Siemens siprotec 7SA6 User Manual page 237

Distance protection

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Determination

C53000-G1176-C133-1

The R-reach may be set separately for the phase–phase faults and the phase–earth

faults to achieve a larger fault resistance coverage for earth faults if this is desired.

For the first zone an additional tilt α exists, which may be used to prevent overreach

resulting from angle variance and/or two ended infeed to short-circuits with fault

resistance. For Z1B and the higher zones this tilt does not exist.

Polygonal characteristic

For each loop an impedance vector is also used to determine the direction of the short-

circuit. Usually similar to the distance calculation, Z

the "quality" of the measured values, different computation techniques are used.

Immediately after fault inception, the short circuit voltage is disturbed by transients.

The voltage memorized prior to fault inception is therefore used in this situation. If the

steady-state short-circuit voltage (during a close-in fault) is even too small for direction

determination, an unfaulted voltage is used. This voltage is in theory quadrilateral to

the actual short-circuit voltage for both phase–earth loops as well as for phase–phase

loops (refer to Figure 6-30). This is taken into account when computing the direction

vector by means of a 90°–rotation. In Table 6-9 the allocation of the measured values

to the six fault loops for the determination of the fault direction is shown.

Line Characteristic

is used. However, depending on

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