GE b30 Instruction Manual page 466

SLOPES AND HIGH SET THRESHOLD
where
T is a primary system time constant
1
T is the secondary CT time constant, which can be estimated by the following equation:
2
where
N is the CT ratio
V is the CT voltage at 10 A exciting current obtained from the CT excitation curve
s
CT limiting factor K
where
I is the maximum CT primary fault current
p
The figure illustrates the K
CT time-to-saturate (t
K CT limiting factor, as shown in the figure.
S_LIM
8
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
– Use equation 8-5 to calculate the corresponding K
– Calculate K
– Compare each K
greater than t
8-8
is defined by the following equation:
S_LIM
CT saturation capability curve and K
s
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
, otherwise it is less than t
i
limiting factor.
S_LIM
, K ...K
S1 S2 Sn
at the given time instance t
S i
i
B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
CHAPTER 8: APPLICATION OF SETTINGS
CT saturation capability curve and
s
is less than K , it means that true t
S_LIM
Eq. 8-6
Eq. 8-7
is
SAT