Stability of synchronous machine with divided-winding rotor

To deal with a divided-winding rotor (d.w.r.) synchronous machine in which the two field windings are not located on the rotor axes and may not have equal numbers of turns, an equivalent machine is introduced with field windings on the direct and quadrature axes. The steady-state stability calculations are carried out by applying the Nyquist criterion to the linearised equations for small oscillations of a system with more than one feedback regulator. This investigation is followed by using the nonlinear differential equations and a digital computer to find the transient response after a change in the tieline impedance. The calculated results are confirmed by experiments carried out on the micromachine equipment at Imperial College, London. The alternatives studied included systems using one regulator at a time, both regulators together and various transfer functions for each regulator. The main conclusion is that the angle regulator stabilises the voltage regulator and has a major influence on the stability of the system; hence the voltage-regulator gain can be varied over a wide range.