Furthermore, the DC time constant (T
dc
) of the fault current is now smaller, assumed
to be 50 ms here.
Assuming the maximum fault current is 30 percent lower than in the bus fault and a
DC offset 90 percent of the maximum.
Ik
max
= 0.7 × 6 = 4.2 (I
R
)
T
dc
= 50 ms
ω = 100π Hz
T
m
= 10 ms
K
r
= 1/(1-0.4) = 1.6667
Equation 114 with these values gives the result:
F K Ik T e
a r dc
T
m
T
dc
> × × × × × − + ≈
−
max
. ( ( ) )0 9 1 1 24
ω
GUID-46A4591B-876D-4E32-A9AB-29F6C1657644 V2 EN
If the actual burden of the current transformer S
a
in the accuracy limit factor equation
cannot be reduced low enough to provide a sufficient value for F
a
, there are two
alternatives to deal with the situation.
1. A current transformer with a higher rated burden S
n
can be chosen (which also means
a higher rated accurate limit F
n
).
2. A current transformer with a higher nominal primary current I
1n
(but the same rated
burden) can be chosen.
Alternative 2 is more cost-effective and therefore often better, although the sensitivity of
the scheme is slightly reduced.
Example 2
Here the actions according to alternative 2 are taken to improve the actual accuracy limit
factor.
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 579
Technical Manual
To Next Page
Loading...
