Setting Guidelines For High Impedance Operation; Figure 84: Hi-Z Ref Protection For A Delta Winding; Figure 85: Hi-Z Ref Protection For Autotransformer Configuration - GE MiCOM P40 Agile Technical Manual

P14D
A
B
C
V00685

Figure 84: Hi-Z REF protection for a delta winding

CTN
V00686

Figure 85: 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 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
power transformer.
P14D-TM-EN-8
1
CT
TN 1 CT
TN 2 CT
TN 3 CT
Varistor
Rst
CT1
Chapter 7 - Restricted Earth Fault Protection
CT2
A
B
C
TN1 CT
Varistor
Rst
a
b
c
a
b
c
157