Woodward
Manual MRIK3-C EN
28
DOK-TD-MRIK3-C Rev. A
1. Mains with isolated star point (setting: "earthing: SIN“, "MTA (fixed): -90°)
Figure 5.7: Phase position between the residual voltage and zero sequence current for faulted and non-faulted lines in case
of isolated systems (sin
)
U
E
- residual voltage
I
E
- zero sequence current
I
C
- capacitive component of zero sequence current
I
W
- resistive component of zero sequence current
By calculating the reactive current component (sin
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 component 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°.
2. Compensated mains (setting: "earthing: COS‘‘, "MTA (fixed): 180°‘‘)
Figure 5.8:
Phase position between the residual voltage and zero sequence current for faulted and non-faulted lines in case
of compensated systems (
cos
)
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
adjustment) 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
