Uninterruptible Power Supplies (UPSs) perform an essential role in
protecting organisations that cannot tolerate any electrical power
interruption. If the mains fails, the UPS’s battery seamlessly takes
over until power is restored. However, UK electricity grid problems
are increasing the likelihood of extended failures that could exceed
the UPS battery’s available autonomy. A standby generator and fuel
supply can solve this, but unlike a UPS, a generator cannot come
online seamlessly when the power fails. Therefore a matched
generator - UPS pair is essential for truly uninterrupted, ongoing
power protection.
To successfully install a generator, it must be matched to the UPS,
sized correctly for the onsite load, and installed into an
environment that is designed to accommodate it. Generator-UPS
matching is important as otherwise each can cause problems for the
other. Generator output voltage is usually acceptable to the UPS,
but its output frequency range, under fluctuating loads, may be too
wide for the UPS to accept. The generator frequency rate of change,
or slew rate, may also be too fast for the UPS to follow safely.
Such problems can be prevented by ensuring the generator is fitted
with an electronic governor maintaining its output within tight
frequency limits.
Meanwhile, UPSs typically have a rectifier charger control circuit
which imposes notches on the power feed, interfering severely with
some types of generator control. Additionally, some charger circuits
draw non sinusoidal input current, creating harmonics, measured as
total harmonic distortion (THDi). These can cause de-rating of the
generators output, especially as some UPSs generate up to 30% THDi.
Also, generators cannot usually tolerate 100% of their rated load
being applied in a single step.
However, careful choice of UPS topology can pre-empt such problems.
Transformerless technology can achieve a THDi of below 3% at full
load, while a separate battery charger circuit, together with a
Generator 'On' signal between UPS and generator reduces the UPS
battery charging current, in turn reducing load, notching and
heating effects on the generator. Step loading on the generator is
controlled primarily by soft start of the rectifier current,
eliminating high current peaks during mains recovery. Sequential
switch-on of UPS rectifiers as a parallel array within the UPS is
another possible way of reducing UPS step loading on the generator.
The generator must be sized for its critical load as well as matched
to the UPS. This may comprise emergency lighting, air conditioning,
building alarm systems and other services as well as the UPS load.
For example, there is no point in maintaining power to ICT equipment
without also supporting the air conditioning essential to
maintaining equipment-acceptable ambient temperature. It is also
good practice to work to the generator’s continuous rating rather
than its higher standby rating, as it can be called upon at any time
to work for any duration.
Planning the chosen generator’s installation is subject to several
environmental and physical considerations. Once the autonomy has
been established, the amount of fuel required and means of storage
can be established. Bunding is essential to ensure fuel oil cannot
leak into the water supply. Positioning of the generator is
influenced by further factors that must all be allowed for. For
example generator start-up and running noise may be acceptable in a
given location during the day, but intolerable at night. As the
mains can fail at any time, either a different location must be
chosen or a ‘bespoke’ acoustic housing used. The cost of this
increases with its attenuation rating.
All generators produce heat as well as power, which is mostly
removed by air cooling. Therefore, generators are typically located
outside, in weatherproof and acoustic enclosures, where a plentiful
supply of cooling air is assured. An outside location also allows
easier exhaust fume venting, obviating problems associated with
installing exhaust pipes within buildings. For internal
installations within buildings ventilation and noise requirements
can be given using specialist acoustic equipment such as attenuators
and exhaust silencers. These are often manufactured specifically for
the application and the complete system assembled on-site by
specialist installation engineers.
Other physical considerations are also important. Even a small 100
kVA standby generator weighs several tonnes, weight which must be
allowed for when planning its delivery and positioning. Special
delivery vehicles and lifting equipment may be needed. The generator
should also be as close as possible to the main electrical
installation to minimize runs of expensive power cable, as well as
volt drop losses.
Adding a generator calls for legislative compliance as well as
satisfying the site’s environmental considerations. Local authority
planning requirements vary with area, so it is essential to check
these before installing a standby generator. If large quantities of
fuel are to be stored on site then compliance with the Environment
Agencies PPG02 regulation is required.
All of these factors are important to the success of adding
generator capability. However, with planning none are particularly
onerous, and the benefits of generator protection can far outweigh
any inconvenience or cost.
Email:
sales@upspower.co.uk