Hydrogenerator Stator Winding Insulation Assessment

Hydroelectric generators generate almost all electricity in Norway. The generator converts together with the turbine the energy in the water into electric energy. A critical part of these generators is the electric insulation around the copper windings where the electric current is generated. If the insulation fails, there is an internal short circuit in the generator and it must be stopped for repair, causing long outage and costly repairs. As the generators becomes older, the insulation undergoes an ageing process. This means deterioration and change in some of the properties and parameters important for the capacity of the materials. A large proportion of the generators are over 40 years and new operations schemes increase the need to have good methods for estimating the condition and predicting the future life of isolation and how this is affected by the operation. This allows for good scheduling of planned repair or replacement of the winding. The project started in August 2016. Through the project-partners NTNU and SINTEF have got several windings after they have been taken out of service. In 2018-20, test objects with known faults have been made in addition and studied in detail. Important in this work is the relationship between the size of defects in the electrical insulation, and the variation in temperature and the frequency of the voltage used during condition assessment tests. The validity of several theoretical models of fault response has been compared to measurements of relevant electrical insulation for generators. Of special value for the project is that Norsk Hydro has given access to a generator in Suldal power plant prior to replacement. SINTEF has conducted a large measurement program autumn 2017 while the machine was in operation. After the replacement, the old windings (160 bars) have been shipped to SINTEF's laboratory in Trondheim. In 2020 several condition monitoring methods have been used to compare the condition between the old generator windings and old spare windings that have not been in service or new windings made for the project- The windings have been heat cycled and tested at different temperatures. In 2021 the windings were tested until breakdown to identify the true strength of the windings and how heat cycling degrades the windings. The main conclusions are that the insulation is degraded little after 50 years of operation and that tough heat cycling did not reduce the breakdown strength significantly. During the heat cycle, increased partial discharge activity was observed when the temperature increased og decreased fast. Diagnostic methods at low frequency give similar values as for 50 Hz when the windings temperature is moderate. The project has coordinated with the FME-Center HydroCen (see https://www.ntnu.no/hydrocen) that started in 2017.

- Faktisk tilstand til isolasjonstypen benyttet i de største generatorene fra midten av 1960-tallet (VPI) er god, selv etter 50 års drift. Dette resultatet kombinert med akselerert aldring av prøveobjekter i laboratoriet viser at viklingen vil tåle mer intermitterende drift enn i dag, men tålegrensen/omfanget er ikke mulig å tallfeste. Det er sannsynlig at en tilstandsdegradering vil finne sted i spolehodet og dette bør overvåkes. Virkningen av disse funnene er at norsk vannkraftgeneratorer kan driftes mer dynamisk enn fryktet og vil kunne bidra til innfasing av mer ikke-regulerbar fornybar energi. - En mer praktisk tilstandskontrollmetode og måling av elektriske delutladninger (PD) ved lavere påtrykt frekvens og temperatur enn i drift har vært testet ut. Sammen med modellering i et PhD-arbeid viser resultatene at PD-måling gir kan gjennomføres ved lavere frekvens og temperatur enn ved normal drift. Metoden og kunnskapen bak vil redusere kostnaden og forenkle disse tilstandskontrollmålinger av både vannkraftgeneratorer og andre elektriske maskiner, og virkningen vil være enda høyere driftssikkerhet og lavere vedlikeholdskostnad for kraftselskaper. - Det er utviklet laboratorieoppsett for videre tester og kunnskapsutvikling om tilstanden til elektrisk isolasjon i roterende maskiner. Dette gjelder både for vannkraftgeneratorer og andre maskiner (motor/generator) i industri og maritim bruk, og vil kunne bidra til utvikling av bedre og billigere elektriske maskiner som er en viktig del av elektrifisering og mindre CO2-utslipp. - Prosjektet har gitt bedre, grunnleggende forskningsbasert forståelse av tilstand og tilstandsovervåkning til isolasjon i elektriske maskiner. Denne kunnskapen kan overføres til nye domener hvor tilstandsprediksjon og pålitelighet er kritisk, blant annet lavutslipps-luftfart hvor utvikling av nye elektriske maskiner er en vesentlig oppgave.

Stator winding insulation is the part of the hydrogenerator experiencing the highest number and the most damaging failures followed by long down times, costly repairs and large economic losses due to interrupted power production. In addition, changing operation patterns from base load to intermittent operation for the Norwegian ageing hydrogenerator population, have created a need for knowledge of the actual condition of the insulation. This calls for research on new condition monitoring methods. This project will accommodate and evaluate promising variable frequency methods currently adopted by other high voltage apparatuses such as transformers and cables. These methods are likely to provide more valuable diagnostic information of the stator winding insulation condition compared to other methods in use today. Hydrogenerator stator bars from storage or service as well as model bars will be included in the project. The measurements will be related to actual failure mechanisms and classified in terms of condition states. The condition state of stator winding insulation is essential in making correct maintenance and renewal decisions. In this work, financial support from the Norwegian Research Council is essential to establish fruitful co-operation between Norwegian and foreign hydroelectric power companies, NTNU, SINTEF Energy Research, Vattenfall, Institut de Recherche d'Hydro-Québec (IREQ), EDF, KTH and the University of Uppsala.