Functions
2.12 Ground Fault Protection 64, 67N(s), 50N(s), 51N(s)
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
233
Logarithmic Inverse characteristic (Inverse Time)
The logarithmic inverse curve is only used for the standard measurement method
cos
ϕ
/ sin
ϕ
(address
130
S.Gnd.F.Dir.Ch
). The curve (see Figure 2-89) is set by parameters
3119
51Ns PICKUP
,
3141
51Ns
Tmax
,
3140
51Ns Tmin
,
3142
51Ns TIME DIAL
and
3143
51Ns Startpoint
.
51Ns Tmin
and
51Ns
Tmax
determine the tripping time range. The slope of the curve is defined in
3142
51Ns TIME DIAL
.
51Ns
PICKUP
is the reference value for all current values with
51Ns Startpoint
representing the beginning of the
curve, i.e. the lower operating range on the current axis (related to
51Ns PICKUP
). This factor is preset to the
value 1.1, analogously to the other inverse time curves. This factor can also be set to 1.0 since in logarithmic
inverse curves the tripping time on a current value, which is identical to the specified pickup threshold, does
not approach infinity, but has a finite time value.
Figure 2-89
Trip time characteristics of inverse time ground fault protection with logarithmic inverse charac-
teristic
Logarithmic inverse
t = 51Ns MAX. TIME DIAL - 51Ns TIME DIAL·ln(
I
/
51Ns PICKUP
)
Note:
For
I
/
51Ns PICKUP
> 35 the time applies for
I
/
51Ns PICKUP
= 35
Logarithmic Inverse characteristic with Knee Point (inverse time)
The logarithmic inverse curve with knee point is only used for the standard measurement method
cos
ϕ
/
sin
ϕ
(address
130
S.Gnd.F.Dir.Ch
) The curve (see Figure 2-90) is set by parameters
3119
51Ns
PICKUP
,
3127
51Ns I T min
,
3128
51Ns I T knee
,
3132
51Ns TD
,
3140
51Ns T min
and
3141
51Ns
T max
.
51Ns T min
and
51Ns T max
determine the range of the tripping time, whereby
51Ns T max
of
current threshold
51Ns PICKUP
and
51Ns T min
of current threshold
51Ns I T min
are assigned. In con-
figuration with knee point
51Ns T knee
, the tripping time is determined at transition of the two curve sections
with different slopes. The transition point is defined by current threshold
51Ns I T knee
.
51Ns PICKUP
represents the minimum pickup limit for the ground fault pickup current of the overcurrent element. As of a
maximum secondary current of 1.4 A, the tripping time takes on a constant value. Parameter
51Ns TD
serves
as a time multiplier for multiplication of the tripping time.
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