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

Eptra Engineering

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<


Simulation & Testing

The mechanical and thermal design, optimisation and testing of components for electrical power transmission requires specialist knowledge, software and elaborate test benches. Benefit from our expertise in this field to optimise your products.

>further information<

Condition Assessment & Failure Analysis

As an independent body, we determine the cause and course of damage, other vulnerable locations and components, as well as measures for damage prevention.

>further information<

Product Development

With our niche expertise, we make your products more reliable and accelerate their time to market. Depending on your needs, we will handle the product development from the concept to the fully designed and tested prototype.

>further information<

About us

After many years in research on the ageing of energy infrastructure at the Swiss Federal Laboratories for Materials Science and Technology (Empa) and ETH Zurich, I now support the practical improvement of reliability issues, especially in electrical power transmission. The focus of my work is on testing and optimisation of mechanical, thermal and electrical aspects in simulation and measurement.
As a small business, we can adapt flexibly to your needs and are open to a broad variety of projects in terms of duration, type and scope.

For a one-on-one conversation, I am available to you via phone (+41 76 652 5116) or email (schillai@eptra.ch). I look forwards to hearing from you.

Simulation & Testing Examples

Material Properties & Modelling

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.

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.

>further information<

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.

Measuring techniques for monitoring the damage progress and checking the damage pattern are essential for the validity of the tests.

>further information<