Page 6 - Uninterruptible Power Supplies

Standby Power Generating Sets
4 Chapter One

Limited-time running power Power limit
Power

Time
Figure 1.3 Illustration of limited-time running power.

Limited-Time Running Power (LTP)
Limited-time running power (Fig. 1.3) is the maximum power which a
generating set is capable of delivering for up to 500 hours per year, of
which a maximum of 300 hours is continuous running, between the
stated maintenance intervals. It is expected that the periods of running
will be long enough for the engine to reach thermally stable conditions.
This may be suitable as a low-cost option for standby supply generating
purposes. It differs from the PRP rating in that it allows the engine and
the generator to run at full capacity for the permissible running time.
The engine wear rate will be greater, the generator will run hotter and
the insulation will deteriorate at a faster rate. The set will therefore
have a shorter life expectancy than a more conservatively rated set.

Power Limit
In addition to the three power ratings described above, ISO 8528 rec-
ognizes a power limit, determined by the fuel rack stop on the engine
fuel-injection system, which is greater than the power required to sat-
isfy the ratings. When the engine is supplying its maximum power to
the generator, this surplus power is available for governing purposes
and is necessary if speed is to be maintained within correct limits.

The Power Unit
Diesel Engines
The development of diesel engines has progressed steadily over many
years due to improved techniques and knowledge of machining, lubri-
cation, metallurgy, combustion, and noise and vibration control.
Engines are available from a few kilowatts upwards at speeds of 750,
1000, and 1500 rpm for 50-Hz supplies and 900, 1200, and 1800 rpm for
60-Hz supplies. Reliability and cost reduce with increasing speed. Standby
sets up to about 1.5 MVAmay run at 1500 rpm provided that a long-term

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.