Eptra - electric power transmission :: Kilian Schillai
electric power transmission engineering endurance & reliability

Simulation & Testing

The analysis of mechanical and thermal loads as well as the experimental testing of materials and components require specialised know-how, software and elaborate test rigs. Our speciality is the acceleration of long-term damage mechanisms in the laboratory. Benefit from our expertise to optimise your systems and products!

Figure: Characterisation of the fatigue behaviour (failure location and lifetime) by simulation and measurement.

Mechanical simulations are often limited by the underlying material models. Especially the typical conductor and fitting materials have a very low yield point, they often tend to creep even at room temperature and can lose strength in unfavourable operating conditions (e.g. due to recovery). We are specialised in the characterisation of such material properties and the modelling to make these data usable for the design.

Finite element simulations have become indispensable in mechanical and thermal design. The correct modelling of complex contact conditions and highly linear material behaviour in particular, however, remains a challenge. Yet simulations allow for a much more efficient design. Many of the relevant parameters cannot be tested experimentally or must be separated from other parameters and the influence of different test conditions. In design, there are usually too many parameters to investigate each one individually in elaborate component tests, and testing the long-term reliability with component tests is also very demanding.

Despite all the progress in design, the validation of simulation results with component tests is still indispensable. However, the experimental effort is much lower compared to the traditional test- and experience-based design, because the main focus is on the validation of the simulation results and the final design.
Such component tests are usually special set-ups, with high requirements for actuators, control systems, mechanics and metrology. However, they are essential in order to know the limits of the simulations.

Electrical contact resistance and fracture pattern cannot be reliably determined with simulations even today. However, the underlying mechanisms only occur during operation over the course of many years, so they must be accelerated in the laboratory for meaningful component tests. Some of the usual component tests, e.g. thermal cycles for type testing, are not very meaningful for the long-term behaviour of components. Component tests therefore also require material models in some cases in order to accurately represent the long-term behaviour.


Material Properties

The experimental characterisation and modelling of material properties is one of our core competences. Especially for electrical applications, highly non-linear material properties such as temperature-dependent plastic deformation and strength, fretting fatigue, annealing and creep often play an important role.
For simulation, modelling is always an essential part of material testing.

Figure: Effect of residual stresses from straightening of wires on the stress-strain diagram as well as the dependence of the measurement result on the measurement location for insufficiently straightened specimens (simulation).

If no standardised tests are available for your damage mechanisms, we will develop suitable test stands for material and component properties for you. In this way, we also support the rapid and reliable expansion of your own test capacities.

Often the accuracy of simulations is limited by the underlying material properties. The measurement of material properties is essential for reliable results. Material models can reduce the inherent scatter of experimental data and allow interpolation between experimental data.

Figure: Optical strain measurement on an aluminium wire (left), and measurement with strain gauges (full bridge with bending and temperature compensation, right).

Accelerated Life Testing

Long-term damage mechanisms that in service often take place over many decades must be accelerated in the laboratory, both for component testing and for the determination of material properties. Grid infrastructure must function reliably over a much longer period of time than can be tested in the laboratory.

Figure: Fatigue test on an overhead line fitting (left) and investigation of oil penetration behaviour in hard fabric (cotton-phenolic resin) with fluorescence microscopy (right).

Models and experimental techniques to accelerate the damage are therefore often essential. For fatigue, for example, it is well established that there is a negligible frequency dependency for metallic materials at low frequencies. For the description of creep and softening behaviour, time-temperature parameters, and thus ultimately higher temperatures, are often used; for the swelling behaviour of plastics, higher humidities are used. The exact procedure therefore differs greatly depending on the damage mechanism.
This approach, known as accelerated life testing, allows the robustness and reliability of products to be tested more quickly and with relatively few components by increasing the load beyond normal operating conditions.

Metrology for determining the damage progress and checking the damage pattern are essential for the validity of the tests. Similar issues arise in asset management. Damages are often due to insufficient information about the condition and properties of aged assets. In order to prevent such failures, we are researching new measuring methods for the condition assessment in asset management. Due to the high investment sums, such measurement techniques have a great influence on the economic efficiency of assets.