Ct Grounding Direction - ABB RELION RET670 Applications Manual

Section 7
Differential protection
7.3.2.6
7.3.3
7.3.3.1
182
A
B
C
REFPDIF (87N)
I3PW1CT1
I3PW1CT2
I3P
ANSI05000214 V2 EN-US
Figure 66: Connection of Restricted earth fault, low impedance function REFPDIF
(87N) in multi-breaker arrangements

CT grounding direction

To make the restricted earth fault protection REFPDIF (87N) operate correctly, the
main CTs are always supposed to be wye-connected. The main CT's neutral (star)
formation can be positioned in either way, ToObject or FromObject. However,
internally REFPDIF (87N) always uses reference directions towards the protected
transformers, as shown in Figure 66. Thus the IED always measures the primary
currents on all sides and in the neutral of the power transformer with the same
reference direction towards the power transformer windings.
The grounding can be freely selected for each of the involved current transformers.
Setting guidelines
Setting and configuration
Recommendation for analog inputs
I3P: Neutral point current ( All analog inputs connected as 3Ph groups in ACT).
52 52
52 52
52 52
Transformer protection RET670 2.2 ANSI
1MRK 504 163-UUS A
ANSI05000214-2-en.vsd
M13048-29 v13
IP14942-1 v1
M13052-3 v3
M13052-8 v10
Application manual