Page 9 - Uninterruptible Power Supplies

Standby Power Generating Sets
Standby Power Generating Sets 7

Standard Reference Conditions for Diesel Engines
The standard conditions for diesel engines are specified in ISO 8528-1
and ISO 3046-1 as:
Total barometric pressure 100 kPa (1 bar)
Air temperature 25°C
Charge air coolant temperature 25°C
Relative humidity 30 percent

The barometric pressure of 100 kPa is equivalent to an altitude of
150 meters above sea level. The altitude at which an engine is working
has an important effect on the engine’s performance. At high altitudes
a smaller mass of aspiration air is drawn into each cylinder, less fuel
can be burnt and less power is produced.
The ambient air temperature of 25°C imposes some limitation of out-
put in temperate and warm climates. It is usual to allow a 10°C rise in
the engine room ventilating air and if the outside ambient exceeds
15°C the temperature within the engine room is likely to exceed 25°C
and some degree of derating will be necessary. The engine manufac-
turer should be made aware of the maximum operating temperature
expected and of its duration.

Gas Turbines
Gas turbines are currently available from about 500 kW upwards; at
the time of going to press units as small as 50 kW were being consid-
ered for combined heat and power purposes so there may be a future
downward trend in the size of gas turbines for standby power applica-
tions. The turbine shaft will run at tens of thousands of rpm and a gear
box is interposed between the turbine and the generator, which will run
at 1500 or 3000 rpm for 50-Hz supplies.
There are two types of gas turbine, the single-shaft machine and the
two-shaft machine. The single-shaft machine has the turbine and the com-
pressor on a single shaft, and is used for standby generation applications.
The turbine and the compressor run at constant speed and the mass flow
of air through the machine is constant. There is always adequate combus-
tion air and any increase of fuel leads to an immediate increase of power;
full load can be accepted in a single step.
The two-shaft machine is of higher efficiency but is not used often for
standby generation purposes because it has a poor step load acceptance.
The turbine and the compressor are on separate shafts, the compressor
being driven by the turbine exhaust gas flow. The speed of the compres-
sor and mass flow of air through it is variable; any increase of load

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