Appendix C. Voltage Regulator Definitions And Adjustments - Kohler KG40 Operation

Appendix C. Voltage Regulator Definitions and Adjustments
The following definitions and adjustment/setting specifications are intended for users planning to adjust the voltage regulator
beyond the default settings in order to customize the alternator for a specific application.
This information is not intended to be a comprehensive explanation of all the terms mentioned. There are numerous documents
available that define these terms more completely than described herein. Any user planning to change the generator set
controller adjustment settings or to apply the generator set to these types of applications should understand these terms.
This appendix contains references to other sections of this manual. Please refer to these sections for further information and
explanation.
Paralleling generator sets can be a complicated and dangerous exercise. Application programming must be performed by
appropriately skilled and suitably-trained personnel.
Definitions
Underfrequency Unloading
Underfrequency unloading is a function used in the alternator excitation control system to improve the overall generator set
system (engine and alternator) response. In particular, underfrequency unloading relates to large-block load applications. When
applied to engine-driven alternators, large-block loads cause a subsequent transient torque load on the engine. This torque load
can reduce the engine's speed below the normal operating point. Typically, the engine speed controller or governor will
compensate for this by commanding an increase in fuel. If, however, the fuel system is inadequate to recover from a relatively
large load, the speed may never recover. In these instances, other measures must be taken. This is where the underfrequency
unloading occurs.
When the excitation control system detects a drop in the speed or electrical frequency below some predetermined point, the
control system enters an unloading condition. This can be described as moving to a lower voltage regulation point. By reducing
the output voltage of the alternator, the load on the generator set is reduced. This can be shown mathematically by Ohm's law,
which states that power is equal to the voltage squared divided by the impedance. As the voltage is reduced, the power delivered
by the alternator decreases by a squared relationship. Since it is the power in the alternator that translates into engine torque,
the engine load is also reduced.
By changing various parameters of this compensation technique, the controlling system can be tailored to match the performance
capabilities of most engine and alternator combinations. The point at which the unloading begins to act or how much unloading
occurs can be adjusted to impact maximum voltage droop, maximum speed droop, or time to recover. Some applications may
not need unloading and, in these cases, set the unloading parameter to disable the function. These parameters are further
described below. An example is provided to help clarify the relationship between these parameters.
Underfrequency Unload Slope
Underfrequency unload slope is the term used to describe the amount that the voltage is reduced, per-cycle-per-second or per-
hertz (Hz), when in an underfrequency condition. The slope or schedule is sometimes called the volts-per-hertz slope. When the
electrical frequency drops below the cut-in point (see below), the excitation control system temporarily reduces the regulated
voltage to reduce the subsequent torque on the engine. The amount that the control system reduces voltage is defined as the
product or multiplication of the slope and the amount of frequency or speed below the cut-in point. For every Hz below the cut-
in point, the control system reduces the line-to-line voltage by an amount equal to the slope.
Because each engine responds differently to the various loads encountered, the slope may be adjusted to improve the system
response. If, when large loads are applied to the generator set, the engine speed drops below the acceptable limit (as determined
by the particular loads applied), the slope may need to be increased. Increasing the slope will cause the voltage to droop more
during load applications, consequently reducing the load torque on the engine and allowing the speed to increase. If, however,
the voltage drops below an acceptable lower limit (as determined by the particular loads connected to the generator set), a lower
slope may work better. The underfrequency unloading function may be disabled by setting the slope to zero.
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