Voltage-Sourced Converters in Power Systems

7

Grid-Imposed Frequency VSC System: Control in αβ-Frame

7.1 INTRODUCTION

Chapter 5 presented dynamic models of the two-level VSC in αβ-frame and dq-frame and briefly introduced the control of a generic three-phase converter system in those frames. Chapter 6 introduced the three-level NPC as a generalization of the two-level VSC and established that the dynamic model of the three-level NPC is conceptually the same as that of the two-level VSC, except that the three-level NPC requires a DC-side voltage equalizing scheme to maintain the DC capacitor voltages, each at half the net DC-side voltage. Chapter 6 also presented a unified model for the three-level NPC and the two-level VSC which are generically called VSC. In this chapter, we introduce and investigate the control of a grid-imposed frequency VSC system in which the three-phase variables are sinusoidal functions of time with a frequency that is imposed by the AC system, for example, a power utility grid. The methodology is for the VSC and therefore covers the control of both the two-level VSC and the three-level NPC. This class of VSC systems is often adopted for real- and reactive-power control, or for DC voltage control. These functions constitute the main operational functions of electronically-coupled distributed generation (DG) units, VSC-based HVDC systems, and most FACTS controllers.

7.2 STRUCTURE OF GRID-IMPOSED FREQUENCY VSC SYSTEM

Figure 7.1 shows the schematic diagram of a grid-imposed frequency VSC system. The ...