Stability of synchronous machines with 2-axis excitation systems

The paper describes analogue- and digital-computer studies of a synchronous machine with various 2-axis-excitation control systems. The steady-state and transient performances of the same machine are analysed, assuming different control schemes, such as rotor-angle control and asynchronised operation, and are compared with a conventional machine. The effects of damper windings, regulator time constants and stabilising circuits on the steady-state performance are shown by regulation curves. It is confirmed that the voltage-regulator loop gain has virtually no effect on steady-state stability, provided the winding with a.v.r. control is aligned with the flux axis by an angle regulator. The improved transient-stability limits obtained with high gains are shown. The fundamentally different transient behaviour of unregulated doubly excited and conventional synchronous machines is explained, and confirmed using accurate mathematical models of the machines. The method of `small oscillations¿ is applied to determine the speed stability of an asynchronised synchronous machine, and the transient performances of three different control schemes are compared in terms of swing curves and switching-time curves.