Technical description
2 Protection functions
2.12 Directional earth fault protection
I0
φ
> (67N)
VM50.EN004
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63
Polarization
The negative zero sequence voltage
U
0
is used for polarization
i.e. the angle reference for I
0
. This
U
0
voltage is measured via
energizing input U
0
or it is calculated from the phase voltages
internally depending on the selected voltage measurement
mode (see chapter 4.7):
LN: the zero sequence voltage is calculated from the phase
voltages and therefore any separate zero sequence voltage
transformers are not needed. The setting values are relative
to the configured voltage transformer (VT) voltage/
3.
LL+U
0
: The zero sequence voltage is measured with voltage
transformer(s) for example using a broken delta connection.
The setting values are relative to the VT
0
secondary voltage
defined in configuration.
NOTE! The U
0
signal must be connected according the connection diagram
(Figure 8.10.2-1) in order to get a correct polarization. Please note that
actually the negative U
0
,
U
0
, is connected to the device.
Modes for different network types
The available modes are:
ResCap
This mode consists of two sub modes, Res and Cap. A digital
signal can be used to dynamically switch between these two
sub modes. This feature can be used with compensated
networks, when the Petersen coil is temporarily switched
off.
o
Res
The stage is sensitive to the resistive component of
the selected I
0
signal. This mode is used with
compensated networks (resonant grounding) and
networks earthed with a high resistance.
Compensation is usually done with a Petersen coil
between the neutral point of the main transformer
and earth. In this context "high resistance" means,
that the fault current is limited to be less than the
rated phase current. The trip area is a half plane as
drawn in Figure 2.12-2. The base angle is usually set
to zero degrees.
o
Cap
The stage is sensitive to the capacitive component of
the selected I0 signal. This mode is used with
unearthed networks. The trip area is a half plane as
drawn in Figure 2.12-2. The base angle is usually set
to zero degrees.