Analysis And Calculation Of Three-phase Power System Based On Improved Fixed-point Iterative Method By Phase-coordinates

With the development and application of power system,the asymmetric lines such as multi-circuit lines have been building increasingly.In addition,single-phase,two-phase load and other factors make the power system three-phase asymmetry more and more serious.Also,the number of new power sources connected to the power system is increasing.It is an important feature of modern power system that the asymmetry of power system includes the above-mentioned new generations,transmission networks and loads.The asymmetric structural parameters,load parameters and types of asymmetric faults caused by the above factors make the symmetric component method lose the rationality in theory and convenience in calculation.The phase-coordinate method can describe the physical meaning of the asymmetric system intuitively and conveniently.This paper employs Fixed-point iteration method by phase-coordinates to analyze and study power-flow and fault problems under asymmetric conditions of power system.The main research work and innovations are as follows:1.The key of the line modeling is to get the correct expression of three-phase coupled circuit.In this paper,the theory of de-mutual inductance is used to deduce the nodal admittance model and the circuit topology of its network equivalent model by phase-coordinates.The model topology will be transformed from electromagnetic coupling to direct electrical connection through branches between nodes of network equivalent circuit.When the number of nodes is not increased,the nodal voltage equation enables to express the coupling components by modeling.This paper also deduces the transformer augmented admittance model including neutral point variables by the operation matrix method,and gives the transformer branch-loop correlation matrix,which solves the difficulty of transformer modeling in augmented calculation.The models of coupling lines and transformers are the basis of calculation and analysis of power system under asymmetric conditions.2.Fixed-point iteration method for power system by phase-coordinates is proposed,and the characteristics of the method are demonstrated.The demonstration results show that the method has non-expansive characteristics when the power is given or the power increases with the increase of voltage.Compared with Newton's method,the method's coefficient matrix is constant coefficient matrix,which need not be updated iteratively.Compared with PQ decomposition method,it is accurate iteration and has similar convergence speed.The essence of this method by searching a set of equivalent admittance approximations obtains a set of linear current injections,and it satisfies the principle of energy minimization first.3.In three-phase power flow,this paper presents the arrangement method and solution technology of Fixed-point iteration method in three-phase asymmetric system,which also includes the access problem of distributed power generation and various methods of neutral point analysis.The proposed method is able to calculate power flow in the different power system networks under the given conditions.The results show that the proposed method has great computational performance and robustness.4.In fault calculation,compared with traditional multi-port Thevenin equivalence and compensation methods,the Fixed-point iteration method and its solution technology are presented to solve the complex faults of power system and the multi-circuit's faults in power system.The calculation results show that the proposed method can solve the above-mentioned fault problems conveniently and effectively,and the calculation has the characteristics of simple embedding and high calculation efficiency.5.The analysis model of transformer under asymmetric magnetic circuit and the method of obtaining phase component parameters are given.The influence of symmetric component parameters and phase component parameters on the system is compared.In the numerical calculation,the results show that the phase component parameters can clearly reflect the mutual inductance between transformers.In this paper,nodal admittance models of three-phase coupled circuits and transformers are established by phase-coordinates at first.And Fixed-point iteration method by phase-coordinates is used to analyze and study the power-flow and the fault calculation in the asymmetric power systems.The method has a clear physical meaning for finding a set of equivalent admittance approximation in iteration to obtain a set of linear current injection,whose iterations first satisfies the principle of minimum energy.And the method has the characteristics of non-expansion under the condition that the given loads or the loads with the positive characteristics of increasing load with voltage raise.The coefficient matrix of the method is a constant coefficient matrix.It does not need time-consuming and tedious updating in the iteration process.Moreover,the iteration formula of this method has not been omitted.And it is calculated accurately and has a non-linear iteration speed.The cases in the paper illustrate the effectiveness and robustness of the proposed method.