Case study: Transformer Differential Incorrect Operation due to System Grounding

Power system grounding is a key component of power system design and can have adverse effects to protection schemes. Particular protection functions that can be impacted are phase, neutral and negative sequence overcurrent, directional overcurrents, restricted ground fault and Transformer differential protection.This paper discusses power system grounding, impacts of an in-zone solidly grounded parallel generator to the system and how this caused the transformer percentage differential to incorrectly operate.Additional security measures added to the percentage differential is also presented to enhance transformer differential security against temporarily system grounding events causing circulating currents.