Page 92 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
90 Chapter Three

which the set is intended to supply, the additional loads to be consid-
ered being:

■ The losses within the generator, which may be 5 or 6 percent
■ The radiator fan if it is not engine driven
■ Any electrically driven ventilation fans, engine room lighting, etc.,
that are not included in the load to be supplied
■ Any engine auxiliaries such as pumps that are driven by electric
motors
■ Switchgear and distribution losses, which should be small (probably
less than 0.5 percent) but should not be forgotten

It will be seen that the total of these loads and losses could be signif-
icant and must be considered in the engine power rating.

The Step Loading Ability of the Engine
Only naturally aspirated diesel engines are capable of accepting full
mechanical load in one step, most modern engines are turbocharged.
The maximum step load acceptable is very much dependent upon the
engine brake mean effective pressure, and manufacturer’s advice must
be sought. ISO 8528 includes guide values which appear elsewhere in
this book. Typically, modern engines will accept a 60-percent step of
load and a portion of the total load has to be separated and the power
supply to it delayed in order to keep within the limit.
When load is applied to the engine there is an immediate decelera-
tion and, until the governor is able to respond, some energy is extracted
from the inertia of the engine, flywheel, and generator rotating system.
The governor responds by moving the fuel rack to increase power and
normal speed is restored.

The kVA Rating of the Generator
The factors to be considered in sizing the generator are:
■ The class of power rating specified for the generating set (continuous
power, prime power, or limited time running power)
■ The voltage performance class G1, G2, G3, G4, or as otherwise spec-
ified (see Table 1.1)
■ The maximum load (kWe) to be supplied
■ The maximum reactive load (kVAr) to be supplied
■ The maximum step of reactive load (kVAr) to be applied

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