Risk-based Research On Power System Transient Stability Assessment And Coordination Control

Power system stability and security are concerned with the system’s capability to withstand all kinds of potential disturbances and they have always been focuses of power system operation and control. In recent years, with the penetration of large-scale renewable energy generation and the frequent occurrence of extreme disaster weathers, the research on the impact of the embedded uncertainties on power system operation and the corresponding measures to improve system performance under these circumstances have become greater concern than ever. In essence, risk-based method can take into account both the occurrence probability and severity of all probable disturbances. The purpose of this dissertation is to introduce a risk-based analysis framework into the transient stability assessment and coordination control. The methods proposed here can not only assess the security level of system operation, but also can consider security and economy tradeoff through a risk-based security control.This dissertation is organized into three parts. In the first part, a new method to assess the transient stability risk of power systems under typhoon weather conditions is proposed, so as to analyze the influence of disaster weather on the security level of power system operation. This method models the contingency rate of transmission lines under typhoon weather, then obtain corresponding short-term contingency probabilities. For the contingency which could not satisfy transient stability constraints or post-contingency static security constraints, an optimization model for emergency control is formulated, and the minimal control cost is used to quantify the severity of the contingency. The New England10-generation39-bus system is used to demonstrate the effectiveness of the proposed method.The second part of this dissertation focuses on the coordination of preventive and emergency controls for transient stability enhancement, which is is a large-scale mixed integer nonlinear programming problem. Two new methods are proposed to solve this problem.1) The first method solves the coordination problem by decomposing it into two less complicated sub-problems. Firstly, a transient security risk index is defined and introduced as a constraint into the coordination problem, in which case the coordination problem can be decomposed into a transient security risk constrained preventive control sub-problem and transient stability constrained emergency control sub-problems. Furthermore, a bi-level optimization model of coordination control is developed, in which on the upper level a risk coordination parameter is adjusted to minimized the total coordination cost; the lower level includes preventive and emergency control sub-problems. Finally, a hybrid method that combines the golden section search method and the successive linear programming method is proposed to solve the bi-level optimization model. The effectiveness of the proposed method is demonstrated by using the New England10-generation39-bus system and a real power system.2) The second method transforms the coordination problem into a continuous problem that can be solved by traditional nonlinear optimization algorithms. By introducing a set of nonlinear complementary problems, discrete emergency control variables are equivalently transformed into continuous variables, and the coordination problem is subsequently transformed into an ordinary nonlinear programming problem, which is solved by the successive linear programming method. Furthermore, Benders decomposition is introduced to speed up the computation efficiency of linear optimization model. The test on New England10-generator39-bus system demonstrates the effectiveness of the proposed method.In the third part of this dissertation, a new UFLS parameter optimization model is developed, which considers both the load shedding amount and the frequency recovery performance during the transient period, and the risk of different operating scenarios is synthesized in addition. Further, a solution approach based on improved differential evolution algorithm is proposed, in which both steady frequency constraints and frequency overshoot constraints in UFLS are judiciously treated. A chaotic behavior is introduced into the algorithm to enhance the population diversity when the stagnation symptom occurs. The feasibility and effectiveness of the proposed model and algorithm are validated by the simulation study based on the UFLS optimization problem in a real power system.