Page 19 - Uninterruptible Power Supplies

Standby Power Generating Sets
Standby Power Generating Sets 17

Rated Droop
speed Steady state band
Transient
deviation
Recovery
time

Load application Time
Figure 1.8 Typical speed governor characteristic (with droop).

2 seconds on application or removal of 25 percent rated load. Figure 1.8
shows a typical response curve for a speed governor.
Load-sensing speed governors increase the speed of response and
reduce the speed dip. The electrical load is monitored and when a step
increase is sensed a signal is passed to the governor control system
which starts to move the fuel rack early, probably before the drop in
speed is sensed.

Voltage and Frequency Performance Classes
for Diesel Engine–Driven Sets
Specification ISO 8528 includes four performance classes for diesel
engine–driven generating sets, the details of which appear in Table 1.1.
Other performance classes are available, such as ISO 3046 (BS 5514)
for speed governing and BS EN 60034 for voltage control but are not
reproduced here.

Modes of Control
There are three modes of control which may be used in connection with
speed governors and voltage regulators. The error is the basic feedback
signal and is the difference between the measured value and the desired
value.
■ Proportional control is the basic mode. Within a proportional band
feedback is applied which is proportional to, and tends to reduce, the
error. This form of control can achieve a stable state within the pro-
portional band but there will be a deviation from the desired value,
known as the offset.

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