An Investigation On Transients In The Power Networks Containing Nonlinear Inductances

Transformer is the most common nonlinear inductance components in power networks. Owing to the structure and material of nonlinear inductance components, their magnetizing characteristics are nonlinear. It will have a great impact on power system when the system running. In the past, in order to avoid nonlinear transient solution and to simplify the circuit model, the nonlinear inductor model is not considered. The paper makes an improvement on the circuit model of transformer in calculation of electromagnetic transients. Two conditions are investigated: exclusion of transformer magnetizing branch and inclusion of hysteresis curve of transformer magnetizing branch. The paper analyzes the difference of switching-voltage on the transmission lines among under these conditions. It also inquires the transient behavior of inrush current cased by energizing unload transformers. Moreover, some waves are given and the calculation results are discussed.A new method for measuring the ferromagnetic material hysteresis loop is presented. This method is simple and feasible. The method was tested with experimental measurements. The accuracy of the result is good.A simplified hysteresis model is proposed in this paper. The multi-valued hysteresis behavior is simulated as a major loop and a set of minor loop trajectories. The minor loop trajectory is created in terms of a linear displacement from the major loop. On the basis of this model, this paper presents an efficient algorithm for calculating the electromagnetic transients of power networks containing ferromagnetic elements. The transient calculation is performed by beginning with magnetizing branches in the network, and the semi-explicit Runge-Kutta method is used to solve the nonlinear ordinary differential equations related to magnetizing currents , then the complete transient solution of the network can be obtained based on the calculated magnetizing currents.This paper discusses the calculation method of operation over-voltage in the transmission system. Using EMTP ,we analysis the reason that cause operation over-voltage in closing free long line,free transformer, then building their mathematical models containing both lumped parameter and distribution parameter elements. The influence of different magnetizing branch models to line over-voltage and inrush current cased by energizing unload transformers are calculated. In the end, the voltage waveforms and calculation results are analyzed.