Functions
2.9 Ground Fault Protection 50N(s), 51N(s)
SIPROTEC, 7SJ61, Manual
C53000-G1140-C210-1, Release date 02.2008
151
Logarithmic Inverse characteristic (Inverse Time)
The logarithmic inverse characteristic (see Figure 2-51) is set in parameters 3119 51Ns PICKUP, 3141 51Ns
Tmax, 3140 51Ns Tmin, 3142 51Ns TIME DIAL and 3143 51Ns Startpoint. 51Ns Tmin and 51Ns
Tmax define the tripping time range. The slope of the curve is defined by 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 (referred 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 go towards infinity, but has a finite time value.
Figure 2-51 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 (see figure 2-52) is set with the 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 tripping time range with 51Ns T max of the current threshold
51Ns PICKUP and 51
Ns T min of the current threshold 51Ns I T min assigned. When configuring the
knee point time 51Ns T knee, the tripping time is determined at transition of the two curve sections with dif-
ferent slopes. The point of transition is defined by the current threshold 51Ns I T knee. 51Ns PICKUP rep-
resents the minimum pickup limit for the ground fault pickup current of the overcurrent element. After a
maximum secondary current of 1.4 A has been reached, the tripping time assumes a constant value. Parameter
51Ns TD serves as a time multiplier for multiplication of the tripping time.
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