Reduction And Reconstruction Of Bulk Power System Practical Dynamic Security Region

Different from traditional "point-wise" approach, the method of Dynamic Security Region (DSR) is a completely new methodology in power system dynamic security analysis. It is a set defined in injection space. Every operation point that lies in it can guarantee transient stability of the system. Through a lot of simulations in real power systems, we have found that DSR can be approximated by a simple hyper polyhedron in injection space , which is named as Practical Dynamic Security Region (PDSR). It is surrounded by hyper planes (HP) that describe the upper and lower limits of the node injection real power and are vertical to the coordinate axes and critical HPs that describe the critical points of transient stability. It can be calculated off-line and applied on-line to determine whether an operation point is secure under a given fault by judging whether the current injection lies inside the PDSR.Critical HP of PDSR is basically constructed by the least-square approach, which uses lots of critical points searched by numerical simulation on the bulk power system. This method, although accurate enough is so time consuming that it cannot meet real time application of bulk power systems. Dynamic equivalent is an effective method for power system reduction. Compared with other dynamic equivalent methods, the one that applies reachability Gram matrix of the power system model can identify coherent generators faster and more clearly by use of some necessary and sufficient conditions. Nonlinear loads are reduced by current sink reduction (CSR) method, which means nonlinear loads are expressed by current sinks and reduced by Gauss reduction method.Based on this technology, the original system with high dimension is firstly reduced to an equivalent system with lower dimension. The critical HP of equivalent system is obtained through numerical simulation and fitting and then the analytic expressions for HP reconstruction are derived on the base of reverse process of dynamic equivalents. At last the reconstructed critical HP coefficients errors caused by the equivalents are corrected to some extent by translation of the HP to a real critical point which is obtained through numerical simulation on the bulk power system with detailed model.The computer program based on this method is developed and the examples on the 10-machine, 39-bus New England System and the 4-machine, 11-bus system of WSCC are given to validate the method. Results of the case studies show that this method can save much time, while keeping the accuracy acceptable in engineering practice and thus PDSR becomes easier to be applied in real time security analysis.