Analysis Of Characteristics Of Nonlinear Load In Distribution Network Based On Parameter Identification

Nonlinear loads cause harmonic pollution and three-phase unbalance in power system, influence the normal operation of equipment and the security and stability of the power system. The reason of these phenomena is that the equivalent parameter of nonlinear load is a function of electric parameter, which reflects load characteristics and the change of the operation status of power system. Load characteristics can be applied to load decomposition, equipment inspection and load forecasting by identifying the equivalent parameters of the load. The existing methods of nonlinear load equivalent parameters identification are deeply influenced by the selection of initial value and can easily cause accumulate error. What's more, the main means of identification is based on the assumption that the equivalent parameters of the system change slowly or unchanged. In addition, for the low order of the identification model, the model is not equal, and a lot of transient information is ignored, so the identification accuracy is affected.In order to accurately identify the equivalent parameters of the load and provide help for the nonlinear load characteristic analysis, this paper proposed a new method for the identification of the equivalent parameters of single phase nonlinear load. In this paper, a single input and single output model is established by using a nonlinear high order differential equation. The paper used the matrix pencil method to fit voltage and current and substitute them into the high order differential equation, then corresponded the equation with the equivalent circuit. Finally, the equivalent parameters of the load are obtained. This method didn't use the piecewise linearization method, and the order of mathematical model is high. What's more, it is a non-iterative algorithm. So, the identification accuracy is improved. This paper introduced the concept of impedance nonlinearity and load nonlinearity, and used them to characterize the fluctuation of load parameter and the harmonic responsibility to PCC.On the basis of the identification of single phase nonlinear load parameters, the method of equivalent parameter identification for three-phase asymmetric load is presented in this paper. The paper used the load line voltage as the model output variable, and the line current as the input variable, established a high order differential equation in the form of a double input single output model. The paper used the matrix pencil method to fit voltage and current, substituted them into the higher order differential equation, corresponded the equation with the equivalent circuit, and finally get the equivalent parameters of the circuit. By using the obtained equivalent parameters, the neutral point displacement and the neutral line impedance of the load can be obtained, which can provide the basis for the three-phase leveling and the protection of the power grid. The paper used the load nonlinearity to further divide the harmonic responsibility, and provided help for harmonic source location.This paper used PSCAD/EMTDC to establish the simulation model, and MATLAB to program the algorithm to analyze the load characteristics of single-phase and three-phase non-linear load respectively. The simulation results showed that this method can get the function of the nonlinear load equivalent parameters under the condition of computer simulation, and has ideal feasibility and accuracy. By using the impedance nonlinearity and the load nonlinearity, the intensity of nonlinear load changing can be obtained, and can be used to divide the harmonic responsibility. The simulation models were built to analyze the load of typical constant power load and three phase AC arc furnace. The experiment results showed that the method proposed in this paper has ideal effect under the condition of computer simulation.