Hydrogen leakage due to stress corrosion cracking of stator conductors in 210 MW generator: a case study

This paper presents a case study conducted to find out the reasons of rupture/failure at bends of water cooled stator bar in a 210 MW generator. The bars containing hollow and solid subconductors were cleaned to remove the insulation. The failed portions were examined under a scanning electron microscope, optical microscope and the corrosion products by X-ray analysis. The investigation revealed the presence of intergranular brittle cracks, cuprous oxide tarnish film, ammonia, and moisture. All these showed the characteristic features of SCC in copper metal in ammonical solution. The ultimate rupture by SCC of solid and hollow conductors took place in three stages: (1) water leaked from inside the hollow conductor, at the lug braze; (2) the water reacted with the insulation and produced ammonia; and (3) the ammonia caused SCC and led to heavy leakage of water and hydrogen in the generator. Each stage involved in the failure process has been described.