Stability analysis of turbogenerator with direct and quadrature excitations and interaction of regulators

A state-space model of a turbogenerator with direct- and quadrature-axis excitation systems is formulated for the study of the stability of, and the interaction between, various regulators. The direct and quadrature excitations are controlled by the signals taken from the terminal voltage and the rotor angle, respectively. It is shown that this model can be extended to the case where the excitation windings are not at right angles. The stability-limit plots and transient responses of the system are determined with the help of a digital computer. The computed stability-limit plots are verified by comparing them with the known results of the experiments carried out on the micromachine equipment at Imperial College, first for the systems with controlled direct-axis excitation only, and secondly, those with controlled quadrature- and adjustable direct-axis excitations. For the second system, the effects of damper windings, the time delay in the regulator and the initial position of the quadrature axis with respect to the infinite busbar on the stability-limit plot are studied. The interactions between various regulators are determined in terms of their effect on the stability-limit plot and on the transient response for small perturbations. The quadrature-axis regulator increases the reactive absorption and is almost independent of the active-power level. An increase in the regulator time delay decreases the maximum value of the negative reactive power up to which the system is stable, while the regulator-gain range is increased The effects of damper windings and the interaction between regulators are marginal. A change in the equilibrium position of the quadrature axis with respect to the infinite busbar voltage, increases the interaction between the direct-and quadrature-axis regulators. The speed governor increases the integral square error of the transient response due to a small perturbation in any of the states. For the system with direct-and quadrature-axis excit- ations, the direct-axis regulator decreases the integral square error.