Calculation Of The Impedances - Siemens siprotec 7SD5 User Manual

2 Functions
2.5.1.3

Calculation of the Impedances

A separate measuring system is provided for each of the six possible impedance loops
L1-E, L2-E, L3-E, L1-L2, L2-L3, L3-L1. The phase-earth loops are evaluated when an
earth fault detection is recognized and the phase current exceeds a settable minimum
value Minimum Iph>. The phase-phase loops are evaluated when the phase current
in both of the affected phases exceeds the minimum value Minimum Iph>.
A jump detector synchronizes all the calculations with the fault inception. If a further
fault occurs during the evaluation, the new measured values are immediately used for
the calculation. The fault evaluation is therefore always done with the measured
values of the current fault condition.
Phase–Phase To calculate the phase-phase loop, for instance during a two-phase short circuit L1-
Loops L2 (Figure2-33) the loop equation is:
with
The line impedance is computed to be
Figure 2-33
118
I · Z – I · Z = U – U
L1 L L2 L L1-E
U, I
Z = R + jX
Short-circuit of a phase-phase loop
L2-E
the (complex) measured quantities and
the (complex) line impedance.
7SD5 Manual
C53000-G1176-C169-1