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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:R.koffler@riello-ups.co.ukmailto:R.koffler@riello-ups.co.uk