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Ozone emission in hydro generators is a sign of surface partial discharge (PD) in the stator windings. Surface PD activity can take different forms: slot PD, corona at the junction of the slot semiconductive coating with the voltage grading coating outside the slot, PD between adjacent coils in the end-winding, PD between adjacent connections and surface tracking. An ozone concentration above the daily exposure limit, for which there is a health hazard for workers, was found at several locations in a 6-hydro generator plant. The ozone level in the vicinity of the generators was in the same range for all of them. This case study presents how this problem was identified and addressed.

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