GE b30 - Slopes and High Set Threshold

9-8 B30 Bus Differential System GE Multilin
9.4 SLOPES AND HIGH SET THRESHOLD 9 APPLICATION OF SETTINGS
9
where
I
p
is the maximum CT primary fault current
The figure illustrates the K
s
CT saturation capability curve and K
S_LIM
limiting factor.
Figure 9–5: DETERMINING CT TIME-TO-SATURATE
CT time-to-saturate (t
SAT
) is determined as a projection of the intersection of the K
s
CT saturation capability curve and
K
S_LIM
CT limiting factor, as shown in the figure.
Practically, CT time-to-saturate can be obtained by either of the following methods:
• Method 1 — Download the "CT Time-to-Saturate Estimator.xlsm" spreadsheet from the GE Multilin web site, located
under Support > Support Documents > B30 Bus Differential System. Enter the required system and CT parameters to
obtain the CT time-to-saturate.
• Method 2
•Define several time instances t
1
, t
2
...t
n
•Use equation 9-5 to calculate the corresponding K
S1
, K
S2
…K
Sn
•Calculate K
S_LIM
using equation 9-7
•Compare each K
S
value with K
S_LIM
. If K
S
at the given time instance t
i
is less than K
S_LIM
, it means that true t
SAT
is
greater than t
i
, otherwise it is less than t
i
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 5.02 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 is detected and the relay uses the current directional principle. It must, however, be taken into account
when setting the high set (unbiased) differential element.
Table 9–4: EXTERNAL FAULT CALCULATIONS ON C-1
CT I
FAULT
(KA) I
FAULT
(A SEC) T
DC
(MS) AC
SATURATION
DC
SATURATION
T
SAT
(MS)
CT-1 14.0 116.67 40 Yes Yes 5.02
CT-2 0 0.00 N/A No No N/A
CT-3 6.0 25.00 5 No No N/A
CT-4 5.0 25.00 30 No Yes 26.37