Studies On Fuzzy Evaluation Based Multi-Objective Optimization In Distribution Networks

Distribution networks connect transmission networks and loads to supply electricity to the customers directly. The characteristics of distribution networks are low voltage grades, great power losses and inferior automatic levels. Operating conditions of a distribution network influence not only the energy quality of customers but also the security and economy of the distribution network itself. Optimal operation of distribution networks includes volt/var optimization and reconfiguration. The volt/var optimization regulates reactive power flow by adjusting tap-changers and switching shunt capacitors to reduce the voltage drop and the active power losses. The reconfiguration alters the topological structures of distribution networks and redistributes loads by changing the open/close states of the sectionalizing and tie switchers. The regulating means of volt/var optimization and reconfiguration, such as tap-changers, shunt capacitors, sectionalizing and tie switchers, are all discrete variables. Therefore, the optimal operation of distribution networks is modeled as a restricted nonlinear combinational optimization. The characteristics of the model include multi-objective, local-optima, nonlinear objective functions and equation restrictions. Furthermore, the objective can hardly be expressed as an explicit function of control variables.Introducing the theory and methods of fuzzy evaluation, intelligent optimization and multi-objective optimization, the distribution networks optimal operation is researched systematically. The main contributions of the dissertation are shown as follows.1. Fuzzy-evaluation based voltage regulation in middle-low voltage networksThe volt/var regulating means in middle-low voltage distribution networks are non-load tap-changers and fixed shunt capacitors, and the regulating frequencies are limited to several times a year. Therefore, traditional static and dynamic volt/var optimization models are not applicable any longer.Considering the regulating frequency restrictions and multi-objective in the volt/var problem, a fuzzy evaluation based multi-objective optimization model and a solution method are proposed for voltage regulation in distribution networks. The model aims to improve voltage profiles and reduce active power losses by adjusting tap-changers and switching shunt capacitors. The maximum and minimum load conditions are both considered in optimization process to represent the load variations in a long period, that is, if the voltage profiles in peak and valley load conditions are eligible, those during this period are believed to be acceptable. The fuzzy evaluated multi-objective model is transformed to a single objective optimization problem, which is solved by reactive tabu search (RTS) algorithm. The solving process is shown as the following. Firstly, membership functions are constructed to evaluate the objectives, so that the satisfactions of different objectives in various operating conditions can be compared. Secondly, the multi-objective optimization is transformed to a single objective optimization by weighted-sum approach. Finally, the single objective optimization problem is solved by RTS. The feedback procedure adjusts the principal parameters in RTS according to the optimization process. The efficiency and applicability of RTS are improved significantly. The simulation on a practical distribution system shows that, the proposed model optimizes the two objectives with the load variations in a long period considered. The solving method evaluates the objectives with different units fairly and obtains the optimal solutions effectively. The regulation results, significantly improved voltage profiles and reduced active power losses, show the efficiency of the proposed method.2. Volt/var multi-objective optimization with distributed generators With the development of small-capacity generation techniques and concern on environmental protection, increasing number of distributed generators (DGs) are connected to distribution networks. This trend will change the current structure, planning and operation of distribution networks. Power injection from DGs influences voltage profiles and active power losses. The output variation and start-stop process of DGs may cause voltage variation.Considering the characteristics of optimal operation in distribution networks with DG connected, a multi-objective volt/var optimization model and a solving method are proposed. The model aims to improve voltage profiles, reduce active power losses and limit the voltage variation by adjusting tap-changers and shunt capacitors, accommodating the influences of DG. The fuzzy-evaluation based RTS is employed to search for the solutions. To distinguish the different importance of voltage eligibility and voltage deviation minimization, two membership functions are constructed to evaluate the voltage profiles respectively. The weights of objectives are determined by the fuzzy judgment matrix, which is constructed according to the relative importance of every two objectives. The fuzzy judgment matrix approach ensures the coherence of the weights and improves the adaptability of the proposed method. The solving process improves the voltage profiles, reduces active power losses and limits the voltage variations without violating voltage restrictions. The simulation results show that the proposed model improves all the objectives with DG influences considered. 3. Distribution networks reconfiguration and comprehensive optimizationTo solve the reconfiguration problem in the intelligent optimization methods, an important step is to code the status of the sectionalizing and tie switchers. The quality of coding affects the solving efficiency and optimization results directly. A topological analysis based decimal coding scheme is presented, that is, to close each tie switch and number all the switches on the looped branch. The scheme is easy to be realized and the codes have a high efficiency in representing the structures. The RTS is employed to solve the reconfiguration model. The simulation results, getting the global best solution with fewer objective-calculation times compared with the other approaches, demonstrates the efficiency of the proposed method.A comprehensive optimal regulation model for distribution networks is presented, which utilizes the volt/var regulating means, sectionalizing and tie switchers to reduce active power losses, improve voltage profiles and balance loads. Fuzzy judgment matrix approach decides the weights of different objectives. The model is solved by RTS based on fuzzy-evaluation. The proposed method optimizes the objectives using various regulating means comprehensively.4. Software development for distribution networks optimal operation According to the practical demand of distribution network operation, a softwarepackage for optimal operating analysis is developed. The software package has several modules, including historical data analysis, data maintenance, power flow calculation, volt/var optimization, reconfiguration and comprehensive optimal regulation. The standard template library provides various algorithms and data structures, which are utilized to program high quality code with good readability. The software package is constructed by object-oriented programming (OOP), which divides and encapsulates the classes according to the data structure and the corresponding functions. OOP improves the reusability of the codes and alleviates the cost of maintenance. The structure of core parts, power flow calculation and intelligent optimization, are introduced in detail.