We are specialised in solutions in the area of electrical power transmission, especially in mechanical aspects and long-term reliability. As a spin-off of the Swiss Federal Laboratories for Materials Science and Technology (Empa) and ETH Zürich, we have experience in the modelling and calculation of damage mechanisms of the relevant materials and continue to foster cooperation with the scientific community.

In damage cases, it is essential to determine the root cause in a failure analysis in order to avoid recurring damage. With a combination of damage/fracture pattern analysis, material analysis (mechanical properties, metallography, composition), simulations, and reproduction of the damage in laboratory tests, the cause of damage usually can be determined unambiguously. Our expertise is centred around the particular problems of materials and components for electrical power transmission.

Major parts of the grid are more than 50 years old. The materials, designs, and processes used in the past differ considerably from the current state of technology, so that the application of today's design methods and fittings can lead to damage. With our analytical techniques we help to develop and review procedures for condition assessment and preventive maintenance in the asset management.

Often the damage mechanisms, such as the progressive loss of strength at temperature, the reduction of the clamping force and increased contact resistance due to creep, and fatigue are very long-term effects. In material and especially in component tests (e.g. type tests), modelling the material behaviour is often unavoidable. We are happy to contribute our experience with accelerated ageing in the laboratory and procedures for fault testing (defect screening) in the laboratory to the testing of your components and fittings.

We are equipped for the experimental measurement and the modelling of material parameters as input data for simulations. For example, we are able to test tensile strength and plastic deformation at room and maximum operating temperature, creep and fatigue on single wires as well as whole conductors or components. With our experience in test stand development, especially for component and conductor/cable tests, we also support the rapid and reliable expansion of your own test capacities.

Mechanical and thermal analysis with finite element simulations, modelling and testing are the backbone of our work. With our experience in material modelling, simulation, accelerated ageing and component testing, we are happy to support you in your product development.

Defects in a cast iron power transmission component:

Different power transmission components made of nodular cast iron failed in service, in some cases after many years of operation. Metallographic cross sections revealed different material defects as the cause.

Edge hardness led to glass-like brittle behaviour (cementite needles and ledeburite, left), the coarse graphite and inhomogeneous microstructure lead to lack of strength (right). To prevent further damage, among other things, the material specification and geometry were changed and measurements were introduced to monitor incoming components.

>Further information on failure analysis of electric power transmission components<

Influences on conductor fatigue and limits in current design procedures:

Local finite element model

Global finite element model

In our contribution to the last Cigre conference, we investigated the influence of a variety of geometry, material and load parameters on the fatigue strength of overhead line conductors under wind-excited vibrations and tested the limits of current design methods. Two different types of finite element models were used, a local model to evaluate stresses at failure locations and a global model in which multiple spans were simulated. A key result is the change in local stresses at the failure location (amplitude, mean stress and their combination) relative to a reference case common in Switzerland (AAAC 550 in commercial suspension clamp). The type of suspension of the fittings has a great influence on the fatigue strength. A rotation axis in the centre of the conductor is optimal; the stresses are even lower only if the conductor is unloaded on one side (e.g. at end clamps).

The significantly better fatigue performance of ACSRs (Aluminium Conductors Steel Reinforced, consisting of a steel core and pure aluminium in the outer layers) compared to AAACs (All Aluminium Alloy Conductors, in Europe almost exclusively made of an E-AlMgSi alloy) cannot be explained by different local stresses alone, but the material properties of the aluminium play a crucial role. The core material, on the other hand, has relatively little influence on the local mechanical loads. Because of the inherent scatter, such information can hardly be derived experimentally without simulations.

Current design methods do not take into account important parameters such as the geometry and suspension of fittings, or do so insufficiently. Therefore, it is important that such parameters are tested by the fitting manufacturers.

Original publication: "Limits of vibration amplitude measurement based conductor fatigue design", Cigre E-Session 2020; paper available as PDF and presentation as video.

>Further information on our services for simulations and measurements<