Chapter 7 - Restricted Earth Fault Protection
Figure 100: Hi-Z REF Protection for autotransformer configuration
SETTING GUIDELINES FOR HIGH IMPEDANCE OPERATION
This scheme is very sensitive and can protect against low levels of fault current in resistance grounded systems. In
this application, the IREF>Is settings should be chosen to provide a primary operating current less than 10-25% of
the minimum earth fault level.
This scheme can also be used in a solidly grounded system. In this application, the IREF>Is settings should be
chosen to provide a primary operating current between 10% and 60 % of the winding rated current.
The following diagram shows the application of a high impedance REF element to protect the LV winding of a