Siemens siprotec 7SA6 User Manual page 232

Functions
Load Area
(only for
Impedance Pick-up)
6-50
gization is activated for all recognized faults in any zone (i.e. with general fault detec-
tion of the distance protection).
On long heavily loaded lines, the risk of encroachment of the load impedance into the
tripping characteristic of the distance protection may exist. To exclude the risk of
unwanted fault detection by the distance protection during heavy load flow, a load
trapezoid characteristic may be set for tripping characteristics with a large R-reach,
which excludes such unwanted fault detection by overload. This load area is
considered in the description of the tripping characteristics (refer to Figure 6-29,
Subsubsection 6.2.4.1).
The R–values 5 ORDG ‘( (address ) and 5 ORDG ‘‘ (address )
must be set somewhat (approx. 10 %) smaller than the minimum load impedance
which may occur. The minimum load impedance results when the maximum load
current and minimum operating voltage exist.
Calculation example:
110 kV overhead line 150 mm
maximum transferrable load
P = 100 MVA corresponding to
max
I = 525 A
max
minimum operating voltage
U = 0.9 U
min N
current transformers600 A/5 A
voltage transformers110 kV/0.1 kV
The resulting minimum load impedance is therefore:
U
min
------------------------ -
R =
Load prim
⋅
3 I
When applying the settings with a personal computer and DIGSI
be entered as primary values. The conversion to secondary values is
N
CT
⋅
R ---------- R
=
Load sec
N
VT
when applying a security margin of 10 % the following is set:
5 ORDG ‘( =   Ω or
primary:
secondary: 5 ORDG ‘( =  Ω.
The spread angle of the load trapezoid ϕ ORDG ‘( (address ) and ϕ load (Ø-
Ø) (address ) must be greater (approx. 5°) than the maximum arising load angle
(corresponding to the minimum power factor cos ϕ).
Calculation example:
minimum power factor
cos ϕ
= 0.63
min
ϕ
= 51°
max
load (Ø-E) = ϕ
Setting
j
2
with the following data:
⋅
0.9 110 kV
--------------------------
= =
⋅
3 525 A
L max
600 A/5 A
------------------------------------- - 108.87 Ω
=
Load prim
110 kV/0.1 kV
+ 5° = °.
max
108.87 Ω
®
4 these values may
⋅ 11.88 Ω
=
C53000-G1176-C133-1
7SA6 Manual