External Faults On C-2 - GE B90 Instruction Manual

10.4 SLOPES AND HIGH SET THRESHOLD
Table 10–4: EXTERNAL FAULT CALCULATIONS ON C-1
CT
CT-6
CT-7, CT-8
The following table presents the results of analysis of an external fault on circuit C-2 (C-2 is connected to the North bus; C-
3, C-4 and C-5 are connected to the South bus).
By comparing the secondary currents (column 3 in the following table) with the limits of linear operation for the CTs (column
4 shown earlier in the Limits of Linear Operations of the CTs table) it is concluded that CT-2 will saturate during this fault
producing a spurious differential signal. All other CTs do not saturate due to the AC components. The amount of the spuri-
ous differential current (magnetizing current of CT-2) can be calculated using the burden, magnetizing characteristic, and
the primary current of the said CT.
For I = 116.67 A, R
s
ure, the solution is I
The magnetizing current of the saturated CT-2 appears to the differential element protecting the North bus as a differential
signal of 27.69 A, while the restraint signal is the maximum of the bus currents (113.3 A). Consequently, the higher slope of
the characteristic should not be lower than 27.69 A / 113.3 A, or 24% and the pickup of the high-set differential elements
should not be lower than 27.69 A, or 2.77 pu.
Columns 6 and 7 of the following table summarize the DC saturation threat for the fault on C-2. CT-4, CT-6, CT-7, and CT-
8 can saturate due to the DC components and can generate spurious differential signal for both the North and South bus
relays depending on the bus configuration. The saturation does not occur before 5.06 ms and is detected by the Saturation
Detector.
Table 10–5: EXTERNAL FAULT CALCULATIONS ON C-2
CT
CT-1
CT-2
CT-3
CT-4
CT-6
CT-7, CT-8
The following table presents the results of analysis of an external fault on circuit C-3 (C-3 is connected to the North bus; C-
4 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 table below summarize the DC saturation threat for the fault on C-3. CT-3, 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 23.68 ms and is detected by the Sat-
uration Detector.
Table 10–6: CALCULATIONS FOR THE EXTERNAL FAULTS ON C-3
10
CT
CT-1
10-8
I (KA) I (A SEC)
FAULT FAULT
3.0 15.00
14.0 58.33
= 1.23 Ω and the characteristic shown earlier in the Approximate CT Magnetizing Characteristics fig-
s
= 27.69 A, I = 113.3 A.
magnetizing relay
I (KA) I (A SEC)
FAULT FAULT
0 0.00
14.0 116.67
6.0 25.00
5.0 25.00
3.0 15.00
14.0 58.33
I (KA) I (A SEC)
FAULT FAULT
0 0.00
B90 Low Impedance Bus Differential System
T (MS) AC
DC
SATURATION
40 No
40 No
T (MS) AC
DC
SATURATION
N/A No
40 Yes
5 No
30 No
40 No
40 No
T (MS) AC
DC
SATURATION
N/A No
10 APPLICATION OF SETTINGS
DC T (MS)
SAT
SATURATION
Yes 61.50
Yes 9.44

10.4.3 EXTERNAL FAULTS ON C-2

DC T (MS)
SAT
SATURATION
No N/A
Yes 5.06
No N/A
Yes 26.37
Yes 61.50
Yes 9.44
10.4.4 EXTERNAL FAULTS ON C-3
DC T (MS)
SAT
SATURATION
No N/A
GE Multilin