Stephan Schlegel

Basic investigations on joints with cylindrical aluminum conductors made by press- and shrink-fit for high-current devices

Electrical joints with cylindrical aluminum conductors are often used in Gas Insulated Switchgears (GIS) and Gas Insulated Lines (GIL). Separable connections have to be used for dealing with relative movement between the conductor and the encapsulation initiated by thermal expansion on one end of the conductor. On the other end of the conductor permanent connections can be used with the intention to reduce the costs and operating power losses. Therefore, technologies like the press- and the shrink-fit are suitable. In order to use those technologies for electrical joints proper joining parameters have to be determined from baseline investigations and the long-term behavior of the joints has to be investigated. This paper presents and compares these two joining technologies, press- and shrink-fit. Joints are made with both technologies and the influence of different parameters like surface properties and forms on the joint resistance are being investigated. The behavior of the joint resistance over time is determined from oven-experiments at a temperature of 105 °C. Furthermore a comparison between the cylindrical contacting of the shrink-fit and the flat contacting like it can be found at bus bar joints is made with focus on the contact resistance.

Effect of Dispersoids on Long-Term Stable Electrical Aluminium Connections

In electrical power systems bolted joints with bus bars made of aluminium are common, whereby the tendency towards higher operating temperatures can be observed. At higher temperatures a reduction of the joint force can occur due to creep and/or stress relaxation processes, which leads to an increasing electrical resistance and, in the worst case, to failed joints. The aim of this project is to increase the creep resistance (and to minimise the stress relaxation) of aluminium conductors for electrical applications without a significant reduction in their electrical conductivity – even after long-term exposure to elevated temperatures. The effect of dispersoids in different aluminium alloys on the longterm behaviour of currentcarrying joints at high temperatures (i.e. 140 °C) was investigated. Longterm tests on bolted joints with force measuring devices were performed to monitor the joint forces and to measure the joint resistances, both with and without current supply.

Investigations on stationary electrical joints with a bare and a silver or nickel coated contact partner regarding the permissible temperature limit according to ANSI IEEE and IEO

In electric power transmission, copper and aluminum are mainly used as conductor materials. To improve the contact behavior, the contact surfaces of e.g. high-voltage switching devices are often coated with silver or nickel. Due to economic reasons, connecting conductors are usually uncoated. In that case, an electrical joint with only one coated contact partner is formed. With respect to the permissible temperature limit for this kind of electrical joints, a contradiction between IEC 62271 1:2007/A1:2011 and the ANSI IEEE C37.100.1:2007 exists. The limiting temperature given in the IEC is the one of the surface material with the higher valid temperature limit. On the contrary, the limiting temperature according to the standard of the ANSI IEEE is the one of the surface with the lower valid temperature limit. One major intention was to investigate the physical temperature limits of connections with only one coated contact partner. Therefore, joints with bus bars made of copper and aluminum and different coatings have been set up and aged at different temperatures. The joint resistance is measured time-dependent and evaluated. Additional investigations of the microstructure and mechanical properties were done to understand the physical aging processes.

Investigations on the long-term behavior and switching function of fuse-elements for NH-fuse-links (gG) at higher thermal stress

In electrical power grids NH-fuse-links (gG) are widely used to protect electrical devices in case of overloads or shorts-circuit currents. New requirements of modern power grids, especially in the context of a growing share of renewable energies, implicate a much higher thermal stress level of the fuse-links. This can lead to an accelerated ageing of the fuse-elements, mainly due to the temperature-dependent ageing mechanisms of interdiffusion. The results are failures like unintended tripping or even a failed tripping of fuse-links. These failures reduce the security of supply, which is expected from the grid, and cause a major economic damage due to the high numbers of fuse-links used in the grid. Thereby the maximum temperature of the fuse-element is the most important criterion for a long-term stable use of fuse-links. Thus current research focuses on the investigation of the long-term behavior of such fuse-elements at higher thermal stress levels. Therefore the fuse-elements temperature distribution at normal operating conditions is investigated as well as extensive long-term experiments with several common used fuse-element types at different thermal stress levels are performed. The paper presents the considerations, on which the experiments are done, and the results of the long-term experiments. Further, a test stand for tripping tests of fuse-elements and first results of tripping tests with new fuse-elements are presented. With that, the impact of the ageing, due to the long-term thermal treatment, on the proper functioning of the fuse-elements should be evaluated and based on that results, conclusions for future limiting temperatures for fuse-elements should be drawn.

Press- and shrink-fit connections with cylindrical aluminum conductors for high-current applications — Contact- and long-term behavior depending on mechanical parameters

In Gas Insulated Switchgears and Gas Insulated Lines, cylindrical aluminum conductors are used for transportation of energy. To connect the conductors with other parts of the system, separable connections with contact elements are the common and most used solution. In many cases, the separable connections can be replaced with permanent connections in order to reduce costs, operating power losses and increase the reliability, especially if short circuit currents occur. To set up these permanent connections, technologies like the press- and shrink-fit are investigated. Based on the preliminary investigations presented in 2015 a broad variety of connections were established with different interferences, grooved and knurled as well as silver-plated contact surfaces to allow for statistic evaluations. The contact and long-term behavior of the connections are investigated with oven-experiments as well as under current load for a period of more than 10,000 h. Mechanical finite element models of the press- and shrink-fit connections are set up to investigate the resulting contact force as well as the mechanical stress distribution in the contact area. Findings are compared to other common connections, e.g. joints with busbars. As a result, specific recommendations for construction are described and the joining technologies are compared with conventional plug-in connections.

The impact of short circuits on contact elements in high power applications

Electrical equipment has to carry the rated normal current and short time withstand current. As shown in previous publications, the voltage-temperature relation allows calculating the temperature of contact elements and connectors for steady- state operating points. In the case of transient electric-thermal load, a finite-element analysis (FEA) is required to calculate the time-dependent distribution of the temperature. This FEA has to be verified for transient load. Therefore a test-environment has been built in which contact elements can be loaded with AC short- circuits. The temperature of the contact system can be measured via infrared thermography during the transient event, so the calculation can be validated quantitatively. Additionally, the benchmark parameters joint resistance and contact force before and after short circuit are discussed. Measurements show that the contact materials soften due to the transient heating. This reduces the electric and thermal contact resistances and affects heat generation as well as heat distribution. Hence, the softening of contact materials was implemented in the FEA. This publication describes the enhanced and validated FEA for transient thermal stress of a high power contact system with contact elements. The performance of the contact elements during and after short circuits are discussed theoretically, via calculation and compared to measurements.