The maximum voltage variation to constant load is determined as follows:
One-half the observed voltage bandwidth (B) is the plus or minus value of the voltage deviation at
Divide each of the values obtained in step (a), above, by the rated voltage of the generator, and
multiply by 100 to convert to percentage.
Determine the maximum overshoot and undershoot at each load step, express as a percentage of its rated
voltage, as follows:
From the meter recording charts, determine the maximum amount that the voltage trace goes
beyond the line (C, Figure 12-9) of the observed voltage band following the load change.
Divide the result obtained in step (a), above, by the rated voltage (as given on the generator
nameplate), then multiply by 100 to convert to percentage.
Do not use the constant operating voltage at each load as the divisor in the computation. Use
only the rated voltage of the generator.
Determine the time required to restore stable voltage conditions after each load change (recovery time) as
(a) The prescribed steady-state voltage bandwidth, extended to the point at which the voltage trace
leaves the prescribed steady-state band, shall be considered as the time at which the transient
conditions begin. The point at which the voltage trace enters and remains within the prescribed band
after a load change shall be considered as the point at which stabilization begins.
(b) Measure the distance (in inches) on the chart from the point where the voltage trace leaves the
prescribed steady-state band to the point where it re-enters, and remains within, the prescribed
voltage band for the next load condition.
(c) Divide this distance by the chart speed (in inches per second). This is the voltage recovery time,
Determine the voltage regulation for all load changes (for example, rated load to no ad, 1/2 rated load to
no load to 1/4 load, etc.) as follows: (a) Using the indicating voltmeter readings, subtract the load value of
voltage from the no load value for each load change (for example step (a) to step (b), above).
For voltage regulators utilizing single-phase voltage sensing, the value of voltage in the
sensed phase only, shall be used in the above calculations. For voltage regulators utilizing
multiphase sensing, the average value of the sensed voltage shall be used.
Convert each of the values obtained in step (a), above, to a percentage of rated voltage by dividing
them by the rated voltage, and multiplying by 100. This is the voltage regulation expressed in
Repeat step d(1), above, substituting frequency for voltage.
The results of the data tabulated above, shall agree with the following requirements:
(a) Frequency regulation. The frequency regulation for zero droop shall not exceed one-fourth of 1
percent of the rated frequency for each load change up to, and including, rated load.
(b) Frequency stability. At each constant load, frequency shall be held within a bandwidth equal to
0.5 percent of the rated frequency. Hunting shall not be permitted within the 0.5 percent band.
(c) Frequency transient response. Overshoot and undershoot shall not exceed 4 percent of the rated
frequency, and the circuit breaker shall not trip when the load is suddenly decreased from rated load
to no load in one step.
(d) Voltage regulation. Voltage regulation shall not exceed 2 percent of the rated voltage.