Study On The Internal Faults Simulation And New Principles Of Power Transformer Protection

Recently with development of power system extra high voltage large capacity transformers are put into operation continuously. Its safe operation would affect the security and stability of power system. High demands to transformer main protection are put forward, so it is very significant of improving the scheme of differential current protection now in use and developing novel transformer main protection, and the simulation of transformer internal faults is the necessary means to reveal the laws of internal faults and improve the performance of protection. The dissertation makes an investigation on some key problems that are not well solved in transformer protection. Based on deep analysis and research, novel methods to solve those problems are presented. The major contributions of this dissertation are as follows:The process of transformer magnetic field calculation based on finite element analysis is studied deeply. The adaptive meshing method and the transformer winding leakage inductances calculation method are presented, the platform for transformer model parameters calculation is developed in which the above computing process is packaged with VC, the computing efficiency and accuracy are raised. The calculation results are compared with experiment data, and the comparison results show that this method is correct.Novel inrush currents identification methods are presented. The phase relation of differential currents and the features in time domain and sampling space under various conditions are studied thoroughly. In the identification method based on the frequency characteristic of Park vector module the content of second harmonic is used to reflect the phase relation among the three differential currents. The method needs less calculation, has definite physical meaning and is not affected by the non-fault phase. The identification method based on the waveform characteristic in time domain is presented. In this method the histogram is used for extracting the local characteristics of inrush current such as backlash and the peak. The method is not affected by the symmetrical inrush and has higher performance. In the identification method based on the distribution characteristic in sampling space the optimum subspace (direction) for identification is obtained with principal component analysis. The reliability and sensitivity are assured as much as possibly.The variable weights synthetic identification of inrush is presented. Based on the statistical results the method to count the interactions among various criterions quantitatively with2-order additive measures is presented, and the results are used to the basis for selecting a optimal criterion combination and setting the constant weight of criterion. According to the characteristic of inrush membership function A new state variable weights function is constructed. Thus the weights reflect the overall importance and the relative importance under certain condition, and the loss in identification performance caused by the invariability of weights is avoided.New method for transformer winding currents at delta side calculation is presented. The similarity relation between zero sequence current at star side and the circulating current at delta side under the inrush condition is analyzed. The proportionality coefficient between zero sequence current and circulating current is determined by analyzing the similarity of the fitted inrush current. The method does not need the parameters of windings, has no requirement on the process of switching on with no load, and has high calculation accuracy.Two new on-line identification methods for transformer winding parameters are presented. The identification model is obtained according to the loop equation. With the method based on UD decomposition least square the stable results can be obtained quickly at the premise that the winding currents are known. The method has high numerical stability. In the nonlinear identification method based on modern interior point method the winding currents are not needed, only the load data are used. The method has high calculation accuracy and is easy to be realized.Two new transformer main protections based on loop equation are presented. In the method based on the generalized fundamental power the phase characteristic between the differential voltage and the differential current is accentuated. The method is not affected by the core loss and copper loss, does not need the winding leakage inductances. A practical scheme of transformer main protection is presented by combining this method and that based on loop equation difference. In the differential voltage protection based on the compensated voltages the amplitude and phase relations between the compensated voltages are considered independently. The operation zero is expanded as much as possible, the sensitivity of the protection under internal fault condition is increased on the premise that the reliability is ensured.