Abstract

The voltage source converter (VSC) excitation system is a novel excitation system based on pulse-width modulation (PWM) voltage source converter, which is proposed as improved alternatives to the conventional thyristor excitation systems. This paper aims to provide theoretical confirmation of power system stability enhancement by the VSC excitation system. The reactive current injected to generator terminals by the VSC excitation system can be controlled flexibly. Its capability of enhancing power system stability is investigated in this paper. The simplified model of VSC excitation system suitable for use in system stability studies is developed. An extended Philips–Heffron model of a single-machine infinite bus (SMIB) system with VSC excitation system is established and applied to analyze the damping torque contribution of the injected reactive current to the power system. This paper also gives a brief explanation on why the VSC excitation system can enhance the transient stability in light of equal area criterion. The results of calculations and simulations show that the injected reactive current of VSC excitation system contributes to system damping significantly and has a great effect on the transient stability. When compared with conventional thyristor excitation systems, the VSC excitation system can not only improve the small-signal performance of the power system, but also can improve the system transient stability limit.

Introduction

The generator excitation system, which provides direct current to the synchronous machine field winding, is the most important and effective means to maintain the stability of power systems. Since the 1960s, the static excitation systems based thyristor converters (thyristor excitation systems) have been extensively used, for its ability of producing almost instantaneous response and high ceiling voltages. This system has a very small inherent time constant and is easily maintainable [1]. However, the modern power systems are interconnected each other to give and take the electric power and have become much more complicated than decades ago. The presence of system instability is becoming more prominent and thyristor excitation systems with conventional PSS are not sufficient to suppress the wide range (0.1–۳٫۰ Hz) power oscillations any more. On the other hand, the long distance power transfer with heavy load seems to be more susceptive to poor damping [2]. Studies show that the thyristor excitation system cannot provide enough damping even if PSS is equipped.