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noted that irrespective of the main CT connections (wye or delta) on-line reading and
automatic compensation for actual load tap changer position can be used in the IED.
Typical main CT connections for transformer differential protection
Three most typical main CT connections used for transformer differential protection
are shown in figure
26. It is assumed that the primary phase sequence is A-B-C.
A
IA
I
A
-
I
B
I
B
-
I
C
I
C
-
I
A
I
A
-
I
C
I
B
-
I
A
I
C
-
I
B
I
A
I
B
I
C
Protected
Transformer
Winding
CT Wye
Connected
CT in Delta
DAC Connected
CT in Delta
DAB Connected
IB
IC
B
C
en06000549_ansi.vsd
ANSI06000549 V1 EN
Figure 26: Commonly used main CT connections for Transformer differential protection.
For wye connected main CTs, secondary currents fed to the IED:
• are directly proportional to the measured primary currents
• are in phase with the measured primary currents
• contain all sequence components including zero sequence current component
For wye connected main CTs, the main CT ratio shall be set as it is in actual
application. The “WyePoint” parameter, for the particular wye connection shown in
figure
26, shall be set ToObject. If wye connected main CTs have their wye point away
from the protected transformer this parameter should be set FromObject.
For delta DAC connected main CTs, secondary currents fed to the IED:
1MRK504116-UUS C Section 3
IED application
109
Application manual