Dynamic P-Q Capability and Abnormal Operation Analysis of a Wind Turbine with Doubly-Fed Induction Generator

After several accidents occurred in wind farms in the United States and around the world, the subsynchronous oscillation (SSO) issues in grid-connected wind farms have gained serious attention. Particularly, these issues have caused significant challenges to wind turbines (WTs) with a doubly fed induction generator (DFIG) because it is connected to the grid via both its stator and rotor paths. Traditionally, a $P$ - $Q$ capability chart is used to assure the safe operation boundary for a synchronous generator. But the energy conversion characteristics of a DFIG WT are completely different. A critical factor to affect the reliable operation of a DFIG WT is the rated current and pulse width modulation (PWM) saturation constraints of its power converters. These constraint conditions can be affected by the WT rotating speed and grid conditions. However, a detailed study of DFIG $P$ - $Q$ capability from these perspectives has not been conducted, which has hindered adequate understanding of many abnormal WT operations reported in the literature and the development of advanced control technologies to overcome the challenges. The proposed study in this article considers vector control implementation to DFIG power electronic converters in the $d$ – $q$ reference frame, and the models and algorithms developed for the $P$ - $Q$ capability study have addressed specific DFIG power converter constraints that are different from those of a traditional synchronous generator. This article especially focuses on exploring the dynamic nature of DFIG $P$ - $Q$ capability under uncertain and variable conditions to explore the root causes of many abnormal operations of DFIG WTs reported in the literature. The proposed study is validated through an electromagnetic transient simulation model of a grid-connected DFIG WT. The proposed study has the potential to lead to the development of new DFIG control technologies that can help overcome the challenges for many abnormal operations of DFIG WTs.