ABB RELION 620 Series Technical Manual page 421

1MAC504801-IB E
620 series ANSI
Technical Manual
Furthermore, the DC time constant (T
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 = 0.7 × 6 = 4.2 (I
max
T = 50 ms
dc
ω = 120π Hz
T 8.33 ms
m
K = 1/(1-0.4) = 1.6667
r
Equation 40 with these values gives the result:
F > K ⋅ Ik ⋅ 0 9 . ( ⋅
a r max
GUID-46A4591B-876D-4E32-A9AB-29F6C1657644 V1 EN
If the actual burden of the current transformer S
cannot be reduced low enough to provide a sufficient value for F
alternatives to deal with the situation.
1. A current transformer with a higher rated burden S
a higher rated accurate limit F
2. A current transformer with a higher nominal primary current I
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.
) of the fault current is now smaller, assumed
dc
)
R
−
T
m
T dc
T ⋅ ⋅ − ( 1 e ) + 1 ) ≈ 24
ω
dc
a
).
n
Protection functions
in the accuracy limit factor equation
, there are two
a
can be chosen (which also means
n
(but the same rated
1n
Section 4
415