Voltage-Sourced Converters in Power Systems

9

Controlled-Frequency VSC System

9.1 INTRODUCTION

Chapters 7 and 8 discussed control and operation of the grid-imposed frequency VSC system in which the operating frequency was predetermined and imposed by the AC system. These chapters implicitly translated the control of the grid-imposed frequency VSC system into the control of real and reactive power that the VSC system exchanges with the AC system, through a current-mode control strategy. This chapter investigates a class of VSC systems in which the operating frequency is not imposed by the AC system, but it is controlled by the VSC system itself. We refer to this class as controlled-frequency VSC system, in which the voltage and frequency at the point of common coupling (PCC) are controlled; thus, the real and reactive power that the VSC system exchanges with the AC system are the by-products.

Typical scenarios where a controlled-frequency VSC system is encountered include

• an electronically coupled distributed generation (DG) or distributed energy storage (DES) unit¹ that supplies a dedicated load, or a cluster of loads, under an islanded (off-grid) condition;
• a VSC-based HVDC converter system that supplies a passive or weak AC system; and
• an uninterruptible power supply (UPS) system that adopts a VSC system as its kernel to regulate the frequency and voltage of a sensitive load, for example, under emergency conditions.

In this chapter, we first present a dq-frame model for the controlled-frequency VSC system and then introduce ...