Effect of excitation regulation on synchronous-machine stability

Abstract

SynopsisThe paper records a detailed study of the effect of a voltage regulator on the stability of an alternator connected through a reactance to an infinite bus. The stability is analysed by means of Nyquist loci calculated for the transfer functions of alternator and regulator. The accuracy and speed of response of the system are also considered. The first part of the paper considers a simple regulator with proportional feedback, and it is shown that such an ideal regulator can extend the region of steady-state stability to a point corresponding to the maximum of the transient power-angle curve.Practical regulators are classified according to the nature of their transfer functions. The analysis provides a means of predicting their behaviour and explains how they affect the stability, accuracy and response. The effect of delay elements, integrator elements and derivative elements in the regulator is considered particularly; e.g. a buck-boost exciter, which effectively introduces an integrator element, gives good accuracy but less satisfactory response, and has a limited effect on stability, and a derivative regulator which gives rapid response and a large extension of the stability region, but has limited accuracy.Experiments performed on a model machine with various simulated regulators agreed well with the computed results. The computations allowed fully for the system parameters, including alternator resistance and alternator damping. List of symbols= alternator transfer function = direct and quadrature-axis currents = field current = moment of inertia = regulator gain = regulator transfer function = constant = -r f \X md = system closed-loop transfer function = power = number of poles with a positive real part = reactive power at the infinite bus = armature resistance = field resistance = slope of steady transient power-angle curve = regulator time constants = turbine, electrical and inertia torques = voltage of infinite bus = generator terminal voltage = voltage induced by field current = X md if = field voltage = direct-and quadrature-axis voltages = leakage reactance = transmission-line reactance = magnetising reactance = direct-and quadrature-axis operational impedances = direct-and quadrature-axis operational admittances = number of zeros with positive real part -load angle with respect to infinite bus = steady load angle as generator (see Section 6.1) = integral square error for a small change = angular frequency of small oscillations -voltage regulation = feedback transfer function for torque methodThe investigation relates to the steady-state stability of a system comprising an alternator connected through a reactance to an infinite bus, as indicated in Fig. 1. The terminal voltage V, is fed back through a regulator, whose transfer function is K(p), to the alternator-field winding. The voltage Vj-applied to the field winding is the difference between the feedback voltage Vf 0 and a reference voltage Vp r The results of a theoretical study have been verified byexperiments on one of the micromachines ...

Stability of synchronous machines with 2-axis excitation systems

“…This shows clearly the advantage of holding the rotor angle at zero, since very high loop gains can be used on the direct axis to maintain high accuracy of regulation. 19 Higher gains on the angle regulator can also be achieved, as shown in curves c, d and e, by using a stabiliser similar to those used with the voltage regulator of conventional machines. 24 The equation for the angle-regulator loop then becomes…”