Basler BE1-CDS220, Instruction Manual

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D-8 Settings Calculations BE1-CDS220
•
Quality of the CTs (If the saturation factor calculated previously is greater than 0.5, additional margin
is recommended. For SF<0.5, we assume that the CTs perform within the 10% limit defined in the
ANSI Accuracy Class. When accounting for CT error, the total CT error should be 10%
∗
(n-1) where n
is the number of restraint current inputs (2 for two restraint relay, 3 for three restraint relay). If more
than one CT is nearing the performance limit, the errors tend to cancel.
•
Relay measuring errors. A total of 4% applies to the BE1-CDS220 (2% per input circuit).
The following errors are not included in the S
i
operating slope value (these sources of mismatch do not
vary with through current so they affect the minpu setting only).
•
Transformer excitation current IE (assumed to be less than 4% of the self cooled rating). This
mismatch does not vary with through current (load) so it tends to not add to the slope mismatch line.
It will offset the slope characteristic from the origin in the operate direction on the operate versus
restraint characteristic (see Figure D-2).
•
Unmonitored loads “Iunmon” such as station service or small capacitor banks in the differential zone
add to the constant excitation current.
When the saturation factor exceeds 0.5 on any of the CTs, CT saturation is likely. For this condition, the
BE1-CDS220 improves security by delaying restrained tripping by two cycles when the transient monitor
function detects operate (differential) current that is a result of CT saturation. For applications where the
saturation factor is greater than 0.5, additional slope margin is recommended. For applications where the
saturation factor exceeds 1.0, severe distortion is likely at high fault current and the maximum slope
setting (60%) is recommended. For more information see Setting Note 4.
In the example, the tap factors were calculated on system nominal ratings so the no load tap changer
position is included as a source of mismatch.
( ) ( ) ( ) ( )
% 29 Re % 4 % 10 % 10 % 5
= + + + =
lay CT LTC NLTC S
i
Equation D-16
AC AB M
o
− =






+ =
offset o
I
S
M
Minpu S
- Minpu
i
offset o
I M
− 





=
S
S
- 1 Minpu
i
Figure D-2. Slope and Operating Margin