GE b30, Инструкция по эксплуатации

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8-8 B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
SLOPES AND HIGH SET THRESHOLD CHAPTER 8: APPLICATION OF SETTINGS
8
where
T
1
is a primary system time constant
T
2
is the secondary CT time constant, which can be estimated by the following equation:
Eq. 8-6
where
N is the CT ratio
V
s
is the CT voltage at 10 A exciting current obtained from the CT excitation curve
CT limiting factor K
S_LIM
is defined by the following equation:
Eq. 8-7
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 8-5: 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 8-5 to calculate the corresponding K
S1
, K
S2
…K
Sn
– Calculate K
S_LIM
using equation 8-7
– 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