Predicting operating times, minimising downtimes
After which operating time should insulators be replaced? Is it possible to replace them together with other components in order to keep the shutdown periods and costs to a minimum and yet nevertheless use the components in an efficient and safe manner? Are there ways to conduct inspections or special tests that increase the guaranteed service life of the insulators? These questions can be answered with a service life forecast based on fracture mechanics.
The main objective of the project is to develop a material-based forecast of the service life for extra-high-voltage porcelain insulators. To this end, the partners from Lapp Insulators, SAG and the Karlsruhe Institute of Technology have formed a team with the grid operators TenneT, 50Hertz and Amprion in order to develop the material and mechanical principles and build a simulation tool based on fracture mechanics and the finite element method (FEM). The developers can then use this tool for evaluating the existing stock and optimising the operating strategy for the overhead transmission lines and substations.
Fracture mechanics is a method that can be used to calculate the formation and propagation of cracks resulting from inevitable initial defects. If the externally applied stresses are large enough so that the so-called fracture toughness is exceeded, the component fails within fractions of a second ("spontaneous breakage"). However, so-called "sub-critical crack growth" can occur below the fracture toughness level. These are cracks that grow until the remaining cross-section can no longer withstand the loads and fails through spontaneous breakage.
The basic laws of sub-critical crack growth have already been known for about 40 years for other ceramics and glasses. New, however, is their application to large components such as insulators. Insulators also generally receive a glaze that causes compressive residual stresses in the surface. Any surface defects are pressed together so they do not usually cause initial defects for the component failure. However, their impact on the sub-critical crack growth is still not known.
Investigations of old and new materials
In a first step, the developers plan to salvage old insulators from high-voltage lines belonging to the grid operators involved and investigate their residual strength in component tests. A detailed analysis of the fracture surfaces makes it possible to determine the crack growth history. Which defect triggered the failure? To what extent has an originally existing defect grown sub-critically?
Furthermore, the constitutive law for the ageing of porcelain based on the fracture mechanics shall be determined for new porcelain. For this the engineers need material samples that are subjected to stresses for longer periods with specific forces below the spontaneous breaking strength. These forces will then lead to time-delayed failure. The correlation between the downtime and load enables the crack growth rate to be calculated.
Parallel to this, the forces that actually act on insulators in the electricity pylons are of interest. A year-long measurement programme is intended to reveal information about their behaviour. For this purpose the project team are selecting representative locations for the snow and wind loads occurring in Germany.
To forecast the service life, a standard software package is being extended to cover "sub-critical crack growth". Based on an assumed defect resulting from the acceptance criteria for the insulators, the sub-critical crack growth can now be extrapolated by applying the determined load. This occurs until a predefined failure criterion is achieved (deterministic method). Alternatively, a statistical error distribution resulting from routine testing of new insulators can be extrapolated over the operating time (probabilistic method). In a final step, the detailed material tests and the new assessment methods shall be used for optimising the porcelain composition.
09/2014 – 08/2017
Lapp Insulators GmbH
95632 Wunsiedel, Germany