Study On Modeling Of Power Transformers And Fast Simulation Algorithm Under Very Fast Transient Overvoltages

It is of great theoretical significance and practical value to research the effect of the very fast transient overvoltage (VFTO) on power transformers in gas insulated substations (GIS). Supported by Natural Science Foundation of Hebei Province (No.503624 and No.503424), based on model order reduced (MOR) techniques, the systematic methodology for setuping the high frequency macromodel of power transformer windings and the fast algorithm for calculating the voltage distributions of power transformer windings under VFTOs are emphatically researched. The dissertation main contributions and works are as follows:1. The systematic methodology is presented to setup the high frequency macromodel of power transformer windings under VFTO in GIS. The proposed methodology could be described in a three-step process. In the first step, the voltage transfer functions are calculated from the measured scattering parameter data or the per-unit-length parameters of the multiconductor transmission line model of power transformer winding at first, then they are approximated by means of Vector Fitting. In the second step, the voltage transfer functions are converted into corresponding state equations and output equations, and through reducing the order of the state equations by the Arnoldi-based MOR technique, the mathematic form macromodels are obtained in reduced-order state equations and transfer functions. These resulting transfer functionsis then used in the third step to synthesize circuit macromodel by network synthesis techniques. the circuit model is composed of resistors, inductors, capacitors and ideal transformers, and the parameters of the elements can be easily calculated by the poles and residues of the voltage transfer function. The circuit model is simple and robust. The simulation results and measured results are presented, confirming the validity of the proposed methodology.2. Considering the frequency-dependent parameters of power transformer winding multiconductor transmission line model, through approximating the frequency-dependent parts by passivity-enforced vector fitting codes and converting them into complex frequency form of state equations, a modified integrated congruence transform approach is proposed for frequency dependent multiconductor transmission lines.3. A fast frequency-domain algorithm is presented to calculate the overvoltage distributions in power transformer windings under VFTO. In order to improve computation efficiency, the concept of effective computation frequencyband is introduced, and the precise integration method and the Chebyshev polynomial approximation technique are used in the proposed algorithm. There is a good agrement between the simulation results and measured results, which confirm the validity of the proposed algorithm.4. The overvoltage distributions are calculated and analysized in high-voltage windings of 800kV transformer used in 750kV Laxiwa to Xining and Laxiwa to Guanting power transmission system and 1000kV power transmission system,respectively.