Physics of microarcs in low-voltage switching devices

该项目旨在描述和了解低压直流(DC)开关设备中的小电流电弧(0.5至20 A)。例如，低压直流开关在电力机动性以及使用光伏系统和电池的本地电网中变得越来越重要。弧光开关器件作为低成本的解决方案或与混合开关设备中的半导体器件相结合很有吸引力，因为它们允许电流分离和高绝缘水平。虽然开关设备已经使用了几十年，但对开关电弧中的物理过程的基本了解仍然不完整，甚至非常糟糕，特别是在小电流情况下。在本项目前期的调查中，发现了短间隙时电弧电压和电弧辐射的意外行为，这是传统的电弧模型所不能描述的。此外，低压开关设备通常使用铜合金等非耐火电极材料。在常压下，电流从非难熔阴极到小电流电弧的转移是一个开放的理论问题，主要是因为即使在接近沸腾温度的情况下，非难熔金属的热电子发射也很低。这些问题应在项目中通过实验和理论研究相结合的方式加以解决。重点放在常压环境空气中扁平圆柱形电极之间的弧形，作为最常见的低压和低成本开关设备的代表。电极材料除纯铜外，还应采用铜复合材料。特别是，应评估小间隙和增大间隙时电流分断过程的伏安特性、包括电极鞘区域的电弧空间结构和电极上的电弧附着性。金属蒸气的主导作用、对局部热力学平衡的明显偏离以及在两个电极上建立斑点模式是预期的，即使在小电流情况下也应该得到确认。为此，将建立一个模型开关，并利用高速成像、层析成像和光谱分析来研究三维弧形结构。建立了包括电极鞘面积和与电极模型的自洽耦合在内的多流体非平衡电弧数值模拟，特别考虑了铜蒸气中的等离子体化学。它的目的是自洽地描述小电流时非耐火阴极上的电流转移和电弧附着，这是到目前为止还不能得到的。该模型应解释电流-电压特性，并应通过模型开关处的测量进行验证。结论在低压开关器件的开关性能、电极的腐蚀、寿命和长期行为等方面应予以重视。

The project is aimed at the description and the physical understanding of low-current arcs (between 0.5 and 20 A) in low-voltage direct current (DC) switchgears. Low-voltage DC switching finds an increasing importance e.g. for electro mobility as well as in local grids that use photo-voltaic systems and batteries. Arc switching devices are attractive as low-cost solutions or in combination with semiconducting devices in hybrid switchgears because they permit a galvanic separation and a high insulation level. Although switchgears have been used for many decades, the basic understanding of the physical processes in switching arcs is still incomplete or even very poor in particular at low currents. An unexpected behaviour of the arc voltage and the arc radiation has been found for short gap lengths in investigations preliminary to this project, which cannot be described by conventional electric arc models. Moreover, low-voltage switchgear typically works with non-refractory electrode materials like copper alloys. The transfer of electric current from a non-refractory cathode to a low-current arc at atmospheric pressure is an open theoretical issue mainly because of the low thermionic electron emission even for temperatures close to the boiling temperature of non-refractory metals. These problems should be tackled in the project by combined experimental and theoretical studies. The focus is on arcs between flat cylindrical electrodes in ambient air at atmospheric pressure as a representative of most common low-voltage and low-cost switching devices. Copper composites as well as pure copper should be used as electrode materials. In particular, the voltage-current characteristics of the current breaking process for small and increasing gap distances, the spatial structure of the arc including electrode sheath regions and the arc attachment at the electrodes should be evaluated. A dominant role of metal vapour, distinct deviations from local thermodynamic equilibrium and the establishment of spot modes at both electrodes are expected and should be confirmed even for low currents. Therefore, a model switch will be set up, and the three-dimensional arc structure will be studied by high-speed imaging, tomography and spectroscopy. A multi-fluid non-equilibrium numerical simulation of the arc including the electrode sheath areas and the self-consistent coupling with models of the electrodes will be developed considering in particular the plasma chemistry in copper vapour. It is aimed in a self-consistent description of the current transfer and arc attachment at non-refractory cathodes for low currents, which is not available up to now. The model should explain the current-voltage characteristic and should be validated by the measurements at the model switch. Conclusions on the switching performance, the erosion of electrodes and the lifetime and long-term behaviour of the low-voltage switching devices should be drown.