ABB RELION RET670 Applications Manual page 554

Section 9
Current protection
548
Single phase-
to- ground fault
ANSI05000493 V3 EN-US
Figure 274: Step 1 fault calculation 1
The fault is located at the borderline between instantaneous and delayed operation of
the line protection, such as Distance protection or line residual overcurrent protection.
This calculation gives the current fed to the protection: 3I
The setting of step 1 can be chosen within the interval shown in equation 433.
× <
3I lowmar I
0fault 2 step1
EQUATION1455 V2 EN-US
Where:
lowmar is a margin to assure selectivity (typical 1.2) and
highmar is a margin to assure fast fault clearance of busbar fault (typical 1.2).
Setting of step 2
The setting of the sensitive step 2 is dependent of the chosen time delay. Often a
relatively long definite time delay or inverse time delay is chosen. The current setting
Three phase CT
summated
3I
0
alt
67N
< ×
3I highmar
0fault1
1MRK 504 163-UUS A
YN/D or YN/Y
transformer
Single CT
ANSI05000493_3_en.vsd
0fault2
(Equation 433)
Transformer protection RET670 2.2 ANSI
Application manual
SEMOD55591-57 v5