Voltage-Sourced Converters in Power Systems

12

Back-to-Back HVDC Conversion System

12.1 INTRODUCTION

This chapter deals with the control of the VSC-based back-to-back high-voltage DC (HVDC) conversion system. Developments of this chapter are based on the dq-frame models and controls for the real-/reactive-power controller and the controlled DC-voltage power port, presented in Chapter 8. Although we concentrate on the dq-frame control, the αβ-frame control approach of Chapter 7 is also applicable to the HVDC system.

12.2 HVDC SYSTEM STRUCTURE

Figure 12.1 illustrates a schematic diagram of a VSC-based back-to-back HVDC system [98]. The HVDC conversion system is composed of two back-to-back connected VSC systems. Both VSC systems employ the three-level NPC as their power converters, labeled as NPC1 and NPC2. Each three-level NPC has a DC-side capacitive voltage divider with two nominally identical capacitors (Fig. 6.18). However, the capacitors of one three-level NPC are not necessarily identical to those of the other one. As discussed in Sections 6.7.2 and 8.5, the partial DC-side voltages of each three-level NPC must be equalized by means of a corresponding DC-side voltage equalizing scheme. Figure 12.1 also shows that NPC1 (NPC2) is interfaced with an AC system, that is, Grid1 (Grid2), through an interface transformer, TR1 (TR2), at the point of common coupling PCC1 (PCC2). As Figure 12.1 illustrates, each grid is represented by a three-phase voltage source, V′_gabc1 (V′_gabc2), and an internal inductance, L′_i1 (L′_i2). To ...