Introduction

Power system sub-synchronous oscillation (SSO) is a complex and long-standing problem. The conventional SSO problem is the sub-synchronous resonance (SSR) in a power system with the series-compensated transmission line. Recently, incidents of sub-synchronous/super-synchronous oscillation caused by the grid-connected wind power generation were reported in many practical power systems and have caused great concerns. In this chapter, the main incidents of the SSO in practical power systems are firstly introduced. This presents an overview about how the SSO problem has evolved so far from the early SSR problem to the recently appeared problems of sub-synchronous/super-synchronous oscillation associated with the grid-connected wind power generation. Afterwards, main methods to study the SSO are reviewed in the chapter. They are the method in the frequency domain, i.e., the complex torque analysis, the damping torque analysis, the impedance model-based analysis, and the time-domain method of modal analysis. In the chapter, those methods are introduced briefly. Finally, the purpose, scope, and organization of the book are introduced. The modal analysis (MA) has been applied to examine the small-signal stability of a power system for over half a century. It is the standard function in all the major commercial software tools for power system stability analysis. Hence, the main method used in the book is the MA. The purpose of applying the MA is twofolds. First, it is to explain why and how the sub/super-synchronous oscillation may occur in the power system as being caused by the grid-connected wind power generation. Second, it is to detect the risk and source of sub/super-synchronous oscillations based on the revelation of instability mechanism.