Siemens siprotec 7SD5 User Manual page 145

Most important for this setting on overhead lines, is the resistance of the fault arc. In
cables on the other hand, an appreciable arc can not exist. On very short cables, care
must however be taken that an arc fault on the local cable termination is inside the set
resistance of the first zone.
The resistance of the line itself does not have to be considered since it is accounted
for through the shape of the polygon provided that the line angle is at least as large as
the inclination angle Distance Angle (address 1540) of the polygon.
Example:
A maximum arc voltage of 8 kV is assumed for phase-phase faults (line data as
above). If the minimum primary short-circuit current is assumed to be 1000 A this cor-
responds to 8 Ω primary. For the resistance setting of the first zone this implies
primary:
or secondary:
Only half the arc resistance was applied in the equation, as it is added to the loop im-
pedance and therefore only half the arc resistance appears in the per phase imped-
ance.
A separate resistance tolerance can be set for earth faults. An arc resistance of 6 Ω
and a tower footing resistance of 12 Ω is assumed. This results in the following
primary:
or secondary:
In this case the least favourable condition was assumed, whereby the earth current
does not return via the measuring point. If all the earth current, or a portion of the earth
current flows via the measuring point, the measured resistance decreases. When
there is an infeed from the remote end, the measured resistance may be increased.
By means of the parameter MODE = Forward or Reverse or Non-Directional
Independent Zones
Z1 up to Z5 each zone can be set (address 1601 Op. mode Z1, 1611 Op. mode Z2, 1621 Op.
mode Z3, 1631 Op. mode Z4 and 1641 Op. mode Z5). This allows any combina-
tion of graded zones - forward, reverse or non-directional -, for example on transform-
ers, generators or bus couplers. In the fifth zone different reach in the X direction can
be set for forward or reverse. Zones that are not required are set Inactive.
The values derived from the grading coordination chart are set for each of the required
zones. The setting parameters are grouped for each zone. For the first zone these are
the parameters R(Z1) Ø-Ø (address 1602) for the R intersection of the polygon ap-
plicable to phase-phase faults, X(Z1) (address 1603) for the X intersection (reach),
RE(Z1) Ø-E (address 1604) for the R intersection applicable to phase-earth faults
and delay time settings.
For the first zone, Z1, an additional tilt α can be set by means of the parameter in
address 1607 Zone Reduction. This setting is required if short-circuits with a large
7SD5 Manual
C53000-G1176-C169-1
2.5 Distance Protection
145