GE B90 Instruction Manual page 305

10 APPLICATION OF SETTINGS
where
I is the maximum CT primary fault current
p
The figure illustrates the K CT saturation capability curve and K
s
CT time-to-saturate (t ) is determined as a projection of the intersection of the K
SAT
K CT limiting factor, as shown in the figure.
S_LIM
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 > B90 Low Impedance Bus Differential System. Enter the required system and
CT parameters to obtain the CT time-to-saturate.
• Method 2
•Define several time instances t
•Use equation 10-5 to calculate the corresponding K
•Calculate K using equation 10-7
S_LIM
•Compare each K value with K
S
greater than t , 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 10–4: EXTERNAL FAULT CALCULATIONS ON C-1
CT I (KA)
FAULT
CT-1 14.0
CT-2 0
CT-3 6.0
CT-4 5.0
GE Multilin
Figure 10–5: DETERMINING CT TIME-TO-SATURATE
, t ...t
1 2 n
, K
S1 S2
. If K at the given time instance t
S_LIM S
i
I (A SEC) T (MS)
FAULT DC
116.67 40
0.00 N/A
25.00 5
25.00 30
B90 Low Impedance Bus Differential System
10.4 SLOPES AND HIGH SET THRESHOLD
limiting factor.
S_LIM
CT saturation capability curve and
s
...K
Sn
is less than K
i S_LIM
AC DC
SATURATION SATURATION
Yes Yes
No No
No No
No Yes
, it means that true t is
SAT
T (MS)
SAT
5.02
N/A
N/A
26.37
10-7
10