Time Domain Simulation Calculation And Experimental Research On The Effect Of VFTO On Power Transformers

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 Province Natural Science Foundation of Hebei (No.503624), in order to meet the requirement of manufacturing 750kV and 1000kV power transformers, the experimental research methods, the fast time-domain algorithm for calculating the voltage distributions of power transformer windings under VFTOs and the resonant analysis method are emphatically researched. The dissertation main contributions and works are as follows:1. To obtain the voltage distributions in the high voltage windings of transformers under VFTO, two kinds of transformer winding models are designed and manufactured. The voltage waveforms of each end windings in the two models are measured; the peak voltage and peak voltage gradient distributions of the transformer windings are mapped and studied. The effects of rise time on the distributions are discussed. It is concluded that, the longer the rise time, the more homogeneous the voltage distributions, contrariwise, the more inhomogeneous; The most inhomogeneous areas exist in the coils close to the lead-in of the transformer windings and the interface of the two different structure of the windings; Internal-shielded structure can improve the overvoltage distributions not only under lightning, but also under VFTO.2. Based on multiconductor transmission-line (MTL) model, a fast time-domain method is presented to calculate the voltage distributions along transformer windings, through combining the compact finite difference (CFD) theory and the backward differentiation formulas (BDF). Vector fitting and recursive convolution are used to dispose the frequency-dependent parameters. There is a good agreement between the simulation and the measurement.3. To further improve the computational efficiency, a simplified model for large power transformer HV windings is presented considering the fact that the insulation faults usually happen in the coils close to the lead-in of the transformer windings under VFTO. The simplified model proposed in this paper is represented by modified Telegrapher's equations and state equations.4. Based on MTL model, a single-input-multiple-output network model is presented for transformer windings. The procedure to determine the transfer function is deduced. The resonant frequencies and the location of resonance are determined by analysis of amplitude frequency response characteristics. For validation, the calculated results are compared with the measurements and the results are satisfactory.