Siemens siprotec SJ62 User Manual page 213

2 Functions
Logarithmic
Inverse
characteristic with
Knee Point (inverse
time)
User Defined
characteristics (In-
verse Time)
211
The logarithmic inverse characteristic with knee point (see figure 2-76) is set by means
of 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 define the range of the tripping time where 51Ns T max is assigned
to the current threshold 51Ns PICKUP and 51Ns T min to the current threshold
51Ns I T min. The knee-point time 51Ns T knee specifies the tripping time in the
transition point of two characteristic segments with different slope. The transition point
is defined by the current threshold 51Ns I T knee. 51Ns PICKUP is the minimum
pickup threshold for the ground-fault pickup current of the overcurrent element. The
tripping time will assume a constant value after reaching a maximum secondary
current of 1.4 A at the latest. The parameter 51Ns TD serves as time multiplier for the
tripping time.
Figure 2-76 Trip-time characteristics of the inverse-time ground fault protection 51Ns with
logarithmic inverse characteristic with knee point (example for 51Ns = 0.004 A)
If a user-defined characteristic is configured at address 131, Sens. Gnd Fault
User Defined PU, it should be noted that there is a safety factor of 1.1 between
pickup and setting value - as is standard for inverse curves. This means that pickup
will only be initiated when current of 1.1 times the setting value flows.
Entry of the value pair (current and time) is a multiple of the settings at addresses
3119 51Ns PICKUP and 3120 51NsTIME DIAL. Therefore, it is recommended that
these addresses are initially set to 1.00 for simplicity. Once the curve is entered, the
settings at addresses 3119 and/or 3120 may be modified if necessary.
The default setting of current values is ∞. They are, therefore, not enabled — and no
pickup or tripping of these protective functions will occur.
Up to 20 pairs of values (current and time) may be entered at address 3131 M.of
PU TD. The device then approximates the characteristic, using linear interpolation.
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015