Siemens SIPROTEC 7SJ81 - Setting Notes for LPCT

15SIPROTEC 7SJ81/7SK81 V4.70, Product Information
E50417-K1150-C462-A2, Edition 02.2012
Setting Notes
Check whether the low-power current transformer used matches the input
measuring range of the 7SJ81/7SK81 (0 V to 17.7 V) and whether the
possible primary currents match.
The maximum measurable current can be determined using the following
equation:
Imax = RELlpct * Vmax
where RELlpct = Iph-LPCT PRIM/Uph-LPCT SEC and Vmax = 17.7 V
Example:
Iph-LPCT PRIM = 50 A
Uph-LPCT SEC = 22.5 mV
Imax = (50 A/22.5 mV) * 17.7 V = 39.3 KA
At rated current, the voltages at the input terminals of the protection
device should not be too small (Vrated min = 45 mV).
The smallest possible rated current can be determined using the following
equation:
Irated_min = RELlpct * Vrated_min
where
RELlpct = Iph-LPCT PRIM/Uph-LPCT SEC and Vrated_min = 45 mV
Example:
Iph-LPCT PRIM = 50 A
Uph-LPCT SEC = 22.5 mV
Irated_min = (50 A/22.5 mV) * 45 mV = 100 A
Note
When setting the parameters, make sure to observe the limiting values.
If the rated current is large (Iph-PRIM ref = 2.5 kA) and
RELlpct = Iph-LPCT PRIM/Uph-LPCT SEC = 100 A/225 mV = 444 A/V
is small, it is possible, that the setting value for I>> = 10 Irated is never
reached because of measuring-range violation (measuring range max.
17.7 V). For example, V
PHYSICAL
= 10 * (2500 A/444.44 A/V) = 56.25 V.