Investigation on charge transport processes in liquid dielectrics on the basis of high-purity paraffins

在高压直流输电的绝缘系统中，电场分布由绝缘材料的电导率决定。液体介质(矿物或天然的变压器油)在绝缘系统中起着关键作用，但目前还没有一个模型能够正确地描述液体介质的电荷传输和导电性。这是因为液体介质的导电性是由多个相互叠加、相互作用的物理和化学过程决定的，而技术绝缘液体是不同化学成分的混合物，本课题选择了一种新的、基本的方法来研究和区分电荷传输过程。石蜡是传统变压器油的主要成分，将首次用于调查。由于定义了烷烃的化学结构，电荷传输过程可以直接与介电液体的物理和化学性质联系在一起。此外，对于研究项目，可以选择在室温下固化的石蜡，以便冻结偏振态并对其进行详细研究。这也使得能够生产最适合预期研究的高纯度去离子石蜡。在研究项目中，应分析相关的电荷产生过程，如场发射(Foweler-Nordheim-Injection)和电极上的电化学解离。此外，还详细研究了化学样品组成以及依赖于场的解离等整体过程。除了与时间、温度和场强相关的电导率测量外，研究工作还包括用所谓的脉冲电声(PEA)方法测量介质内的定量载流子密度和场强，以及基于克尔效应的激光光学测量系统。这些方法用于验证应创建的物理和化学模型。这项研究的目的是为了更好地了解液体介质的不同导电过程，并以定量的方式了解相关过程。为了实现这一目标，需要在高压测试和诊断以及化学领域拥有丰富的知识和实验室设备。因此，预计电力工程和高压技术研究所(FHWS)将与材料合成化学技术教席(维尔茨堡大学)进行跨学科合作，将不同的能力聚集在一起。

In insulation systems of high voltage direct current transmission the electric field distribution is determined by the electrical conductivity of the insulation materials. Liquid dielectrics (transformer oil on mineral or natural basis) play a key role in insulation systems, whereas at the moment there is no model that is able to describe the charge transport and hence the electrical conductivity of liquid dielectrics correctly. The reason is, that several physical and chemical processes, which may be superimposed and interact with each other, contribute to the conductivity of liquid dielectrics and technical insulation liquids are a mixture of different chemical components.In this project a new and fundamental approach is chosen to investigate and differentiate the charge transport processes in detail. For the first time, paraffins, that are the main component of conventional transformer oil, will be used for the investigations. Due to the defined chemical structure of the paraffins, charge transport processes can directly be linked to the physical and chemical properties of the dielectric liquid. Moreover, for the research project paraffins can be chosen that solidify at room temperature so that states of polarization can be frozen and be investigated in detail. This also enables to produce high purity deionized paraffins that are best suited for the intended investigations. In the research project, the relevant charge generation processes, like the field emission (Foweler-Nordheim-Injection) and electrochemical dissociation at the electrode shall be analyzed. Furthermore bulk processes like the chemical sample composition as well as the field dependent dissociation are investigated in detail. The research work contains beside the time-, temperature- and field strength-dependent measurement of electrical conductivity also the determination of quantitative charge carrier densities and field strength within the dielectric by the so called pulsed electroacoustic (PEA) method, as well as a laser optic measurement system based on the Kerr-effect. These methods are used to verify the physical and chemical models which should be created. The goal of the research is to contribute to a better knowledge of the different conduction processes of liquid dielectrics and to understand the relevant processes in a quantitative manner. To reach this goal extensive knowledge and laboratory equipment in the field of high voltage testing and diagnostics as well as chemistry is needed. Consequently an interdisciplinary cooperation of the Institute for Power Engineering and High Voltage Technology (FHWS) with the chair for Chemical Technology of Material Synthesis (University of Würzburg) is foreseen, to bring together the different competences.