Real-time simulator studies and model development for time-down voltage stability analysis.
Problems of voltage stability and voltage collapse have become a major concern in power system planning and operation in recent years, often as a result of power systems being operated under much more stressed conditions than was usual in the past. Factors that are responsible for this trend include: environmental pressures on transmission expansion; increased electricity consumption in concentrated heavy loads where installation of new generation is not feasible; new system loading patterns. Voltage stability problems are characterised by either slow or sudden voltage drops, sometimes escalating further to a collapse in voltage, leading, in some cases, to system wide blackouts. The power engineering community has devoted significant effort to developing new analysis tools and methods to control this type of instability. The main methods that have been developed and used for analysis of voltage stability are steady-state methods (power flow: analysis via PV and Q-V curves); dynamic analysis (time-domain simulations); modal analysis of system jacobian matrices and optimization (special optimal power flow). This thesis investigates the use of a particular tool, real time simulation, as a method for voltage stability analysis and testing of voltage control strategies. The particular simulator used is the Real-Time Digital Simulator (RTDS) from RTDS Technologies. The real-time simulator software environment provides generalized models of generation, transmission, distribution plant and loads that can be used to develop accurate models of power systems for analysis in real-time. The broad objectives of this thesis are to assess the suitability of the RTDS as a tool for time domain voltage stability analysis and to develop additional real-time models of particular power system controllers that are known to playa key role in voltage stability phenomena. In particular the thesis considers development of custom real-time models of a transformer on-load tap changer (OLTC) controller, detailed generator excitation controls (automatic voltage regulators), a static var compensator (SVC) controller and a synchronous condenser reactive output controller. The thesis then describes the development of real-time models of two benchmark systems for the voltage stability studies: a well known II-bus voltage stability benchmark system and a smaller (4-bus) benchmark system. These two benchmark systems are used to establish the validity and correctness of the custom real-time models and to investigate simple compensation and control strategies for voltage stabilization. In particular the thesis considers the following stabilizing techniques on the II-bus system: switched shunt capacitor compensation, voltage control using a synchronous condenser and finally the use of an SVc. Finally, the thesis demonstrates the ability of RTDS to investigate the performance of actual hardware controllers on the plant in the real-time model of the 11 bus system in a full closed loop arrangement. The custom-developed real-time software model of the OLTC controller in the II-bus benchmark system is replaced with an actual external hardware controllers connected in closed loop with the real-time simulation. This thesis has successfully confirmed the known characteristics of individual power system plant using the models provided in the RTDS environment and developed additional customized software models of controllers for voltage stability studies on the RTDS. The results of the RTDS simulations of voltage stability benchmark systems have been found to agree with documented results of these systems. The thesis has shown that the RTDS provides a suitable platform on which time-domain voltage stability studies can be conducted. The thesis has also shown that real-time digital simulation is a practicable technique for the analysis and investigation of control strategies for voltage stability, particularly when interactions between real hardware controllers and their impact on system stability are of concern.