Siemens SIPROTEC 7SJ62 Manual page 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
Logarithmic inverse
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.
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
Trip time characteristics of inverse time ground fault protection with logarithmic inverse charac-
teristic
t = 51Ns MAX. TIME DIAL - 51Ns TIME DIAL·ln(I/51Ns PICKUP)
2.12 Ground Fault Protection 64, 67N(s), 50N(s), 51N(s)
Functions
233