变压器绕组内VFTO波过程的建模与计算

Modeling and Calculation of the Wave Process in Transformer Windings Under VFTO

为了研究变压器绕组在快速瞬态过电压(VFTO)下的波过程,减少频域内求解时频谱混叠和泄漏带来的结果误差,建立了变压器绕组多导体传输线(MTLs)模型;推导了基于时域有限差分法(FDTD)在时域内求解变压器绕组多导体传输线模型的迭代公式。应用矢量匹配法采用一阶有理函数逼近传输线频变阻抗,用拉普拉斯反变换处理传输线阻抗的频变问题来解决FDTD处理频变难的问题。实测和仿真计算一个180匝连续式变压器绕组模型的结果表明,二者波形较吻合,计算模型和计算方法正确和可行。

Switching operations in gas insulated substations （GIS） are known to produce Very Fast Transient Overvoltage （VFTO） which are dangerous for the insulation of the transformer. The experience shows that VFTO has a very short rise time（less than 0.1μs） and its main oscillating frequency which depends on the length of GIS is between 1 MHz to 40 MHz. When VFTO invades into the transformer, an unbalance of voltage distribution in transformer windings can be caused. Another problem is the resonance in the transformer windings, which may occur when the frequency of the incoming VFTO matches some of the resonance frequencies of the transformer windings, and the inter-turn insulation is particularly vulnerable to high-frequency oscillation. This paper deals with the calculation of the inter-turn voltage distribution in transformer windings under VFTO. A model of transformer windings based on the MTLs theory is developed and calculated based on the Finite-Difference Time-Domain method （FDTD）. The Vector Fitting method is adopted to treat the frequency-dependent parameters by expanding the timedomain form of internal impedance of the transformer windings into a sum of finite exponential functions, which are easy for the time-domain convolution. An actual transformer windings model is studied, the calculated results obtained by this method are compared with those by measurement.