GE B90 Instruction Manual page 432

SLOPES AND HIGH SET THRESHOLD
CT time-to-saturate (t
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 8-5 to calculate the corresponding K
– Calculate K
– Compare each K
greater than t
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 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 5.02 ms and is detected by the Saturation
Detector.
The transient saturation of the CTs due to the DC component can be neglected when setting the slopes of the
characteristic 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.
8
Table 8-4: External fault calculations on C-1
CT I
FAULT
CT-1 14.0
CT-2 0
CT-3 6.0
CT-4 5.0
CT-6 3.0
CT-7, CT-8 14.0
8-8
Figure 8-5: Determining CT time-to-saturate
) is determined as a projection of the intersection of the K
SAT
, t ...t
1 2 n
using equation 8-7
S_LIM
value with K . If K
S S_LIM S
, otherwise it is less than t
i i
(kA) I (A sec)
FAULT
116.67
0.00
25.00
25.00
15.00
58.33
, K ...K
S1 S2 Sn
at the given time instance t is less than K
i
T (ms) AC saturation
DC
40 Yes
N/A No
5 No
30 No
40 No
40 No
B90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
CHAPTER 8: APPLICATION OF SETTINGS
CT saturation capability curve and
s
, it means that true t
S_LIM
DC saturation t (ms)
SAT
Yes 5.02
No N/A
No N/A
Yes 26.37
Yes 61.50
Yes 9.44
is
SAT