Governing: An Examination of a Torque Feedforward Loop

Abstract

The inclusion of a load sensing loop in a governor is known to counteract the effects on engine speed of changes in the external load. To derive the maximum benefit from load sensing, it is necessary to possess a detailed mathematical representation of the individual elements within the governing loops. In this paper, a method of identifying the dynamic behaviour of a small generating set is described. A governor is designed on the basis of this dynamic model, and results are presented of tests performed on the generating set controlled by the governor. The governing action is a combination of the feedforward of a load signal, and the feedback of a speed signal. Careful selection of the gain of the feedforward path results in improved governing with only proportional action in the feedback loop. A simple criterion, derived from the mathematical model, predicts a range within which the feedforward gain should lie. The feedback loop is designed by standard control techniques. The experimental results compare favourably with the theoretical predictions. In the tests on the rig, a steady-state droop of ±1 per cent was held over the full-load range. A 100 per cent step change in the load produced a transient overshoot of less than 1·5 per cent, and a settling time of 1·5 s.