Analysis of an Autotransformer Restricted Earth Fault Application

Restricted earth fault, or zero-sequence differential protection, is beneficial in transformer applications. Because it does not respond to load current, it offers a significant improvement in sensitivity over traditional differential protection. Ground current in the transformer neutral is used as a reference and is compared to zero-sequence current at the terminals to determine if a fault is internal to the transformer. The predictability of the neutral current phase angle is critical to it being a stable reference. It is not well understood how the magnitude and direction of circulating zero-sequence current in a delta tertiary relates to the zero-sequence current in the autotransformer neutral. This technical paper derives that relationship, explains restricted earth fault protection, and uses a real- world, unexpected relay operation to demonstrate these concepts and make relay settings recommendations.