Synchronous Machine Damping by Excitation Control.

The low inherent damping of the electromechanical system formed by the rotor of a synchronous machine operated in conjunction with an a. c. power supply system, is augmented by variation of the field current of the synchronous machine. The control operates at a relatively low power level, in some respects as an alternative to a damping winding, and although the damping obtained varies with the ope rating conditions it is shown that a satisfactory transient response is possible for a wide range of loadings. The system is given a simple theoretical treatment using phase plane diagrams, small signal linearisation of equations and an analogue computer simulation, and the results show the feasibility of excitation control for damping purposes. A practical system is described which uses an electronic exciter operated in conjunction with current feedback to give field forcing. Load angle and load angle rate signals are derived using pulse rate techniques employing a commercial shaft mounted incremental digitiser. The control scheme is applied to a small laboratory machine and the results of some comprehensive testing with transient disturbances from both the mechanical and the electrical sides of the system are displayed. The use of current negative feedback in conjunction with the exciter suggests that a simple second order approximation to the system should give worthwhile predictions of the transient behaviour for small disturbances. The experimental results, although masked by noise, appear to confirm this conclusion. The most appropriate application of the scheme would appear to be to synchronous motor drives where the load provides limited damping and transient torque changes are likely to produce objectionable oscillations. The scheme could also be applied to synchronous generators to minimise hunting and ensure oscillatory stability but its use in conjunction with a voltage regulator would require careful optimisation of the relative regulator gains