New methods for monitoring neutral grounding resistors

Electrical power systems control arcing current and electrical shock hazard by proper neutral grounding such as high resistance grounding. The high resistance neutral grounding resistors fail due to vibration, intermittent arcs, corrosion, etc. and cause the risk of the system being ungrounded, or solidly grounded. In this paper, two efficient solutions are introduced that provide continuous monitoring of such resistors installed at neutral of two most common configurations of the unit-connected generators. The first proposed method relies on the third harmonic of neutral and residual voltages, and the second technique employs the sub-harmonic injection based generator stator ground protection. The proposed methods show satisfying performance under different conditions of the resistor and generator, observed through comprehensive software analysis and further hardware validations. The first proposed monitoring method has been retrofitted to an industrial generator protection relay which no longer maloperates due to failed-short neutral grounding resistor. Proposed techniques can be incorporated into digital protective relays, which will monitor and alarm in case of the failed grounding resistor.