Page 7 - Uninterruptible Power Supplies

Standby Power Generating Sets
Standby Power Generating Sets 5

base load type of operation is not envisaged. Some installations may be
required to continue running for long periods after normal supplies
have failed (e.g., some military installations) and in such cases 1000-rpm
engines should be considered above approximately 1 MW. Most diesel
engines in use are of four-stroke type, but two-stroke engines may occa-
sionally be encountered. Diesel engines are designed to run on Class A
fuel to BS2869 which has a calorific value of about 42.7 MJ/kg; before
running on any other fuel, advice should be obtained from the engine
manufacturer. The efficiency of a modern turbocharged engine may be
about 40 percent but this does not take into account any auxiliary drives
or the generator losses; the overall efficiency of the generating set will
be less.
The useful energy produced by the engine passes through the cou-
pling to the generator but, depending on the arrangement of the set, it
is not always possible to use all the energy to supply the intended load.
Sets up to a few hundred kVA are usually self-contained but larger sets
may require power for auxiliary items such as cooling and ventilation
fans. The requirement will be small, only a few percent of the genera-
tor rating. There may also be incidental extras such as engine room
lighting and small power and fuel pumps.
A naturally aspirated diesel engine is capable of accepting full load
in a single step but, in order to reduce the size and cost, many modern
engines are turbocharged. This reduces the step load capability and a
modern turbocharged engine with a high brake mean effective pres-
sure (see below) will probably accept only 60 or 70 percent of its rated
load in one step. It follows that for most installations a load switching
sequence has to be followed after the starting procedure.
With a naturally aspirated engine the quantity of combustion air
within the combustion space is constant and there is always sufficient
oxygen for combustion of the maximum amount of fuel. With a tur-
bocharged engine the quantity of surplus combustion air available at
any one time is limited by the turbocharger. A step load change may
cause a sudden increase in the amount of fuel injected but there will be
inadequate combustion air until the turbocharger has had time to
accelerate.
Generating set manufacturers recognize four categories of load accep-
tance and categories 1, 2, and 3 are typical of the sets used for standby
generation:

Category 1 100 percent
Category 2 80 percent
Category 3 60 percent
Category 4 25 percent

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