Studies On Load Restoration After Power System Major Blackout

As modern society is dependent on the power supply excessively, the widespread blackout has a serious negative effect to the social economy. In order to speed up the restoration process and reduce outage cost, proper scheme for system restoration should be determined in a reasonable way. As one of the optimization objectives in system restoration, load pickup should be focused on which exists in each stage of restorion process. The effective load reatoration is in favor of the unit start-up and network reconfiguration. Based on current researches, a detail study on load restoration has been carried out in this dissertation, which includes maximizing the restorable load amount for specific substation, load pickup optimization for different stages in practical restoration, coordination strategies for multi objectives and etc. The whole research refers to many specific aspects, such as mathematical model, sovling algorithms, checking constraints, restoration strategies and coordinative control. The dissertation presents a fully study on load restoration, which perfects the whole framework for power system restoration. Main contributions and innovations are described as following:1) During power system restoration, the maximum load amount that a substation can pick up at one time is a critical parameter to be determined. Many factors should be considered, such as frequency constraint, transient voltage-dip constraint, cold load pickup and steady-state voltage constraint. A general model is proposed for calculating the maximum restorable load amount, in which proper checking methods are presented to deal with these constraints. Frequency deviation is calculated by the average system frequency model or an estimation formula. The transient voltage-dip constraint can be dealt with by two checking methods according to different requirements of calculation speed and precision.Cold load pickup characteristics are addressed using derived formulas. The proposed model is adaptive to various scenarios and can speedup computations by neglecting unnecessary simulation. A modified bisection algorithm is proposed to solve the complex problem efficiently. The effectiveness is demonstrated by case studies performed on the IEEE14-bus system and a practical power system.2) After a major blackout, the primary units of system should be restarted first. In this process, it is necessary to pick up reasonable loads to keep system stability and improve restoration efficiency. According to characteristics of this stage, this dissertation presents detailed contents of load restoration and related restoration strategies, which are coordinated with unit start-up. For load restoration of stabilizing generators, a multi-objective model is established. According to the principle of greedy algorithm, a method based on load priority is proposed to solve the problem efficiently. For load restoration during energizing high voltage line, two different situations are discussed. The optimization strategy based on bisection algorithm is described for determining reasonable load amount and a basic principle is given for choosing different load feeders. The effectiveness is demonstrated by case studies performed on the IEEE9-bus system and a practical black-start test system.3) During last stage of network reconfiguration after major blackout, loads should be picked up in sequence with considering certain constraints, which is a multi-constraint, non-linearity, integer programming problem. A combinatorial optimization model is proposed with considering the sequencing problem of load pickup, in which effects of pickup sequence are taken into consideration in both objective function and constraints. As the large optimization scale of restorable loads during last stage of network reconfiguration, a new adaptive particle swarm optimization algorithm is adopted to solve the proposed model. The adaptive strategy of parameters is carried out by comparing individual optimum in iterative process. Case studies performed on the actual Shandong power system demonstrate that the proposed model effectively reflects sequencing problem and the adopted algorithm can satisfy the practical requirement on computation speed.4) In order to achieve independent modules for different restoration actions or strategies, issues during system restoration are classified from different aspects. The whole optimization goal in certain stage is divided into many local objectives, in which one or several operation actions can be optimized by local restoration strategies. And a cooperation framework for system restoration is established in an interactive way. According to specific criterion, different system restoration states are defined with employing the corresponding local restoration strategies, which include unit start-up, network reconfiguration, load restoration, paralleling operation, DC restoration and etc. The implementation flow corresponding to specific local restoration strategy is also given in the proposed cooperation framework. From the perspective of both space and time dimensions, the present strategy can be carried out in a parallel processing way, which improves the availability and feasibility of restoration schemes. Case studies demonstrate that the proposed framework and strategies can deal with different emergency blackouts effectively.