Supervisor:
Professor Zhe Chen

Co-Supervisor:
Yanbo Wang

Assessment Committee:
Sanjay Chaudhary
Paola Verde, Department of Electrical and Information Engineering, University of Cassino and South Lazio
Terence O'Donnell, School of Electrical and Electronic Engineering, University College Dublin

Moderator:
Sanjay Chaudhary

Abstract:

Analysis of Transients in Power Electronic Dominated Power Systems

Abstract

Driven by global climate change, renewable energy is gradually becoming an essential component of the energy resources of human society. Due to the diversity of renewable energy, such as wind, solar, chemical, and mechanical energy, almost all renewable energy must utilize power electronic converters as interfaces to achieve energy collection, pooling, and injection into the power system. Therefore, the development of renewable energy generation (REG) has ignited a significant revolution in power system generation, from traditional synchronous generators to inverter-interface generations (IIG). However, the IIG has an entirely different dynamic response than conventional synchronous generators: 1) The IIG does not have physical rotating components, which results in a breakneck response speed so that IIGs cannot provide inertia to the power system disturbed by faults. 2) Because of the highly flexible and powerful control system, IIGs with different control systems and parameter configurations have significantly different dynamic responses. 3) As the core components of IIG, the semiconductor switches cannot withstand high overload currents. Thus, IIGs cannot firmly provide voltage and reactive power support to the disturbing power system. Consequently, the high penetration of REG presents a different transient response for the power electronic-dominated power system (PEPS) and presents enormous challenges for the transient analysis of the PEPS.

This Ph.D. project focus on the transient analysis methodology of PEPS. First, this project investigates the fault modeling and analysis method of grid-following (GFL) and grid-forming (GFM) inverters and furtherly summarizes the fault characteristics of GFL and GFM inverters. Furthermore, upon the fault analysis, this project develops a new mathematical model for GFL inverters with fault-ride-through (FRT) control, thereby studying the transient stability region of GFL inverters. Details of the work are summarized below.

This project proposes a fault analysis method for GFL inverters in terms of the fault modeling method and the fault response analysis. The proposed method transforms the fault analysis problem of GFL inverters into the response analysis problem in the classical control theory by introducing the appropriate simplification and system modeling method. The proposed method thereby constructs and solves the time-domain analytical expression of the GFL inverter using the Laplace transformation. Based on the proposed method, this project investigates the influence of control parameters, hardware parameters, system delay, and FRT control on the fault response of IIG inverters. Furtherly, this study summarizes the fault characteristics of GFL inverters. This study found that reasonable controller parameters and the filter inductance design can effectively reduce the inrush current of GFL inverters. The conclusion of this study provides an essential theoretical and methodological basis for the FRT control and fault analysis of IIG inverters in PEPS.

Different from GFL inverters, the interaction between GFM inverters with the power system challenges the fault response of GFM inverters. This study proposes a dynamic-phasor-based model for the fault analysis of GFM inverters to face the challenge. A state-space equation model of GFM inverters is first developed according to the dynamic-phasor theory. After that, the fault response of GFM inverters is discussed and divided into the Homogenous State Equation (HSE) solution and the Inhomogeneous State Equation (ISE) solution. It found that the fault characteristics of GFM inverters determined by the HSE solution can only be affected by the control and hardware parameters, and characteristics driven by the ISE solution are the primary carrier of the interaction between inverters and the power system. According to the finding, this study investigates the effects of control parameters, hardware set-ups, and interaction between inverters and power grids on the fault response of GFM inverters.

This project also studies the stability region estimation of the PLL-driven GFL inverters with the FRT control. FRT control introduces switch features into the mathematical model of GFL inverters, which challenges the stability analysis of the GFL inverters. This study presents the differential geometry and the topology theory to overcome the challenges and thereby investigate the stability region of GFL inverters. First, A new subsystem-based state-space equation model of PLL-driven GFL inverters with FRT control is developed for the transient stability analysis. This study systematically investigates the existence of equilibrium points (EP) by introducing the real-root classification method of semi-algebraic systems. Secondly, this study presents a transient stability region estimation method for GFL inverters based on differential geometry theory. Compared with the conventional Lyapunov energy function-based method, the proposed method overcomes the challenges introduced by switch features and the error caused by the non-differential items in the model. Therefore, the proposed method can yield an accurate transient stability region of GFL inverters. Finally, based on the proposed transient stability region estimation method, the effects of PLL parameters, FRT control, and active- and reactive-power injection on the transient stability are discussed in this study.