Research On Equivalent Method For Large-scale Power System

With the trend of inter-connection and deregulation among the power systems, the relationship among the different areas is intensified. When fault occurs in one area, it is likely to lead to other areas'instability. So, the impact from the external power systems has to be taken into consideration when conducting security assessment on its own. This results in two aspects, one is improving the precision and the accuracy of the analysis, the other is slowing the calculating speed, which is unbearable in the recent aspiration of real-time security analysis.Network reduction is the process of reducing the dimension of the power system, while keep the researched area unchanged and preserve the external networks'influence on the researched area. The large-scale power system equivalence is very important for the steady state power flow analysis, transient stability analysis, and the real-time security analysis.Coherent equivalent algorithm is a classical equivalent method in the large-scale power system off-line transient stability analysis. It is typically done in the next three steps: identification of the groups of coherent generators, reduction of the external network, and aggregation of the coherent generators. In the first step, coherent generators are identified using linear simulation or manual identification; in the second step, the network reduction is based on the constant megavolt ampere (MVA) technology, in this process we often use Ward equivalent method or REI equivalent method to reduce the nodes in the external network; in the last step, we keep the equivalent generator's transfer function identical with the set function of the single generator's transfer function in the frequency domain.The effectiveness of the dynamic equivalent method is based on the model of the equivalent generators and its'parameters. For generators we try to select the model with most possible high order, such as transient model and sub-transient model. For exciter model, in the practical application, we only need to classify the major aspect (such as E- model and F- model in the BPA software), and the result is mostly acceptable. For primer mover and its controller, they can be neglected because the equivalent generator's moment of inertia is always great.In the process of the network reduction and the generators'transient simulation, we have to solve great amount of matrix calculation. Besides, the dimension of the matrix of the power system is always high, it is important to use the sparse matrix technique. When solving the sparse matrix, we conduct LU decomposition to form L, D, U matrix. Finally we can use the L, D, U matrix to solve the linear function and the inverse matrix, even the sensitivity calculation.The simulation results on the inter-connected power system (the data given by the Tianjin power grid in the winter, 2005) validate the efficiency of the coherent equivalence.