Multiple criteria decision processes for voltage control of large-scale power systems

Abstract

As modern power systems become more and more stressed, voltage instability has become a major concern. Coordinated voltage control is an advanced technique which provides emergency voltage control by sequencing and timing various types of controllers within a system that mitigates voltage instabilities. Owing to the highly nonlinear, constrained, and dimensional characteristics of power systems, optimal coordinated voltage control is thus a complex combinatorial optimization problem. In large scale power systems, it is difficult to find a sufficiently fast and effective optimal method for optimal coordinated voltage control. Traditional optimization techniques, modern heuristic algorithms, and knowledge-based controls have been tried to find feasible solutions in a reasonably fast time. However, in practical application, they all have their own difficulties in providing efficient solutions that would meet various operational constraints. A new voltage control strategy that can sustain voltage stability in a rapid manner is proposed in this thesis. Considering the analysis difficulties of the optimal coordinated voltage control, the new coordinated voltage control scheme is presented based on the idea of improving searching efficiencies by focusing only on a reduced solution space. The novelty of the proposed control strategy is to improve searching efficiencies by utilizing the control knowledge of power systems. Control knowledge is gained from off-line studies and then further accumulated from on-line experiences. With the prepared control knowledge, the searching solution space can be significantly reduced. Hence, the timely on-line control can quickly achieve a good system performance. The main contributions of this thesis are listed as follows: Applying multi-objective optimization to the optimal coordinated voltage control. Coordinated voltage control is considered as a multi-objective optimization problem. As such, the obtained solution from the Pareto-optimal solution set can be traded off with one or the others that are dependent upon the performance requirements. By properly selecting the design objectives, useful control knowledge can be extracted from the Pareto-optimal solution front. Hence, a landscape of control options can be formed. Providing flexible voltage control. Control flexibility may also need to consider the requirement of operational cost. A critical on-line decision making scheme is integrated into the control design strategy. A multiple criteria decision making technique is devised for selecting the appropriate solution to meet the design task. This multiple criteria decision process serves as a theoretical framework throughout the thesis. Accumulating control knowledge from off-line and on-line search. Based on MOO, the control knowledge is acquired in two ways. First is the off-line, long-term search which is carried out by the newly developed jumping gene genetic algorithm that searches for some anticipated fault cases, while second is the learning scheme adopted to accumulate knowledge for unexpected faults. The combination of both can gradually improve the overall control performance. Developing a fast on-line search scheme. The accumulated control knowledge can be used to form a reduced searching space in which only effective solutions are involved. Within this limited area, a fast and new local search technique can be developed. It can find a good solution with a rapid system response. Proposing a comprehensive coordinated voltage control system. A new coordinated voltage control system is proposed by combining all the above schemes. By considering the optimal coordinated voltage control as a multi-objective optimization problem, this control system is composed of off-line search, on-line knowledge accumulation, on-line search, and flexible control. Based on the control knowledge from off-line preparation and on-line accumulation, a flexible control for any fault which may cause voltage instability can be achieved immediately. The effectiveness of the proposed control system is demonstrated based on the New England 39-bus power system. Each of the proposed algorithms and schemes is verified. The overall power voltage control performance is found to be responsive and in good order, providing an efficient scheme for emergency voltage control of large-scale power systems.