Voltage-Sourced Converters in Power Systems

10

Variable-Frequency VSC System

10.1 INTRODUCTION

This chapter introduces a class of VSC systems that we refer to as variable-frequency VSC system. A variable-frequency VSC system constitutes the kernel of an electromechanical energy conversion system in which the VSC AC side is interfaced with the stator terminals of an electric machine¹. The VSC system controls the machine voltage and frequency such that the torque is controlled and the flux is regulated at its nominal value. The torque control in turn permits control of the machine power. In a variable-frequency VSC system, the machine speed is usually variable and thus the VSC system operates in a variable-frequency environment. Typical applications of a variable-frequency VSC system include variable-frequency wind-power units, gas microturbine units, and industrial regenerative drive systems. The first of the aforementioned applications is discussed in Chapter 13.

In this chapter, we first adopt the model developed in Appendix A for a symmetrical three-phase AC machine. Then, based on the model, we present the methodology of vector control (or vectorial control) for fast torque control and robust flux regulation. We introduce proper control schemes for the asynchronous machine, the doubly-fed asynchronous machine, and the permanent-magnet synchronous machine (PMSM).

10.2 STRUCTURE OF VARIABLE-FREQUENCY VSC SYSTEM

Figure 10.1 shows a schematic diagram of a variable-frequency VSC system to control the asynchronous machine or ...