GE MiCOM P141 Agile - Setting Guidelines for High Impedance Operation; Figure 100: Hi-Z REF Protection for Autotransformer Configuration

a
b
c
A
B
C
CT1
CT
2
Varistor
Rst
TN
1 CT
CTN
V0
0686
Figure 100: Hi-Z REF Protection for autotransformer configuration
4.3.2 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 pr
ovide a primar
y 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
power transformer.
Chapter 7 - Restricted Earth Fault Protection P14x
178 P14xEd1-TM-EN-1