Development of a toolbox for assessing Frost and Rime ice impact on overhead Transmission Lines

The design of power lines exposed mountain terrain in Norway is challenging. The knowledge about many important processes related to the ice accretion and its different effects on line components is limited. Experiences with damages on power lines that were caused by atmospheric icing during the last years have illustrated the need to improve the knowledge of atmospheric icing and to develop calculation tools to give better estimates of climatic loads on power lines to avoid future damages. The underlying idea of this project has been to develop a toolbox that will combine state-of-the art methods and models to calculate the impacts from frost and rime ice on overhead transmission lines. The first objective was to develop models of ice accretion on power lines, to be applied both for design purposes and for monitoring lines under operation, including forecasting of potentially critical events. Every winter the generation of hoar frost on conductors causes significant energy loss in the transmission lines. The second objective of the project was to develop a pilot forecasting system for hoar frost and corona losses which aimed to reduce the power transmission losses. Three measurement stations for icing have been established in relation to this project. The stations have been in operation for 3-4 years and contain ice load sensors in the power lines and test-spans as well as meteorological sensors, web-cameras and an ice load sensor designed according to the ISO12494. The stations are located at elevated and exposed sites with harsh environmental conditions during the winter. This has given some challenges regarding availability and maintenance, but we have during the measurement period gathered unique and necessary data for the further development of numerical models for icing on power lines. An instrument for the measurement of icing, which is the first instrument constructed according to the ISO12494, has been developed in this project and has been installed at two of the stations. With this instrument it is possible to derive icing rate and cloud liquid water content during the winter season. With this data we are closer to understand the icing processes and to validate some of the parameters from our weather prediction model. Simulations with high resolution WRF simulations (333m and 500m) have been carried out for some areas in Norway. Methods have been developed in order to combine these relatively short model simulations (1-2 year time series) with longer term coarser model simulations (39 years) in order to achieve modeled data which are representative for both high resolution and for long time series. These methods have been important in order to more correctly simulate the climatic conditions for the power lines. Studies have also been carried out in order to reach the most optimal setup of the WRF model to be used for simulations of icing situations. This will be used for the generation of a new long term hindcast reference data set to be used in all future icing studies in the Nordic countries. A selection of wind tunnel experiments has been conducted at VTT in Finland. This wind tunnel allows for the inclusion of super cooled cloud droplets in order to study how the ice will build up under as realistic icing conditions as possible. The experiments have shown that there are limitations to the theoretical calculation models that we use today in the ISO12494 and that these conditions will be relatively common for power line icing. With the aid of CFD calculations we are today closer at understanding these limitations and to develop new analytical formulations to describe the ice buildup under such conditions. A new method based on these experiences has been implemented and will be used for the calculation of design loads for new power lines. Models have been developed to estimate the influence of hoar frost on power lines and the consecutive energy loss caused by hoar frost corona. Data on the voltage and current from transformation stations for two power lines have been studied. One line is a 420 kV line of 115 km, the other a 300kV line of 142 km. A method has been developed to indentify periods of corona losses corresponding to hoar frost situations on the power lines and to estimate the magnitude of the losses along the lines. The periods have been validated with observed meteorological conditions from met-stations and WRF model data. We have implemented a hoar frost model which are coupled with operational forecasts from the Meteorological Institute. The studies show that the forecasts successfully predicted 86% of the observed corona loss episodes.

1. Det er lagt grunnlag for utvikling av instrument for målinger av is på i kraftlinjer og dannet grunnlag for videre samarbeid med Statnett om å utvikle et varslingsystem til operasjonell bruk. 2. Økt fokus på isingsproblematikken hos Statnett og finansiering av nytt Fou prosjekt for å videreføre resultatene (IceBox) 3. Det er utviklet instrument for måling av ising som følger ISO12494. 4. Prosjektet er et viktig referanseprosjekt for KVT har bidratt til økt anerkjennelse i bransjen og bidratt til økt oppdragsmengde for Kjeller Vindteknikk 5. Økt oppdragsmengde hos KVT har medført at staben har økt fra 24 (2015) til 30 (2018). 6. Prosjektet har bidratt til økt anerkjennelse internasjonalt og Kjeller Vindteknikk har fått oppgaven med å lede arbeidet med ny Eurocode på ising på strukturer. 7. Prosjektet har bidratt til en kunnskapsøkning på meteorologi og ising for alle deltakerne i prosjektet.

The design of power lines exposed mountain terrain in Norway is challenging. The knowledge about many important processes related to the ice accretion and its different effects on line components is limited. Experiences with damages on power lines during the 2013/2014 winter season have illustrated the need to improve the knowledge of atmospheric icing, to develop calculation tools to give better estimates of climatic loads on power lines and to verify the modelling and calculations to avoid future damages. In addition, the generation of hoar frost on conductors cause significant energy loss every winter due to corona discharges. This issue requires update of knowledge, modelling and verification by testing. The underlying idea of this project is to develop a toolbox that will combine state-of-the art methods and models to calculate the impacts from frost and rime ice on overhead transmission lines. The first objective is to the models for ice accretion on power lines, to be applied both for design purposes and for monitoring lines under operation, including forecasting of potentially critical events. The second objective is to develop a pilot forecasting system for hoar frost and corona losses which aim to reduce the power transmission losses. To achieve the required development it will be essential to carry out field measurements of key meteorological parameters at relevant sites and to carry out laboratory experiments to better understand and describe the ice accretion processes for large ice loads. The measurements will in part be used to validate the models and assumptions used in the models and partly to develop new formulations of ice accretion which is expected to move the research front forward. The partners in the project are composed experienced and leading scientists, institutions and companies which by the combined effort are expected to move the research front forward.