Voltage-Sourced Converters in Power Systems

8

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

8.1 INTRODUCTION

Chapter 5 presented dynamic models for the two-level VSC in αβ-frame and dq-frame and briefly discussed its control based on generic block diagrams of Figures 5.5 and 5.7. Chapter 6 introduced the three-level NPC as an extension of the two-level VSC and established that the dynamic model of the three-level NPC is identical to that of the two-level VSC, except that the three-level NPC requires a DC-side voltage equalizing system to maintain DC-side capacitor voltages, each at half the net DC-side voltage. Thus, Chapter 6 presented a unified model for the three-level NPC and the two-level VSC (Fig. 6.18 and 6.19). Chapter 7 introduced a class of VSC systems referred to as grid-imposed frequency VSC systems. On the basis of the unified model of Chapter 6, Chapter 7 presented αβ-frame models and controls for two members of the family of the grid-imposed frequency VSC systems, namely, the real-/reactive-power controller and the controlled DC-voltage power port. In parallel with Chapter 7, this chapter presents dq-frame models and controls for the real-/reactive-power controller and the controlled DC-voltage power port.

As discussed in Chapter 7, compared to the abc-frame control, the αβ-frame control of a grid-imposed frequency VSC system reduces the number of plants to be controlled from three to two. Moreover, instantaneous decoupled control of the real and reactive power, exchanged between the VSC system and ...