Siemens siprotec 7SA6 User Manual page 243

Independent Zones
Z1 up to Z5
7SA6 Manual
C53000-G1176-C133-1
care should 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 need not be taken into consideration since it was considered
through the shape of the polygon, provided the line angle in address  /LQH
$QJOH (see Subsection 6.1.3, margin heading „General Line Data") had been set
correctly.
Example:
In the following example a maximum arc voltage of 8 kV is assumed for phase–phase
faults (line data as above). If the minimum primary fault current is assumed to be
1000 A this corresponds to 8 Ω primary. This results in the following setting for the
resistance reach of the first zone:
primary:
1
⋅
R1 -- - R
= =
prim arc
2
or secondary:
N
CT
⋅
R1 ---------- R1
=
sec prim
N
VT
Only half the arc resistance was applied in the equation, as it is added to the loop
impedance and therefore only half the arc resistance appears in the per phase
impedance.
A separate resistance margin 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:
R1E = R + R
prim arc
or secondary:
N
CT
⋅
R1E = ---------- R1
sec
N
VT
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. If the
infeed comes from the remote end, the measured resistance may be increased.
By means of the setting parameter 02'( each zone can be set )RUZDUG or 5HYHUVH
or 1RQ'LUHFWLRQDO (Address  2S PRGH =,  2S PRGH =, 
2S PRGH =,  2S PRGH = and  2S PRGH =). This allows any
combination of forward, reverse or non-directional graded zones, for example on
transformers, 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
,QDFWLYH.
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, Z1, these
are the parameters 5 = (address ) for the R intersection of the polygon
applicable to phase-phase faults, ; = (address ) for the X intersection of the
polygon (reach), 5( = (address ) for the R intersection of the polygon
applicable to phase-earth faults as well as the relevant delay time settings.
1
-- - 8 Ω ⋅ 4 Ω
=
2
600 A/5 A
------------------------------------- - 4 Ω ⋅
=
110 kV/0.1 kV
6 Ω 12 Ω
= + =
tower
600 A/5 A
------------------------------------- - 18 Ω ⋅
=
prim
110 kV/0.1 kV
0.44 Ω
=
18 Ω
1.96 Ω
=
Functions
6-61