【作者】 刘刚；

【导师】 李琳；

【作者基本信息】 华北电力大学 ， 电工理论与新技术， 2012， 博士

【摘要】 换流变压器是高压直流输电系统中的重要设备之一,其阀侧绕组在正常工况下承受含有高次谐波成分的交直流复合电场,在潮流反转时又承担极性反转瞬态电场,准确计算其油纸绝缘结构中的电场是换流变压器绝缘设计的基础,深入研究换流变压器交直流复合电场和极性反转瞬态电场分布特征对换流变压器的设计、制造和维护具有重要意义。本文对换流变压器油纸绝缘结构交直流复合电场和极性反转瞬态电场计算方法进行了深入的研究,主要内容如下：(1)交直流复合电场的频域有限元计算方法研究换流变压器阀侧绕组承受的交直流复合电压中不仅含有直流分量和基频交流分量,还含有高次谐波分量。目前变压器设计人员普遍采用的分析方法只将直流电场叠加交流电场作为交直流复合电场,忽略了高次谐波的影响。本文采用频域有限元方法分析绝缘介质电导率线性条件下的交直流复合电场,考虑了高次谐波电压分量的影响。对绝缘介质电导率-电场强度非线性条件下的交直流复合电场,提出将定点法和线性化迭代方法相结合,然后采用频域有限元方法进行计算。分别采用频域有限元方法、瞬态法和谐波平衡有限元方法分析了典型油纸绝缘结构的交直流复合电场,对比结果表明频域有限元算法既可以比瞬态法减少计算时间,又可以比谐波平衡有限元法节省计算机内存,适合实际换流变压器的交直流复合电场分析。本文同时对油纸绝缘结构典型模型在电阻率为线性、非线性和非线性各向异性条件下的交直流复合电场进行了计算,对计算结果作了比较分析。(2)绝缘介质电导率-电场强度非线性条件下换流变压器极性反转过程持续时间对极性反转完成时刻电场分布的影响目前国内外学者在换流变压器极性反转电场分析时没有同时考虑绝缘介质电导率非线性和短时极性反转过程的影响,本文详细研究了绝缘介质电导率-电场强度非线性条件下,短时极性反转过程持续时间对极性反转完成时刻电场分布的影响。分析结果表明,在绝缘介质电导率-电场强度非线性条件下,极性反转过程持续时间对反转完成时刻的电场分布影响很大,尤其是绝缘裕度较低的变压器油中电场强度值变化很大。如果同时考虑电导率-电场强度非线性及2分钟的短时极性反转过程,则反转完成瞬间油中最大电场强度值要比瞬间完成的电场强度值小50%,这对绝缘设计具有重要意义。(3)分析换流变压器极性反转电场的节点电荷(密度)-电位有限元方法目前国内外学者在分析换流变压器极性反转过程时,往往只是定性地用空间电荷在极性反转前后不能突变或变化量很小来解释电荷及其电场对换流变压器极性反转电场的影响,没有对油纸绝缘结构中的空间电荷做定量地研究。本文提出一种能够直接计算换流变压器极性反转瞬态过程中空间电荷及其电场分布的数值计算方法—节点电荷(密度)-电位有限元法,该方法直接以节点电荷(密度)和电位为待求量,计算精度较高。在得到空间电荷密度的基础上,提取导体表面电荷密度,从而可以准确地计算换流变压器金属结构件(比如静电环)表面法向电场强度,为结构设计提供依据。(4)一台±500kV换流变压器极性反转电场和电荷分布的计算利用本文提出的节点电荷(密度)—电位有限元方法,对一台±500kV换流变压器在极性反转电压下的瞬态电场、电荷及其电荷电场分布进行了计算分析。对比了绝缘介质电导率各向同性线性／非线性、各向异性线性／非线性条件下的瞬态电场和电荷分布,分析了瞬态过程中绝缘介质典型位置电场、电荷和静电环表面法向电场强度的变化特征。更多还原

【Abstract】 The valve siding of the converter transformer, which is one of the most important equipment in HVDC (High Voltage Direct Current) project, is imposed on periodic AC-DC hybrid electric fields under the operation condition, which contains higher harmonic voltage, and PR (polarity Reversal) electric field when the power flow reversing. To accurately compute the electric fields in oil-paper insulation structure under the above conditions is the foundation of insulation design and the further study on the distribution characteristics of the AC-DC hybrid electric field and PR electric field has great significance to converter transformer’s design, manufacture and maintenance. This thesis is focus on the algorithms of calculating the AC-DC hybrid electric field and PR electric field in oil-paper insulation structure and the main content is as follow:(1) Study the algorithm of computing the AC-DC hybrid electric field by FEM in frequencyThe voltage imposed on the valve siding of converter transformer not only contains dc and ac component, but also has higher harmonic components. It is a common method for the designers to obtain the AC-DC hybrid electric field by directly superimposing the dc electric field and static electric field and neglecting the higher harmonic components. In this thesis, the FEM in frequency is adopted to analyze the hybrid electric fields in the linear dielectrics and the FEM in frequency combing the fixed point method and linearization method is proposed to get the periodic electric field distribution in the electric dependent dielectrics. The FEM in frequency is fully compared with transient method and the HBFEM (Harmonic Balance FEM) in analyzing the steady hybrid electric fields in linear and nonlinear dielectrics of a typical oil-paper model. The results show that FEM in frequency can not only reduce computational time compared to transient method, but also save computer memory compared to HBFEM and can be adopted to obtain the AC-DC hybrid electric field distribution of the actual converter transformer. The electric fields in linear, nonlinear and anisotropic nonlinear dielectrics are obtained and analyzed in this paper.(2) Study the influence on the electric field distribution at the end of each PR by the duration of the PR process in the electric dependent dielectricsBecause the current analysis of PR electric fields has not took the electric dependent nonlinearity and the duration of the PR process into consideration at the same time, this paper deeply study the influence on the electric distribution at the end of each PR by the short duration of PR process in the electric dependent nonlinear dielectrics. The computational results show that the duration time of the PR process has great influence on the electric field distributions in the electric dependent dielectrics, especially for the oil, which has low insulation margin. The maximum electric field of oil in the nonlinear dielectrics, which PR duration is2minutes, is about50percent less than the corresponding ones assuming the PR taking place without any delay, and the less electric strength is benefical for the insulation design.(3) The node charge density-potential finite element method for the PR’s electric fieldIt is usually qualitatively pointed out that the charges and their induced electric fields could not change suddenly or have small variations and have influence on the PR electric field when analyzing the PR’s transient electric field, and should be quantitatively study the charges and their charge induced electric fields. This paper propose an algorithm to obtain the charge and potential at the same time, which is called node charge (density)-potential finite element method, to study the transient electric field and charge. The method directly uses the node charge and potential as variables and has higher accuracy. The charge densities on the metal structure, such as static ring in converter transformer, are adopt to obtain the normal electric field accurately, which can provide guidelines for the structure design.(4) Compute the electric field, charge and charge induced electric field of a±500kV converter transformer under PR voltageThe transient electric fields and charge distributions of a±500kV converter transformer under PR voltage are obtained by the proposed node charge(density)-potential finite element method. The transient electric fields, charges and charge induced electric fields in isotropic linear dielectrics, isotropic nonlinear dielectrics, anisotropic linear dielectrics and anisotropic nonlinear dielectrics are compared and study the variety characteristics of the electric fields, charge distributions and the normal electric fields of the static ring.更多还原