Abstract
An undetected stator ground fault on a large turbine generator can cause millions of dollars in damage and a larger amount of lost operating revenue during the time the generator is out of service being repaired. As Minnesota Power has been upgrading the protection on their generating units the grounding has been being upgraded from low impedance grounding to high impedance grounding. 100% stator ground protection has been being implemented. From a past project of relay upgrades at Minnesota Power's Laskin Energy Center the two generators do not have 100% stator ground fault detection. Due to the design of the generators, the use of third harmonic voltage to detect the stator ground fault is not reliable. When it was time to upgrade Minnesota Power's Hibbard Energy Center Unit 3 and Unit 4 generators, which are similar in design and size as the units at Laskin Energy Center, it was decided that different options needed to be explored to achieve 100% stator ground protection.
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