Load Flow Calculation And Unit Commitment For Integrated AC/DC Power Systems

With several DC transmission systems completed and put into operation, the electric power system in China is being transformed into a large-scale integrated AC/DC power system, which poses challenges to the administration of power system operation. Given this background, this thesis explores the application of step size optimization technique in the load flow calculation and the security-constrained unit commitment problem for integrated AC/DC systems. Besides, this thesis also develops interval prediction methods to quantify the uncertainty of load demand prediction. Based on the interval prediction results of load demand, the implication of the uncertainty of load demand prediction on the security-constrained unit commitment is discussed. A detailed introduction is as follows:In chapter two, a new comprehensive index to evaluate the quality of prediction intervals is proposed, namely, Prediction Interval Satisfaction Index, as well as two new methods to build the prediction intervals, namely, Interval Extension Method and Scalar Method. Case studies regarding short-term and ultra-short-term load demand interval prediction are conducted, results of which illustrate that the proposed two simple methods are able to build high-quality prediction intervals and also applicable to different conditions while maintaining robust performance.Chapter three compares three step size optimization multipliers, namely, optimal multiplier, quasi-optimal multiplier and mixed multiplier. With AC and DC power flow equations solved simultaneously, optimal multiplier and quasi-optimal multiplier, which have been widely used in AC power flow calculation, are extended to the AC/DC power flow calculation in the polar coordinates and then mixed multiplier is proposed based on the quasi-optimal multiplier to overcome the intrinsic drawbacks of optimal multiplier and quasi-optimal multiplier. Test results show that the improved method with mixed multiplier has the best convergence accuracy, convergence precision and robustness, which is a promising method with only a little adjustment on the ordinary method.The mathematical model of security-constrained unit commitment problem for integrated AC/DC systems is built in chapter four, and Benders decomposition is applied to solve this large-scale non-linear mixed-integer programming problem, which decomposes the original problem into a master problem considering unit commitment and a series of subproblems used to check the network security during each time period. The master problem and the subproblems iterate alternatively until achieving convergence when the optimal solution of the original problem is obtained. Considering the uncertainty of load demand prediction, the modifying strategy based on scenarios is then proposed, which adds scenario-feasibility-checking subproblems into the deterministic model to narrow the solution space of unit commitment to make the optimal solution more robust, as well as the modifying model which adds the unbalaced power in each scenario into the object of the deterministic model as the penalty term.