The Effect of Reactive Compensators and Coordination With Volts/Hertz Limiting

It is important to evaluate all of the variables necessary in the setup and commissioning of the generator excitation system to ensure predictable behavior of the synchronous machine during all conditions of operation. The digital voltage regulator has several programmable features that enable setup for proper operation. A few of the many controls include reactive droop compensation, line drop compensation, and volts/hertz limiting. Often, for reactive sharing, reactive droop compensation is utilized with either a small percentage of droop or no droop when connected to a large step-up transformer. Line drop compensation regulates the terminal voltage to a point beyond the generator terminals into a step-up transformer by using a small percentage of terminal voltage rise at the generator output. The benefit of line drop compensation is its greater reactive contribution from the generator during a disturbance. Volts/hertz limiting ensures safe operation of the synchronous machine for either underfrequency operation or generator overvoltage. Whereas the use of reactive droop compensation is fairly predictable with volts/hertz limiting, line drop compensation also needs to be coordinated with volts/hertz limiting. In this case, it is necessary to carefully examine the settings of the line drop compensator versus the setting of the volts/hertz limiter. If unchecked, the two settings may overlap unfavorably. For example, if the volts/hertz limiter set point were lower than the line drop compensation setting, there could be undesired reduction in excitation. This paper discusses the two types of reactive compensators and the volts/hertz limiter to provide a better understanding of their effect on the generator excitation system.