Lessons learned from generator event reports

Generators are critical components of the power system. In addition to being some of the most expensive pieces of equipment on the system, they are also extremely critical to power system stability. A false trip can lead to extensive testing in search of a nonexistent fault, while a delayed trip can result in unnecessary additional damage to equipment. Both misoperations result in excessive equipment outages, increasing the utility's outage costs and placing additional strain on the system. Therefore, the misoperation of protective relay schemes at generating stations is very expensive and must be avoided. This paper analyzes real-world event report data in the interest of uncovering valuable lessons for setting and commissioning generator relays. The paper reviews the application of phase differential relays for protection of the generator stator windings against phase faults and the application of third-harmonic voltage elements for 100 percent stator ground protection. Additionally, the effects of negative-sequence currents and overexcitation on a generator are discussed, as well as the importance of detecting and removing both conditions. With event report data, the paper teaches important lessons to be used in applying each function to protect generators. The paper discusses application limitations and considerations for each case.