GE B30 UR Series, Instruction Manual

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GE Multilin B30 Bus Differential System 10-9
10 APPLICATION OF SETTINGS 10.4 SLOPES AND HIGH SET THRESHOLD
10
10.4.3 EXTERNAL FAULTS ON C-2
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
s
= 116.67 A, R
s
= 1.23
Ω
and the characteristic shown earlier in the Approximate CT Magnetizing Characteristics fig-
ure, the solution is I
magnetizing
= 27.69 A, I
relay
= 113.3 A.
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.
10.4.4 EXTERNAL FAULTS ON C-3
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.
CT-6 3.0 15.00 40 No Yes 61.50
CT-7, CT-8 14.0 58.33 40 No Yes 9.44
Table 10–5: EXTERNAL FAULT CALCULATIONS ON C-2
CT I
FAULT
(KA) I
FAULT
(A SEC) T
DC
(MS) AC
SATURATION
DC
SATURATION
T
SAT
(MS)
CT-1 0 0.00 N/A No No N/A
CT-2 14.0 116.67 40 Yes Yes 5.06
CT-3 6.0 25.00 5 No No N/A
CT-4 5.0 25.00 30 No Yes 26.37
CT-6 3.0 15.00 40 No Yes 61.50
CT-7, CT-8 14.0 58.33 40 No Yes 9.44
Table 10–6: CALCULATIONS FOR THE EXTERNAL FAULTS ON C-3
CT I
FAULT
(KA) I
FAULT
(A SEC) T
DC
(MS) AC
SATURATION
DC
SATURATION
T
SAT
(MS)
CT-1 0 0.00 N/A No No N/A
Table 10–4: EXTERNAL FAULT CALCULATIONS ON C-1
CT I
FAULT
(KA) I
FAULT
(A SEC) T
DC
(MS) AC
SATURATION
DC
SATURATION
T
SAT
(MS)