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10.4 SLOPES AND HIGH SET THRESHOLD
10.4SLOPES AND HIGH SET THRESHOLD
To set the higher slope and threshold of the high set (unbiased) differential operation, external faults must be analyzed.
Consider an external fault for the North bus relay. It is justified to assume bus configurations that give maximum stress to
the maximum number of CTs. For this purpose we will assume the tie breaker, B-7 closed; all the circuitry capable of sup-
plying the fault current to be in service; moreover, they are connected to the South bus in order to analyze the CT-7 and CT-
8 carrying the fault current.
The table below presents the results of analysis of an external fault on circuit C-1 (C-1 is connected to the North bus; C-3,
C-4, and C-5 are connected to the South bus).
For security reasons, it has been assumed that the fault current being a sum of several contributors (C-3, C-4, and C-5 in
this case) has a time constant of the DC component of the maximum among the time constants of the contributors. The
fault current is supplied from circuits C-3, C-4, and C-5 connected to the South bus, thus through CT-3, CT-4, and CT-6.
The current passes through the tie breaker threatening saturation of CT-7 and CT-8.
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 earlier), it is concluded that CT-1 will saturate during this fault, producing
a spurious differential signal for the North bus zone differential protection. All other CTs will not saturate due to the AC com-
ponents. The amount of the spurious differential current (magnetizing current of CT-1) can be calculated using the burden,
magnetizing characteristic and primary current of the noted CT by solving the following equations:
For I
= 116.67 A, R
s
ure, the solution is I
The magnetizing current of the saturated CT-1 will appear to the differential element protecting the North bus as a differen-
tial signal of 29.73 A, while the restraint signal will be the maximum of the bus currents (112.8 A in this case). Conse-
quently, the higher slope of the characteristic should not be lower than 29.73 A / 112.8 A, or 26%, and the pick up of the
high set differential elements should not be lower than 29.73 A, or 2.97 pu.
The CTs identified as operating in the linear mode as far as the AC components are considered may, however, saturate due
to the DC components. Saturation will not occur if
If the above condition is violated, saturation will occur but not before:
Columns 6 and 7 of the table below summarize the DC saturation threat for the fault on C-1. CT-4, CT-6, CT-7, and CT-8
may saturate due to the DC components and may generate spurious differential signal for both the North and South bus
relays depending on the bus configuration. The saturation will not occur before 4.7 ms and will be detected by the Satura-
tion Detector.
The transient saturation of the CTs due to the DC component may be neglected when setting the slopes of the characteris-
tic as the saturation will be detected and the relay will use the current directional principle. It must however, be taken into
account when setting the high set (unbiased) differential element.
Table 10–4: EXTERNAL FAULT CALCULATIONS ON C-1
CT
CT-1
CT-2
CT-3
10
CT-4
CT-6
CT-7, CT-8
10-6
I
relay
I
relay
= 1.61 and the characteristic shown earlier in the Approximate CT Magnetizing Characteristics fig-
s
= 29.73 A, I
= 112.8 A.
magnetizing
relay
I
(KA)
I
(A SEC)
FAULT
FAULT
14.0
116.67
0
0.00
6.0
25.00
5.0
25.00
3.0
15.00
14.0
58.33
B90 Low Impedance Bus Differential System
2
2
I
I
=
–
s
magnetizing
R
V
=
s
magnetizing
T
V
I
R
1
+
sat
s
s
dc
T
=
sat
T
(MS)
AC
DC
SATURATION
40
Yes
N/A
No
5
No
30
No
40
No
40
No
10 APPLICATION OF SETTINGS
10.4.1 DESCRIPTION
10.4.2 EXTERNAL FAULTS ON C-1
, where is radian system frequency (2f).
1
V
I
R
–
sat
s
s
T
ln
1
--------------------------------------- -
–
–
dc
T
dc
DC
T
SATURATION
Yes
No
No
Yes
Yes
Yes
(EQ 10.4)
(EQ 10.5)
(MS)
SAT
N/A
N/A
N/A
15.19
35.25
4.70
GE Multilin
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