Incorrect Transformer Differential 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 LV power system grounding, impacts of a solidly grounded generator to the system with a solidly grounded wye transformer winding and how this caused the transformer percentage differential to incorrectly operate. Additional security measures added to the percentage differential are also presented to enhance transformer differential security against temporary system grounding events causing circulating currents, ensuring correct percentage differential operation.