White paper: UPS and generators together –
the only true 24/7 power solution
Fully integrated power protection solutions for today’s
critical loads.
Providing continuous power to critical processing and communication applications such as data centres,
online banking and emergency services is essential, and nothing less than 24/7 can be tolerated.
However, the user’s confidence in the reliability of the utility power supply has deteriorated due to the
fragility of the electrical supply infrastructure and potential supply and demand issues. Consequently the
recognised solution nowadays to meet 24/7 continuous power lies in using a combination of a standby
generator and an uninterruptible power supply (UPS).
While a UPS alone will protect against short term utility power loss and supply quality problems the UPS
will eventually shutdown when its batteries reach the end of their discharge period. Increasing the size
or number of batteries will of course extend the autonomy time but this is becoming more expensive as
the rising cost of lead directly affects battery pricing. Irrespective of this, the exposure to blackout times
exceeding the UPS battery autonomy will always remain a reality.
Conversely, using a standby generator alone will serve as an alternative source of power but will not
provide a no-break solution in the event of a loss of mains power. However, a generator with a secure
fuel supply can provide a source of power, far exceeding the duration of a long term mains blackout and
the back-up capability of a static UPS system.
It is therefore clear that the respective limitations of utilising a UPS or a standby generator alone are fully
overcome by operating these two different sources of back-up power in tandem as shown below.
Enclosed
Enclosed
generator
generator set
set
PowerWAVE
9000DPA UPS
PowerWAVE
9000DPA UPS
Figure 1 – tandem back up system
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During normal operation the utility mains feeds the critical load via the UPS, during which time the UPS
battery is also float charged. The UPS will protect against breaks and disturbances in the mains supply
and after a pre-set time, typically between 2 to 10 seconds, the Automatic Mains Failure (AMF) panel
sends a signal to the generator to start. The UPS battery serves as an alternative source of supply to
support the critical load while the generator starts up. Once the generator has stabilised the UPS accepts
the generator as a mains replacement, continuing to supply the critical load and recharge the battery for
the duration of the mains failure. When the AMF senses that the mains supply is restored and stable it
switches back from the generator, with the UPS battery once again covering the power interruption
caused by the changeover. This sequence ensures uninterrupted clean power to the load and demands
no user intervention.
Close up of battery
However, a standby generator and UPS do not have natural plug ‘n’ play compatibility; they must be
carefully matched with one another to ensure reliable operation. Although reliability is of course a critical
consideration, it is not the only one. There are large cost implications in ensuring that the generator is
optimally sized for the UPS, as well as many physical and environmental factors.
UPS Considerations
Constant voltage and frequency are directly proportional to the size and type of generator. The
generators used by the power generation companies produce consistent power because they are
extremely large. A standby generator, by comparison, is quite small and cannot supply such consistent
power. Any increase in electrical load requires an instantaneous increase in mechanical power to supply
it and whereas in a large generator many of these variations are ‘absorbed’ by the inertia of the rotating
parts, a small generator set with less inertia will actually slow down until the engine governor
compensates. The resultant instability in generator voltage and frequency must be accommodated by a
correctly chosen UPS system.
UPS Compatibility
There can sometimes be compatibility problems between the generator and the UPS system.
The generator output voltage may be acceptable to the UPS, but often the generator’s frequency range
is wider than the UPS is designed to accept. In the worst case the frequency variations of the generator
will be such that the UPS cannot synchronise with it, either because the frequency is outside limits or it is
varying too quickly for the UPS to follow (slew rate) causing, in some cases, the UPS to revert to battery
supply and putting the integrity of its output supply in jeopardy.
This problem can be overcome by ensuring that the generator is properly matched to the UPS and fitted
with an electronic governor that allows the generator to operate within tight frequency tolerances.
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The type of load presented to the generator by the input
stage of the UPS can also cause problems.
Typically a UPS utilises a phase controlled rectifier which
imposes notches on the power feed, interfering severely
with some types of generator control systems. Phase
controlled rectifiers also draw a non sinusoidal input
current, creating harmonics, measured as total harmonic
distortion (THDi). These can cause excessive heating in
the generator alternator, especially as some
manufacturers’ UPS generate up to 30% input THDi.
