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Application and Setting Notes
Parameter:
Gn 87T Common > Wn ICT Multiplier
•
Default setting:
1x
(1 ⋅)
The current transformer primary ratings cannot be matched exactly to the rated currents of the protected
object so internal ratio correction current multiplying factors are applied to the CT secondary currents
flowing into the relay.
For load or through fault conditions the output of the
ICT Multiplier
for each winding must be
equal, notwithstanding variations in the OLTC position. Where possible the output of each
ICT Multi-
plier
is set to 1 A at transformer full load rating when the transformer OLTC is on its mid-tap position.
At mid-tap a balanced relay should have virtually no differential currents. The bias currents will vary with
the load level. The multipliers are used to adjust the CT secondary currents to accommodate for any
mismatch between the winding 1 and winding 2 CT ratios. The multiplier is used to correct any CT ratio
mismatch so that p.u. current (ICTOUT = 1 p.u.) is applied to the biased differential algorithm. Balancing
ICT Multiplier
outputs (ICTOUT) to 1 A at transformer rating ensures that the relay operates at the
levels indicated by its differential protection settings.
i
i
NOTE
Achieving balance at ICTOUT = 1 is not always possible; here the effects on settings must be taken into
account. The effect of applying ICTOUT < 1 is to de-sensitize current dependent differential settings,
applying ICTOUT > 1 makes the effected elements more sensitive. To compensate for the resultant ICTOUT
value the following settings must be multiplied by ICTOUT:
87HS Setting
87BD Pickup
87BD S1L
A single reference rated power is nominated for all inputs (S
rated
). The power of the highest rated winding is
used as the reference rated power.
Parameter:
Gn 87T Common > Wn ICT Connection
•
Default setting: Yy0
Applies the correct vector compensation to the line CT input currents. The nominal current ratio of the
virtual interposing CT is 1:1. Note that where Yd settings are applied some current distributions will result
in a √3 multiplying factor being applied. See ‘Applications Guide’.
Internal interposing current transformers are used to correct the CT secondary current phase relation-
ships in line with any phase change introduced by the transformer connections. Vector-group correction
replicates the magnetic coupling of the transformer coils and makes the measured currents comparable.
As a general rule, the phase angle
ICT Connection
setting to correct the phase angle difference is
applied to the star side winding. A table showing the settings to apply for all standard transformer vector
groups is included on the following page. The table assumes that all line CTs are ‘star’ connected.
Note that the choice of interposing CT vector group will modify the effective operating levels of the
protection due to the current distribution for the various fault conditions, the effects on settings must be
taken into account.
Interposing CT Selection Guide – Power Trans-
former Vector Group
W1 ICT Connection
W2 ICT Connection
Yy0, YNy0, Yyn0, YNyn0, Ydy0, Yndy0, Ydyn0,
Yndyn0, Dz0
Ydy0,0°
Ydy0,0°
Yd1, YNd1
Yd1,-30°
Yy0,0°
Yd1, YNd1 + Earthing Transformer
Yd1,-30°
Ydy0,0°
Yy2, YNy2, Yyn2 YNyn2, Ydy2, YNdy2, Ydyn2,
Yndyn2, Dz2
Ydy2,-60*
Ydy0,0°
Yd3, YNd3
Yd3,-90°
Yy0,0°
5.22.4
Protection and Automation Functions
5.22 87T Transformer Differential Protection
Reyrolle 7SR5, Transformer Protection Device, Device Manual
313
C53000-G7040-C015-1, Edition 11.2019
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