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8/12/2019 [] Transformer or Transformerless Ups[2003]{Koffler}
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PEE
ROBIN
KOFFLER
ASSESSES THE MERITS OF TWO DIFFERENT ON-LINE
UPS TECHNOLOGIES
otal power loss can be one of the most
devastating events for any business.
Critical systems can no longer function,
manufacturing ceases and, perhaps worst of
all, crucial data is at risk. The solution is at
T and however, by specifying a suitable
uninterruptible power system
UPS).
But, with so
many different alternatives to choose from, which is
most appropriate for your circumstances?
Nowadays, the traditional transformer-based on-
line UPS is being usurped by the latest transformerless
design, at power ratings below 40kVA. The advent of
sophisticated microprocessor controlled electronics
and advancements in power electronics have allowed
UPS
manufacturers to produce ever more capable
devices at lower costs. And, because the latest breed of
transformerless on-line UPSs dispense with
conventional bulky and heavy transformers, they offer
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Power management
a highly compact and cost-effective solution for many
modern power protection environments.
Make no mistake, in most circumstances these
smaller, lighter and lower cost UPSs offer an ideal
solution for any organisation looking to provide back
up power for the ir computer and telecoms equipment.
As with most things, however, there is a trade off
between certain characterist ics, some of which are an
essential consideration for particular applications,
while others assume a lesser importance.
An average transformerless on-line UPS with a
reasonable specification can provide adequate power
protection for a wide range of crit ical devices. Because
of the recent proliferation of t ransformerless UPSs,
most customers will be able to find a suitable device
within their budget, boasting a specification that
appears to meet their needs. However, comparative
specifications can be misleading and users that operate
in high risk mission critical sectors such a s telecoms
and military environments, o r those operating highly
sensitive equipment such as mobile communications
base stat ions and file servers, should examine exactly
what they require from their UPS, before committing
to
a
particular technology
Briefly, a transformer-based on-line UPS boasts
several intrinsic advantages compared to the more
recent transformerless technology For many years,
experienced electrical engineers have been aware of
the fact that transformerless UPS devices are
susceptible to interference from spikes and transien ts
caused
by a
variety of devices such as photocopiers,
laser printers and motors. The dirty mains that this
produces can be transferred through
a
UPS,
and feeds the connected load. This
installations (BS-EN 60601 and 61558-2) require that
critical devices be connected through a Galvanic
Isolation transformer, rather than directly to the raw
mains.
Any electrical installation that requires the utmost
reliability needs to deploy proven solutions to
minimise the likelihood of potential equipment
failure. One of the major advantages of transformer-
based UPS devices is their higher mean time before
failure (MTBF). Because these devices use traditional
electrical engineering principles instead of a high
concentration of electronic components, they are
inherently more reliable than a corresponding
electronic c ircuit board assembly
employ sophisticated electronic
circuitry to mimic the on-line
characteris tics of a transformer-
based device, as well
as
providing suitable standards
of mains filtering. As such,
they can be more prone to
failure on sites where
By contrast, transformerless UPSs
it can do nothing to improve the quality
of raw mains tha t can be variable to say the least.
In contrast, because the transformer itself
constitutes
a
physical barrier, a transformer-based on-
line UPS completely isolates the load from the worst
vagaries of the mains supply Therefore, in
circumstances where th e load is likely to be affected
by
a very large variation in its power supply, a
transformer-based UPS provides a safer and more
robust solution than transformerless technology-
simply because its size and construction affords some
inert ia between the input and output waveforms, with
no additional electronic filtering required.
Also, because transformer-based UPSs inherently
contain Galvanic Isolation, the power supply fed to the
load is invariably superior to the mains supply itself.
This attr ibute alone can be a major consideration for a
number of crucial applications and installations. In
fact, the latest electrical standards for medical
within industrial and/or critical environments such as
medical and transportation infrastructure.
However, it should still be noted that, because
transformerless designs use plug-in circuit boards,
they are actually quicker to repair than transformer-
based units. In contrast, faul ts with transformer-based
designs can be more difficult to diagnose on-site.
Of course there a re some very compelling reasons to
specify a transformerless UPS. Among these is the fact
that conventional transformer-based uninterruptible
power systems are significantly larger than their
transformerless brethren. The main reason for this is
that th e transformer itself t akes up a large amount of
space within the cabinet, which results in the
transformer-based UPS being significantly heavier,
and therefore not as convenient or portable as the
transformerless variant.
