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.
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.