double-conversion on-line UPS continuously operates with
instantaneous and will result in short interruption of supply to the efficiency between 85% and 92%, depending on the particular
load. With additional filtering, the LI can handle a limited range of design, compared to 96% to 98% for a line-interactive UPS. In
sags and over-voltages as well as voltage spikes and transients. addition to the cost, this loss results in an additional heat load
that must be removed from the environment, incurring additional
Double conversion (DC) cooling costs that will vary depending on the particular cooling
Double conversion has become the effective standard for IT and system’s efficiency. This becomes a significant factor in the UPS
industrial applications, for several reasons: total cost of ownership. In comparison, a similarly loaded line
• Seamless transfer from grid supply to UPS supply interactive UPS will incur less than a third as much energy cost per
• Built in bidirectional power conditioning, which isolates the load power. 2

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