These parameters are used to define the direction characteristic of the stage. The direction characteristic to
use is dependent on the neutral-point treatment of the system.
Note that, for the direction determination, basically only the component of the admittance perpendicular to
the set direction-characteristic curve is decisive, see chapter 6.15.8.1 Description. This admittance component
is compared to the threshold value Polarized G0/B0 threshold. In contrast, the absolute value of the
current 3I0 is compared with the 3I0> release thresh. value parameter.
System Type/
Neutral-Point Treat-
ment
Description
Grounded In the arc-suppression-coil-ground system, the watt-metric residual current 3I0 · cos φ
of the arc-suppression coil is decisive for the direction determination.
To evaluate the watt-metric residual current, set the parameters as follows:
•
Dir. measuring method = G0
•
φ correction = 0.0°
The direction determination for a ground fault is made more difficult in that a much
larger reactive current of capacitive or inductive character is superimposed on the small
watt-metric residual current. Therefore, depending on the system configuration and
the fault evaluation, the total ground current supplied to the device can vary consider-
ably in its values regarding the magnitude and the phase angle. However, the device
should only evaluate the active component of the ground-fault current.
This requires extremely high accuracy, particularly regarding the phase-angle measure-
ment of all the instrument transformers. Furthermore, the device must not be set to
operate too sensitively. A reliable direction measurement can only be expected with
connection to a core balance current transformer. For the setting of the Polarized
G0/B0 threshold parameter, the following formula applies:
where:
k
s
: Safety margin, k
s
= 1.2 (cable networks), k
s
= 2.0 (overhead lines)
I
0active
: Active component of the ground-fault current (watt-metric residual current) of
the protected line
V
rated
: Secondary rated voltage in the healthy case
I
0min
: Min. ground current in the healthy case, 5 mA to 10 mA (core balance current
transformer), 50 mA to 100 mA (Holmgreen transformer)
V0>: Pickup threshold of the residual voltage ≈ 0.1
If a parallel resistor Rp is used on the arc-suppression coil, the threshold value G0 must
also be smaller than:
where:
k
s
: Safety margin ≥ 1.5
I
Rp
: Secondary rated current of the parallel resistor
V
rated
: Secondary rated voltage in the healthy case
The 3I0> release thresh. value parameter can be set to half of the expected
measuring current and here, the entire zero-sequence current can be put to use.
Protection and Automation Functions
6.15 Sensitive Ground-Fault Detection
SIPROTEC 5, Overcurrent Protection, Manual 597
C53000-G5040-C017-8, Edition 07.2017
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