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Moreover, transformer-based uninterruptible power transformer-based device. This is why several
systems operate typically
10
less efficiently than transformerless on-line UPS manufacture rs offer
their transformerless stable mates, as the transformer additional cabinets to house filter s and isolation
itself causes heat to build up within the vicinity of the transformers so that their systems can approach the
device, leading to greater care in terms of airflow and benefits required for industrial and heavily polluted
air conditioning. Some transformerless on-line UPSs environments.
also incorporate an extra economy mode to further In addition, a traditional transformer-based UPS
boost efficiency up to
99 .
will prove more reliable over an extended period than
The potential anti-soc ial character isti cs of the sophisticated but sensitive electronic component based
transformer-based UPSs are heightened by the noise filters. This makes the transformerless UPS more
they can make -frequently a loud and intrusive suitable for circumstances where the incoming mains
hum- which can often be compounded by thei r cooling supply is less polluted. Equally, they are more suited to
fan noise. In practice this means that moder n office-based environments
thi s type of UPS is often unsuitable for
THE USERS HAVE
where cost and footprint are more
siting in an office or small computer and
significant considerations, and where
telecoms rooms, particularly where the
TO BALANCE the UPS may be likely to be sited in full
levels of noise and heat emitted might
view of employees and visitors alike.
cause a problem for staff.
AND
Of course, there ar e certain situations
Transformerless UPSs are quieter in
TERM BENEFITS
where a transformerless UPS will have
operation, and ru n considerably cooler.
an advantage. For example, where there
Conversely, the heat and noise generated
by transformer-based UPSs makes them significantly
less efficient in operation than transformerless
devices. This makes them more costly to run, with a
higher current drain than the electronic switching
circuitry in a transformerless UPS.
Although transformerless UPSs are more complex
in design, because the majority of components are
electronic, they are cheaper to manufacture than the
transformer-based variety Rather like mass-produced
electronics, many transformerless UPS manufac turers
can take advantage of significant economies of scale in
their manufacturing processes, further reducing the
costs of designing, developing and producing this type
of uninterruptible power supply. Transformer-based
UPS devices differ significantly, in that they a re more
specialist electrical items, and therefore cannot benefit
to the same degree from mass produced off the shelf
electronic components. This makes them typically
up to 30 more expensive to purchase than
transformerless devices.
What this suggests therefore is that, while the
transformer-based UPS might be the ideal choice in
certain circumstances, economic considerations,
together with issues of space and environmental
impact, may sway the decision towards a
transformerless model. There is no doubt that
transformer-based devices are ideal for sites
that experience heavily polluted mains supplies -
particularly industrial, rural and complex
infrastructure locations, such as hospitals. In these
circumstances, any UPS would be expected to offer
dependable long-term protection from repetitive
transients and electrical noise.
Despite containing sophisticated electronic circuits
tha t smooth out such anomalies, transformerless UPSs
cannot fully isolate the load from the mains, unlike a
are sudden step load changes on the
UPS
a transformerless device can react immediately while a
transformer-based unit will take a little time to
respond. Whether this dynamic weakness affects
the load depends very much on
its
individual
characteristics.
In addition, a transformerless unit can provide
input power factor correction capabilities as an
integra l part of its electronic circuitry This is a major
advantage for certa in types of load. In this case, power
factor leading or lagging can be caused by the
connected load, which a transformer-based UPS tends
to delay As such, this type of
UPS
requires a separate
filter
or
power factor correction device.
The corollary of this is that -because of the
principle of Galvanic Isolation
- a
transformer-based
uni t is incapable of generating DC current tha t might
be passed to the connected load. In contrast, a
transformerless UPS will pass on the DC current.
Because DC current causes up to a lox heating effect
compared
to
an alternating current, this inevitably
stresses the unit. Additionally, a transformer-based
UPS will allow
a
load that has a high neutral current
requirement to take such a cu rrent, by simply passing
this through from its input.
In summary, therefore, choosing the most suitable
UPS for your needs is rather like deciding between a
diesel and petrol car. When selecting the right
technology to employ, the user has to balance short and
long term benefits against budget an d overall life cycle
costs. Consultation from a supplier that can offer an
impartial recommendation from both technologies
should lead to the adoption of a secure and robust
power solution.
Robin Koffler i s general manager of UPS manufacturer
Riello Galatrek R.koffler@riello ups.co.uk
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mailto:[email protected]:[email protected]