Evaluation And Computation Of Available Transfer Capability In Hybrid AC/DC Power Systems

Under the competitive power market environment and in the smard grid era, the power grid has been extensively used and operated close to its limit. This results in a new challenging task for power system engineers in managing the security of the power system. Available Transfer Capability (ATC) is a significant technical index in the power market environment. The precise and fast computation or evaluation of ATC is crucial to reliable operation of the power system and fair distribution of the power market participants’benefits. Along with the interconnection of bulk power grid and extensively used of high voltage direct current (HVDC) in modern transmission network, there is an urgent need to study the theory and methods on ATC assessment problems arising from AC/DC interconnection modes of bulk power grid. Based on thoroughly researching of the existing literature, this dissertation deeply focuses on the computation and evaluation problems of ATC in AC/DC hybrid systems from the definition and computation requirements of ATC, the main aspects are as following.Considering the uncertainties, DC system and its flexible control modes, and their effects on the AC system, a novel mathematical model is constructed to assess ATC in the AC/DC hybrid systems. The objective function and corresponding constraints are modified according to the electricity transfer profile selected and the control and operation of DC systems. Because of time and space varying of ATC, the probabilistic methods are suitable for ATC evaluation. The non-sequential Monte Carlo simulation based methods are presented to calculate the five-number summary and other statistical indices of ATC for the long time transfer capability assessment of the network. Effective numbers of Monte Carlo simulations can be obtained by the given acceptable error and the confidence level. A kernel density estimation based method is proposed to approximate the probability density function of ATC. In the process of calculation, the optimal power flow model is added into the repeated power flow method to ensure both safe and economical operation. In general, the ATC is the difference between the Total Transfer Capability (TTC) and the Existing Transfer Capability (ETC) without considering system margins since various operating margins can be accounted for separatedly. When using such a definition, all the models and methods applicable to evaluation of ATC are also applicable to the evaluation of the TTC.In order to avoid too many times of Monte Carlo simulations and promote the efficiency of the assessment, generalized Monte Carlo methods are proposed to deal with the stochstic behavior of the power system by incorporating the traditional Monte Carlo method and resampling, parallel computing, and robust statistics methods. All of these methods can not only establish the basis for realtime assessment of ATC/TTC, but also provide methodology support for applications of Monte Carlo method in other fields.Based on the analysis to the traditional modeling of available transfer capability, one method is proposed to calculate ATC for AC/DC hybrid power grid considering transient stability constraints. The optimal power flow (OPF) model is employed to solve the problem. In this method, different kinds of constraints including voltage limit, transient stability limit, generator operation limit, etc. can be considered comprehensively. Some contingency states are considered. Furthermore, the flexible control functions of DC systems were involved in the calculation by rationality and security principles.Numerical bifurcation analysis techniques are very powerful and efficient in physics, biology, engineering, and economics. Power system engineering has become a classical application field of bifurcation theory since1990’s. Local bifurcations are readily evident in power systems as important elements of voltage instability. Apart from considering the static limits, application of bifurcation analysis would be a novel idea in the computation of dynamic ATC. By integrating the saddle-node bifurcation (SNB) and Hopf bifurcation (HB) into the ATC model, both the static and transient security are considered. For ATC determination, the static and dynamic ATC corresponding to SNB and HB respectively and they are obtained by the extended systems with respect to specific singular points. Furthermore, the direct method and the optimization method to determine the bifurcation point are compared and the equivalency of them is proved from a mathematical viewpoint. All of these can act as theoretical and technical support for fast and accurate computation of ATC.