TD_MRI1_06.05_GB
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Figure 4.6:
Phase position between the residual voltage and zero sequence current for faulted and non-faulted lines in case of isolated
systems
(sin
ϕ
)
U
E
- residual voltage
I
E
- zero sequence current
I
C
- capacitive component of zero sequence cur-
rent
I
W
- resistive component of zero sequence current
By calculating the reactive current component (sin
ϕ
adjustment) and then comparing the phase angle in
relation to the residual voltage U
E
, the ER/XR-relay
type determines whether the line to be protected is
earth-faulted.
On non-earth-faulted lines, the capacitive compo-
nent Ic(a) of the total current precedes the residual
voltage by an angle of 90°. In case of a faulty line
the capacity current I
C(b)
lags behind the residual
voltage at 90°.
Figure 4.7:
Phase position between the residual voltage and zero sequence current for faulted and non-faulted lines in case of
compensated systems (
cos
ϕ
)
U
E
- residual voltage
I
E
- zero sequence current
I
L
- inductive component of zero sequence current
(caused by Petersen coil)
I
C
- capacitive component of zero sequence current
I
W
- resistive component of zero sequence current
In compensated mains the earth fault direction cannot
be determined from the reactive current components
because the reactive part of the earth current depends
upon the compensation level of the mains. The ohmic
component of the total current (calculated by cos
ϕ
ad-
justment) is used in order to determine the direction.
The resistive component in the non-faulted line is in
phase with the residual voltage, while the resistive
component in the faulted line is opposite in phase with
the residual voltage.
By means of an efficient digital filter harmonics and
fault transients in the fault current are suppressed. Thus,
the uneven harmonics which, for instance, are caused
an electric arc fault, do not impair the protective func-
tion.