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Chapter 12 Helpful information
as an unstable power source. If this occurs, the UPS remains on battery permanently,
which will ultimately cause the battery to fail and drop your load.
2. Increase the generator size. Using the proper size of generator is essential to ensuring
the safety of both the UPS load and an organisation's personnel during a lengthy
outage. Many companies expect the generator to accommodate air conditioning,
emergency lighting, communications and other vital services. Yet if the generator isn't
large enough, it won't be able to hold voltage and frequency within input tolerances
when the UPS comes online.
The bigger the generator engine, the more stable the frequency and therefore the more
it can handle as the UPS comes off battery. With that in mind, whenever budgets
permit, it is wise to increase the generator size and also allow for some potential growth
with the generator-friendly UPS. The general recommendations for UPS models up to
25 kW are as follows:
2 times the total load (including the UPS, A/C, and all other equipment that must
•
remain online) for natural gas-powered and mechanical governor generators.
1.5 times the total load for propane or diesel-powered generators and those with
•
an electronic governor.
Attention! When dimensioning a UPS unit and / or a power generator always consider the
3. Consider the fuel source. The most common generator fuel options include propane,
natural gas and diesel, each of which comes with its own set of own advantages and
disadvantages. Natural gas-operating generators can be slower to respond and may
need to be sized larger than their propane counterparts. Diesel is widely considered the
best fuel and is predominately used for 50 kW and larger generator solutions. However,
it has a short storage life and its cost can strain budgets.
4. Don't overlook the governor. Portable generators are equipped with a governor which
limits the speed of fuel being delivered to the engine to a safe level amid load changes.
Electronic governors are quick to respond, while mechanical governors are slower and
can also cause calibration challenges.
5. Consider the UPS topology. The type of UPS will also affect UPS generator
compatibility and configuration, as not all can compensate for frequency variations
without relying on the battery. Both standby and line-interactive UPS units use battery
power to prevent frequency variations from affecting the protected load. A double-
conversion, online UPS, on the other hand, recreates the sine wave and filters frequency
6U Rack or Tower UPS RT8kVA and RT11kVA IUOMG
reactive load and not only the active load!
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