Stability Assessment of Inverter-fed Power Systems

As the increase of renewable energy utilization, the power electronic-enabled power plants such as wind farms and photovoltaic plants are becoming promising architectures to integrate renewable energy sources. However, the new emerging power electronic-fed power plants also give rise to potential challenges. Stability issue is matter of concern. The low frequency oscillation phenomenon may be caused by droop-based power controllers, constant power loads, and grid synchronization loop of grid-connected inverters. In addition, the interaction of the wideband control system of power converters with passive components maybe brings about in harmonic-frequency oscillation phenomena. These phenomena are challenging the system stability and power quality. Hence, it is essential to develop the modeling and analytical technique for the stability assessment of power electronic-fed power systems. The aim of this project is to develop the stability assessment methods and reveal the oscillation mechanism of inverter-fed islanded power system as well as paralleled grid-connected inverters system. The main contents of this thesis are organized as following. Chapter1 introduces the motivation and background of this project. Also, the problem statements and structure of the thesis are described. Chapter 2 presents a state-space-based harmonic instability assessment method for inverter-fed power system, where the effect of time delay is considered in details. Furthermore, the participation analysis is developed to evaluate and identify the contributions of different components on harmonic-frequency oscillation modes. Chapter 3 develops a component connection method-based stability analytical approach to assess low frequency oscillation and high-frequency resonance of inverter-fed power systems. Chapter 4 develops a state-space-based impedance stability analytical method for paralleled grid-connected inverters system, where the resonance mechanism of paralleled inverters with grid-current feedback and converter-current feedback is investigated. In addition, a frequency scanning-based impedance analysis for stability assessment of grid-connected inverters system is presented, which can be easily performed for stability assessment without using the detailed mathematical model. Main conclusions of this project are drawn and future research topics are explained in Chapter 5. The main contributions and conclusions are summarized as following. (1) The influence of time delay of digital controller on harmonic instability is considered in proposed stability analysis. (2) Participation analysis is adopted to assess and identify the contributions of different components on harmonic-frequency oscillation phenomenon. (3) The component connection method (CCM)-based small signal model of voltage-source inverters with multiple control loops is proposed, where the frequency response and eigenvalue analysis explain the contributions of different control parameters on terminal characteristic in a wide frequency range. (4) A statespace-based impedance stability analysis for multiple paralleled grid-connected inverters system is presented, which establishes the bridge between the state-spacebased modelling and impedance stability criterion, and combines the advanced merits of the two methods. Finally, the application of these stability analysis methods in different situations is demonstrated.