Bus Differential Settings; Description; External Faults On C-4 - GE B90 Instruction Manual
Low impedance bus differential
system
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BUS DIFFERENTIAL SETTINGS
8.4.5 External faults on C-4
The following table presents the results of analysis of an external fault on circuit C-4 (C-4 is connected to the North bus; C-
3 and C-5 are connected to the South bus).
By comparing the secondary currents (column 3 in the table below) with the limits of linear operation for the CTs (column 4
in the Limits of Linear Operations of the CTs table shown earlier), it is concluded that none of the CTs saturate due to the AC
currents during this fault.
Columns 6 and 7 of the following table summarize the DC saturation threat for the fault on C-4. CT-4, CT-6, CT-7, and CT-8
can saturate due to the DC components and can generate a spurious differential signal for both the North and South bus
relays depending on the bus configuration. The saturation does not occur before 10.03 ms and is detected by the
Saturation Detector.
Table 8-7: External fault calculations on C-4
CT
I
FAULT
CT-1
0
CT-2
0
CT-3
6.0
CT-4
9.0
CT-6
3.0
CT-7, CT-8
9.0
8.4.6 External faults on C-5
The following table presents the results of analysis of an external fault on circuit C-5 (C-5 is connected to the North bus; C-
3 and C-4 are connected to the South bus).
By comparing the secondary currents (column 3 in the table below) with the limits of linear operation for the CTs (column 4
in the Limits of Linear Operations of the CTs table shown earlier), it is concluded that none of the CTs saturate due to the AC
currents during this fault.
Columns 6 and 7 of the following table summarize the DC saturation threat for the fault on C-5. CT-4, CT-5, CT-7, and CT-8
can saturate due to the DC components and can generate a spurious differential signal for both the North and South bus
relays depending on the bus configuration. The saturation does not occur before 9.45 ms and is detected by the Saturation
Detector.
Table 8-8: External fault calculations on C-5
CT
I
FAULT
CT-1
0
CT-2
0
CT-3
6.0
CT-4
5.0
8
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.
8-10
(kA)
I
(A sec)
FAULT
0.00
0.00
25.00
45.00
15.00
37.50
(kA)
I
(A sec)
FAULT
0.00
0.00
25.00
25.00
55.00
45.83
T
(ms)
AC saturation
DC
N/A
No
N/A
No
5
No
40
No
40
No
40
No
T
(ms)
AC saturation
DC
N/A
No
N/A
No
5
No
30
No
30
No
30
No
B90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
CHAPTER 8: APPLICATION OF SETTINGS
DC saturation
t
(ms)
SAT
No
N/A
No
N/A
No
N/A
Yes
10.03
Yes
61.50
Yes
22.23
DC saturation
t
(ms)
SAT
No
N/A
No
N/A
No
N/A
Yes
26.37
Yes
9.45
Yes
11.54
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