Characteristic Study Of Electric Field In Valve Winding And Line Winding Of Converter Transformer

With the rapid development of the national economy, HVDC transmission hasbecome the trend of power network in China as its advantage of long-distance, largecapacity and low-loss. As an important device of HVDC transmission system, theoperation reliability of converter transformer directly affects the overall systemperformance. Due to the valve side winding working condition particularity and thematerial complexity, there are many uncertainties in internal electromagnetic field of theconverter transformer. In the process of transformer design and manufacturing, theinsulation structure design is the focus and one of the difficulties. Therefore, studingconverter transformer internal electric field analysis method and developing systematicanalysis software, has very important theoretical significance and practical value in guidingthe transformer design.The same as the power transformer, converter transformer longitudinal insulationassessment is the calculation of winding wave process. To simplify the calculation processusing the equivalent circuit model of single-winding, ignoring the impact of low-voltagewinding parameters resulting in the assessment of the insulation is not accurate enough. Onthe basis of the study of single-winding equivalent circuit model derived two-windingequivalent circuit model which consider the impact of low-voltage winding parameters andbased on the double-winding equivalent circuit model, a practical wave processengineering calculation software is developed in this paper. In the transformer maininsulation, the electrical conductivity of pressboard is an important parameter for theconverter transformers DC electric field calculation accuracy. Nonlinear and anisotropic ofpressboard has not been thoroughly studied in previous, this paper presents a mathematicalmodel of nonlinear anisotropic DC electric field and compiling the calculation program ofthe converter transformer DC electric field through large number of theoretical analysisand mathematical derivation. The main work is as follows:(1) Based on the analysis of single-winding transformer equivalent circuit model, the shortage of the equivalent circuit model of single winding is pointed out, and theequivalent circuit model of double winding is built. To find the vulnerable of the insulationof winding on guiding transformer design, the impulse voltage distribution on each disk ofthe transformer windings and the maximum oil channel potential gradient is calculated bythe differential equation.(2) According to the shortcomings of existing calculation software, the wave processcalculation software basing on the double winding equivalent circuit model is developedfor the engineering staff. The software can be used not only to analyze a variety of voltagelevels, structural capacity transformers, and the initial distribution of large transformers butalso calculate the oil channel impulse response and the maximum potential gradient.(3) The circuit model of three-phase transformer under different conditions into waveequivalent is established, the wave process of three-phase transformer is researched underthe single-phase into the wave and three-phase into the wave, the wave process of law isdiscussed through the form into the waves and winding connection way. Taking the shieldwinding for example, considering the influence of the non-uniform electric field to thecapacitance parameters, analytical methods were applied, the impedance circuit andnumerical calculations method to calculate the disk-to-disk capacitance and the groundcapacitance, and compared with the manufacturers, then provide the parameterscalculations form for this type of winding.(4) Considering the anisotropic and nonlinear of material characteristics, providing thematerials DC steady-state electric field mathematical model when the two characteristicsare both taken into account, deriving from the corresponding discrete form, the DCsteady-state electric field calculation program is programed. Based on different materialcharacteristics, the regularity of electric field distribution is analysised.A real converttransformer DC steady-state is calculated and the result of ANSYS is compared, thecorrectness of the DC steady-state electric field application is verified.(5) Considering the uncertainty factors when the electric field is calculated, polarityreversal electric field of the converter transformer valve side is calculated through thetransient field, then the calculation way of maximum electric field strength is provided.The electric field calculation accuracy is improved.