Siemens siprotec 7SA522 - Calculation of the Impedances

2.2 Distance protection
65
7SA522 Manual
C53000-G1176-C155-3
2.2.1.2 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 0LQLPXP,SK!. The phase-phase loops are evaluated when the phase current
in both of the affected phases exceeds the minimum value 0LQLPXP,SK!.
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
Loops
To calculate the phase-phase loop, for instance during a two-phase short circuit L1-L2
(Figure 2-11), the loop equation is:
I
L1
· Z
L
– I
L2
· Z
L
= U
L1-E
– U
L2-E
with
U
, I the (complex) measured quantities and
Z
= R + jX the (complex) line impedance.
The line impedance is computed to be
Figure 2-11 Short-circuit of a phase-phase loop
The calculation of the phase-phase loop does not take place as long as one of the con-
cerned phases is switched off (during single-pole dead time), to avoid an incorrect
measurement with the undefined measured values existing during this state. A state
recognition (refer to section 2.20.1) provides the corresponding block signal. A logic
block diagram of the phase-phase measuring system is shown in Figure 2-12.