Siemens SIPROTEC, Руководство пользователя

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Functions
2.5 Distance Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
137
The resulting minimum load impedance is therefore:
This value applies for phase-to-phase measurement. The setting for ground loops must also consider the con-
dition when the parallel line has a single pole open condition. In this state, the load current on the „healthy line“
will increase in the phase with single pole open condition as well as in the ground path. To determine the
minimum load resistance in the ground loops during this state, the magnitude of the load current in the ground
path must be set. For the calculation, it is given as a ratio relative to the load current
I
max
calculated above.
Ratio between
I
E
on healthy line and
I
max
when parallel line has a single pole open condition:
This ratio depends on the line length as well as on the source and line impedances. If it is not possible to de-
termine this value from power system simulations, a value between 0.4 for long double lines (200 km) and 0.6
for short lines (25 km) may be assumed.
The resultant minimum load impedance for phase-to-earth loops is therefore:
This value may be entered as a primary value when parameterizing with a PC and DIGSI. Conversion to sec-
ondary quantities is:
when applying a security margin of 10% the following is set:
R load (Ø-Ø)
=
108
Ω
primary =
10.8
Ω
secondary
R load (Ø-E)
=
53.5
Ω
primary =
5.35
Ω
secondary