Voltage-Sourced Converters in Power Systems

APPENDIX B

Per-Unit Values for VSC Systems

B.1 INTRODUCTION

It is often more convenient to express a power-electronic converter system in a per-unit term. This can be achieved based on the following per-unitization system.

B.1.1 Base Values for AC-Side Quantities

The base values for a VSC system AC-side quantities are given in Table B.1. As shown in the table, the base voltage for a VSC system is chosen as the peak value of the line-to-neutral voltage of the point of common coupling (PCC); this is in contrast to the conventional power system for which the rms line-to-neutral voltage represents the base voltage. The rated three-phase power is selected as the base power.

B.1.2 Base Values for DC-Side Quantities

The DC-side base values are determined based on those of the AC side. The base power is the same for both DC and AC sides. However, the DC-side base voltage is defined to be two times the AC-side base voltage. This is to obtain the AC-side voltage of 1.0 pu from the DC-side voltage of 1.0 pu, at unity modulation index. The base values for DC-side quantities are summarized in Table B.2.

EXAMPLE B.1 Model of Three-Phase VSC-Based Rectifier

Figure B.1 illustrates a schematic diagram of a three-phase VSC system that operates in the rectifying mode of operation and supplies a DC RL load. The open-loop model of the VSC system is described by

TABLE B.1 Based Values for VSC AC-Side ...