Bus Differential Settings; Description - GE b30 Instruction Manual

CHAPTER 8: APPLICATION OF SETTINGS
Table 8-8: External fault calculations on C-5
CT I (kA)
FAULT
CT-1 0
CT-2 0
CT-3 6.0
CT-4 5.0
CT-5 11.0
CT-7, CT-8 11.0

8.5 Bus differential settings

8.5.1 Description

Taking the previous analysis from this chapter into account, the settings have been calculated as follows.
Table 8-9: North bus differential protection settings
Setting Value
PICKUP 0.1 pu
LOW SLOPE 25%
LOW BPNT 1.79 pu
HIGH SLOPE 60%
HIGH BPNT 8.96
HIGH SET 5.94
Table 8-10: South bus differential protection settings
Setting Value
PICKUP 0.1 pu
LOW SLOPE 25%
LOW BPNT 1.83 pu
HIGH SLOPE 60%
HIGH BPNT 9.13 pu
B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
I (A sec) T (ms)
FAULT DC
0.00 N/A
0.00 N/A
25.00 5
25.00 30
55.00 30
45.83 30
Comments
Default value. Lower or higher values can be entered upon security/dependability
requirements. The pu value is for the base of 1200A. This means the actual pickup is 120 A
primary.
Default value. Lower or higher values can be entered upon security/dependability
requirements.
None of the CTs saturate for AC currents below 1.79 pu even with 80% remanence. The DC
component, however, combined with the remanence can saturate some CTs even for currents
below 1.79 pu. The B30 copes with saturation using the current directional principle.
Default value. Lower or higher values can be entered upon security/dependability
requirements. The value of 60% ensures that the differential characteristic alone (without the
directional principle) work correctly under AC saturation of the CTs (26% of spurious
differential during the fault on C-1 saturating CT-1).
None of the CTs saturate for AC currents below 8.96 pu. The DC component, however, can
saturate some CTs even for currents below 8.96 pu. The B30 copes with saturation using the
current directional principle.
The maximum spurious differential current is 2.97 pu. Due to limited accuracy of analysis and
the effect of DC saturation a security factor of 2 has been adopted. The highest internal fault
current is 14kA, or 11.67 pu giving a good chance to clear a number of faults by the unbiased
differential operation.
Comments
Default value. Lower or higher values can be entered upon security/dependability
requirements. The pu value is for the base of 1200A. This means the actual pickup is 120 A
primary.
Default value. Lower or higher values can be entered upon security/dependability
requirements.
None of the CTs saturate for AC currents below 1.83 pu even with 80% remanence. The DC
component, however, combined with the remanence can saturate some CTs even for
currents below 1.83 pu. The B30 copes with saturation using the current directional principle.
Default value. Lower or higher values can be entered upon security/dependability
requirements. The value of 60% ensures that the differential characteristic alone (without the
directional principle) work correctly under AC saturation of the CTs (24% of spurious
differential during the fault on C-2 saturating CT-2).
None of the CTs saturate for AC currents below 9.13 pu. The DC component, however, can
saturate some CTs even for currents below 9.13 pu. The B30 copes with saturation using the
current directional principle.
BUS DIFFERENTIAL SETTINGS
AC saturation DC saturation
No No
No No
No No
No Yes
No Yes
No Yes
t (ms)
SAT
N/A
N/A
N/A
26.37
9.45
11.54
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