Page 16 - Uninterruptible Power Supplies

Standby Power Generating Sets
14 Chapter One

It is important to note that the voltage regulator on its own does not
determine the recovery time of the generator output voltage, there are
other and longer time constants in the system. The regulator can
respond very quickly to a voltage dip by increasing the voltage across
the exciter field circuit, but the exciter field circuit is inductive and
introduces a time constant. The exciter in turn increases the voltage
across the main field which is highly inductive and introduces an addi-
tional and longer time constant. The overall response time is improved
by field forcing, or applying say twice the normal voltage to the exciter
field thereby increasing the rate of rise of current in the main field.
For voltage rises the regulator can do no more than reduce the
exciter field voltage to zero, leaving the main field current to circulate
in the highly inductive loop formed by the field and the associated rec-
tifier. The current will decay at a rate determined by the time constant
which will be fairly long, but flux reduction is encouraged by the
demagnetizing component of the load current (i.e., lagging vars).
Figure 1.6 shows a typical response curve for a voltage regulator.
There are four voltage regulating performance classes for diesel
engines defined in ISO 8528 and the data relating to them appear in
Table 1.1. These performance classes may be regarded as being equally
applicable to gas turbine driven generating sets.

Speed Governors
The speed governor maintains the speed of the engine and generator
within the specified limits. It has developed from the simple Watt gov-
ernor to the sophisticated electronic governors available today.
The simple Watt governor has no amplifying mechanism and depends
on the centrifugal force acting on two or more rotating weights, the force

Rated voltage Steady state band
Transient
deviation

Recovery
time

Load application Time
Figure 1.6 Typical voltage regulator characteristic (with zero droop).

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