Research On Novel Methodologies Of Coordinated Voltage Control In Electric Power Systems

At present, the electric power industries of the whole world have entered into a new times which can be characterized as very large grid, generator units with huge capacity, ultrahigh voltage level, heavy loading condition and very long transmission line. Because of those mentioned points, power grids are operated closing to their stability margin increasingly, and the problems of voltage stability get the focus much more. Keeping the voltage stability by using real-time, automatic and coordinated voltage control is always an important direction of research and practice. Based on the existing coordinated voltage controls, in this work, a series of new scheme and its algorithm are proposed.A new voltage control scheme that combines the secondary voltage control (SVC) and tertiary voltage control (TVC) of the triditional three-level voltage control is presented here. This scheme is based on regional optimal power flows which implemented periodically in real time, and its objective is to update the set-points of generators' automatic voltage regulators etc. Comparing to the traditional secondary and tertiary voltage control, the advantages of the proposed method are listed below:â‘ no need for the selection of pilot bus, and because the proposed method is software-based, the redefinition of voltage control areas is easier than that for the kind of secondary voltage control that implemented by PI controllers.â‘¡the voltage profiles at all buses, particularly generator terminal voltages, are better overall, with all voltages being maintained within required steady-state limits.â‘¢the implementation of TVC is simplified by the regional division of the associated OPF problem, since the corresponding optimization sub-problems are numerically and computationally significantly less demanding.Under the scenarino of long-term voltage stability, and based on quasi steady-state (QSS) approximation, an optimal coordinated voltage control (OCVC) model including continuous and discrete time differential-algebraic equations (DAEs) is established. This model considers long-term voltage stability and costs of controls synthetically,which can coordinate different kinds of controls. This OCVC problem can be solved by using indirect method with radau collocation. The algorithm has high accuracy and good numerical stability. To handle the discrete control variables, a quadratic penalty function mechanism was incorporated to the OCVC control model.For the long-term voltage stability, and based on the QSS approximation, an OCVC model is proposed in which secondary voltage controllers are embedded. This model designed control strategies from dynamic view, and its control goal was to keep long-term voltage stability by coordinating the reference voltage of pilot buses and other control means. Its objective function considered voltage stability and control-costs, and its equality constraints were continuous and discrete time DAEs. Direct method with switching technique is applied to solve the proposed models. The advantages of the proposed models and algorithm have advantages as:â‘ the proposed models accord with hierarchical coordinated voltage control well because of the embedded secondary voltage controllers;â‘¡the time cost for calculation is less than the response time of SVC which typically set as 1min, and so on-line computation speed is realized;â‘¢the switching technique enhances the iteration stability of line search interior method, then the better robustness for system's operation condition;â‘£because of the coordination between different kinds of voltage control means and the consideration of system's dynamic procedure, the proposed control scheme has better control ability than traditional SVC.