Design synthesis of LCC HVDC control systems.
From the early days of HVDC system applications, the importance of mathematical modelling of the dynamics of Line Commutated Converter (LCC) HVDC systems has been appreciated. There are essentially two methodologies used to develop mathematical models of dynamic systems. One methodology is to define the properties of the system by the “laws of nature” and other well-established relationships. Basic techniques of this methodology involve describing the system’s processes using differential equations. This methodology is called “Deductive Modelling”. The other methodology used to derive mathematical models of a dynamic system is based on experimentation. Input and output signals from the original system are recorded to infer a mathematical model of the system. This methodology is known as “Inductive Modelling”. A review of the current state of the art of modelling LCC HVDC systems indicates that majority of the techniques utilized to develop mathematical models of LCC HVDC systems have used the “Deductive Modelling” approach. This methodology requires accurate knowledge of the ac systems and the dc system and involves complicated mathematics. In practice, it is nearly impossible to obtain accurate knowledge of the ac systems connected to LCC HVDC systems. The main aim of this thesis is to present an “Inductive Modelling” methodology to calculate the plant transfer functions of LCC HVDC systems. Due to the uncertain nature of the effective short circuit ratio of rectifier and inverter converter stations, generic ranges of parametric uncertainties of the developed plant transfer functions were determined. Based on the determined range of HVDC plant parametric uncertainty, Quantitative Feedback Theory (QFT) methodology was used to design the parameters of the LCC HVDC control system. The stability of the start-up and step responses for varying ac system conditions validated the “Inductive Modelling” technique and the QFT design methodology. The thesis presents the following, which are considered to be scientific advancements and contributions to the body of knowledge: · Novel LCC HVDC Step Response (HSR) equations were developed using an “Inductive Modeling” technique. · The range of parametric variations of the LCC HSR equations were determined for various rectifier and inverter ac system effective short circuit ratios. · The LCC HSR equations were used to develop the LCC HVDC plant transfer functions for various rectifier and inverter effective short circuit ratios. · The LCC HVDC plant transfer functions were utilized to design an LCC HVDC control system for varying ac system conditions using Quantitative Feedback Theory (QFT) methodology. The main contributions of this thesis relate to LCC HVDC systems. This thesis does not attempt to advance control theory however this thesis does apply existing classical control theory to LCC HVDC control systems. Index Terms: Line Commutated Converter, HVDC, inductive modelling, power system, transient analysis.