Data centres rely on 24/7 power protection
Some UPS manufacturers introduce a capacitor and inductor filter at the input to the UPS to attenuate
the level of input current harmonic distortion. However, this is often a passive filter tuned to optimum
load conditions, which can create a leading power factor if the UPS is subsequently lightly loaded. When
a leading power factor is presented to the generator the alternator over-excites and the output voltage
rises. To compensate, the voltage regulator reduces exciter power to reduce the strength of the
magnetic field. Under such circumstances the magnetic field can fail causing the generator to shutdown
or even result in catastrophic alternator damage.
Generator Sizing Guidelines
In addition to the UPS load, the generator may also be required to power emergency lighting, air
conditioning, building alarm systems etc. (see Figure 1). As a very general ‘rule of thumb’, when
assessing the rating of a generator the following multipliers should be applied:
1.5† x the nominal UPS capacity should be allowed for a transformerless UPS
2.0 x the nominal UPS capacity should be allowed for a transformer based UPS
3.0 x the nominal air conditioning running capacity should be allowed for the air conditioning.
† With the UPSL transformerless 9000DPA series, this figure drops to 1.2 times the nominal UPS
capacity, thanks to the much reduced THDi imposed on the generator. Significant savings in both cost
and space arise from this reduction in generator size.
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In practice, it is usually necessary to size the generator to handle more than just the UPS. While the
UPS is typically running data processing equipment, the air conditioning power to cool the equipment
must be maintained. Emergency lighting, communications and alarm systems must also stay
operational.
The above are only guidelines and it is recommended that advice should be sought from the UPS and
generator supplier if independently attempting to size the generator.
Step Loading
Most generators cannot accept 100% of their load rating in one single step. For example, an Auto Mains
Fail (AMF) panel cannot present a 100kVA load to a 100kVA generator in one single ‘hit’. A generator’s
ability to take large load steps is a function of its design and turbo charged generators can, typically, take
larger load steps than standard generators.
It is good practice to not introduce the entire load to the generator when this load is >60% of the
generator's standby rating. This can be achieved by either over-sizing the generator, which is a
potentially expensive option, or by ensuring that load equipment such as the UPS and air conditioning
has a ‘soft start’ facility or by the use of time delay contactors on the essential services distribution board.
Generator Location
A key decision involves whether to site the generator within the user’s building or outdoors. There are
pros and cons to both choices. Indoors, a generator requires significant environmental management; for
example, exhaust ducting, adequate airflow for cooling, noise attenuation, space etc. Outdoors, these
requirements can be easier to handle, but the generator itself must be environmentally protected.
Smaller generators of up to 750 kVA usually have a combined weatherproof and acoustic canopy, while
larger generators are typically installed into a container for out of building operation.
Uninterruptible Power Supplies Ltd. provides full site survey and installation
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The fuel storage tank must be double skinned or bunded, or have a drip tray to contain oil spillage in
accordance with environment contamination regulations. Most standby generators have a base tank,
often referred to as a ‘day tank’ fitted as standard. For longer autonomies a separate bulk fuel storage
tank is required which adds to the cost, space and complexities of the installation.
Electrical Installation
As a minimum, a power cable rated to carry full generator power and a signal cable to carry generator
start/stop signals must be run between the generator and the AMF panel and/or the essential services
board. If the cable from the generator is long then it may be necessary to increase the cable cross
sectional area to reduce the ‘volt drop’ along it. This increases the electrical installation costs of the
generator, so the generator should be located as close as practically possible to the AMF panel and/or
the essential services board.
Uninterruptible Power Supplies Ltd. provides full commissioning and maintenance services
Conclusion
There are obvious reasons and advantages for choosing a single source vendor such as
Uninterruptible Power Supplies Ltd (UPSL) who are experienced in supplying both UPS and
generator equipment and the associated installation and environmental requirements. They
can ensure that the complexities of matching a UPS and generator are taken care of,
removing the responsibility from the consultant and contractor and alleviating any concerns
for the end user. In addition to providing a one-stop solution, features within the UPSL
products also contribute to improving operating efficiency, thereby offering significant
savings in running cost.
A correctly configured and installed UPS and generator combination can give years of
trouble free service and true 24/7 year round power availability even in the event of
extended power failures.
Contact
Uninterruptible Power Supplies Ltd
Bacchus House
Calleva Park
Aldermaston
Berkshire
RG7 8EN
Phone:
Email:
Web:
UPS 245-01-00
0118 981 5151
[email protected]
www.upspower.co.uk