is fed directly to the loads via the static
switch. This mode is most often used
if there is a problem with the rectifier
or inverter, or if maintenance is being
carried out on the UPS.
A UPS with ESS has exactly the same
four power function blocks. No new
blocks that could increase complexity
or reduce reliability are introduced.
The ESS UPS makes better use of the
blocks, however, by providing a fourth
operating mode – ESS mode. In this
mode, the static switch is closed and
power is fed direct from the mains
supply to the loads, just as it is in an
ordinary UPS operating in bypass
mode.
The big difference, however, is that in
ESS mode, the rectifier and inverter are
held in a state of ‘extraordinary system
preparedness’. This means that, if the
mains power quality falters, the UPS
can switch to full double-conversion
operation in less than two milliseconds.
This is so fast that the transition is
invisible even to the most sensitive of IT
equipment.
ESS UPSs will normally operate
in ESS mode almost all the time,
switching to double-conversion mode
only occasionally to deal with mains
disturbances. In ESS mode, their
efficiency is a truly impressive 99 per
cent as the losses associated with the
rectifier and inverter are eliminated.
The potential for energy savings is
huge and, as an added bonus, because
ESS UPSs run cooler most of the time
than conventional UPSs, they are more
reliable and they have longer working
lives.
Testing
While VMMS and ESS technologies
offer effective ways of significantly
increasing the efficiencies of UPSs
during normal operation, there is
another aspect of energy efficiency
that needs to be considered. To ensure
that the batteries used in UPSs are still
capable of supporting the load for the
required run time, they must be tested
regularly. The most conclusive way of
doing this is with a discharge test that,
as the name suggests, involves almost
fully discharging the batteries while their
performance and capacity is monitored.
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The latest technologies make it possible to reach new energy efficiency levels.
The traditional way of doing this is to
discharge the batteries into a load bank
– essentially a bank of resistors – which
means all the energy the batteries have
stored is converted to heat and wasted.
In a big UPS installation this can be
a significant amount of work – with a
three MW installation recently tested,
for example, the cost associated with
annual battery testing was €50,000.
Another advanced technology –
Easy Capacity Test (ECT) – provides
a solution for regular maintenance.
With ECT, the UPS is temporarily
reconfigured so that its own power
modules can be used to feed the energy
from the batteries back into the supply
system. This eliminates the need for
external load banks and all associated
costs and, as a bonus, no waste heat
is generated and the energy taken
from the battery during the discharge
test is put to good use. In redundant
systems the testing can be performed
concurrently