(e) Voltage stability. The voltage shall remain within a bandwidth equal to 1 percent of the rated
voltage at every constant load.
(f) Voltage transient response.
1 The instantaneous ms voltage shall not drop to less than 80 percent of the rated voltage, and
shall reach stable condition within 3 seconds with sudden changes from no load to half load, and
half load to full load. Overshoot or undershoot of the final voltage shall not exceed the initial
voltage transient in amplitude.
2 The instantaneous rms voltage shall not rise to more than 130 percent of the rated voltage, and
shall reach stable conditions within 3 seconds with sudden changes from half load to no load.
Overshoot or undershoot of the final voltage shall not exceed the initial voltage transient in
3 When a balanced three-phase 0.4 pf, or less, lagging static load with an impedance of 0.5 per
unit is applied to the output terminals of the set. Operating at no load, rated voltage, and rated
frequency, the instantaneous ms voltage shall not drop to less than 60 percent of the rated
12-24. FREQUENCY ADJUSTMIENT RANGE AND UNDERFREQUENCY TRIP.
General. The frequency of the generator set must be adjustable to provide rated frequency at various load
conditions, and to synchronize two or more generator sets for parallel operation.
Apparatus. Instrumentation for measuring load conditions, field voltage and current, and ambient temperature
shall be as illustrated in Figure 12-3 and Figure 12-5.
Preparation for test. Connect the load and field instrumentation in accordance with Figure 12-3 and Figure
Start and operate the generator set and allow it to stabilize at rated load, voltage, and frequency.
During this period, readings of the load and field instrumentation shall be recorded at minimum
intervals of 10 minutes. If necessary, adjustments to the load, voltage, and frequency may be made
to maintain the rated load at the rated voltage and frequency. Adjustments to the voltage and
frequency, however, shall be limited to those available to the operator: specifically, adjustments to
the voltage and frequency adjust devices. Adjustments to load, voltage, or frequency shall be noted
on the stabilization data sheet. Stabilization will be considered to have occurred when four
consecutive voltage and current readings of the exciter field either remain unchanged, or have only
minor variations about an equilibrium conditions with no evident continued increase or decrease in
value after the last load, voltage, or frequency adjustment has been made.
No further adjustments shall be made to any set control for the remainder of this test except for the
control panel frequency adjust device.
For each of the conditions in the following steps, allow approximately 2 minutes between each
adjustment and the subsequency instrument readings.
Adjust the generator set frequency for the specified minimum frequency (57.5 at 60 Hz, 47.5 at50
Hz) at rated load. Read and record all instrument readings.
Reduce the load to zero.
Adjust the generator set frequency for the maximum attainable frequency. Read and record all
instrument readings. If the over-frequency or overspeed protection device acutates, read and record
all instrument readings just prior to the point of actuation, and note on the data sheet that the
protection device actuated.
Adjust the generator set frequency for the minimum attainable frequency. Read and record all
instrument readings, and trip point.
Repeat steps c(1) and (2), for each frequency.
Results. The data sheet shall show the maximum and minimum frequencies attained at rated load, the
maximum and minimum frequencies attained at no load, and actuation of the protection devices. Compare
these results with the requirements of Table 12-3.