Rotor one-point grounding is one of familiar faults of the turbine generator. This article elaborated and analyzed an incident that A rotor grounding protection device of 1030MW generator alarmed when the generator was running. After check, no grounding points were found in the exterior of the generator, and it was sure that sparks existing in the copper grounding braids of the shaft in the turbine end were the reason for this alarm by a test. This article built mathematic models according to protection devices, elaborated the reason for malfunction of different protection devices, and indicated what should be lay emphasis on in the check when alarm appeared. It will be beneficial to the operation and maintenance of the same-type generator.
