Setting Guidelines - ABB RELION 650 Series Applications Manual

Section 8
Current protection
8.5.3

Setting guidelines

The sensitive earth fault protection is intended to be used in high impedance earthed systems,
or in systems with resistive earthing where the neutral point resistor gives an earth fault
current larger than what normal high impedance gives but smaller than the phase-to-phase
short circuit current.
In a high impedance system the fault current is assumed to be limited by the system zero
sequence shunt impedance to earth and the fault resistance only. All the series impedances in
the system are assumed to be zero.
In the setting of earth fault protection, in a high impedance earthed system, the neutral point
voltage (zero sequence voltage) and the earth fault current will be calculated at the desired
sensitivity (fault resistance). The complex neutral point voltage (zero sequence) can be
calculated as:
=
U
0
+
1
EQUATION1943 V1 EN-US
Where
U
phase
R
f
Z
0
The fault current, in the fault point, can be calculated as:
=
I 3I
j 0
EQUATION1944 V1 EN-US
The impedance Z
point apparatus) the impedance is equal to the capacitive coupling between the phase
conductors and earth:
= -
Z jX
0
EQUATION1945 V1 EN-US
Where
I
j
X
c
In a system with a neutral point resistor (resistance earthed system) the impedance Z
calculated as:
156
U
phase
×
3 R
f
Z
0
is the phase voltage in the fault point before the fault,
is the resistance to earth in the fault point and
is the system zero sequence impedance to earth
×
3 U
= phase
+ ×
Z 3 R
0 f
is dependent on the system earthing. In an isolated system (without neutral
0
×
3 U
= -
phase
j
c
I
j
is the capacitive earth fault current at a non-resistive phase-to-earth fault
is the capacitive reactance to earth
1MRK 505 363-UEN A
SEMOD171961-4 v9
(Equation 102)
(Equation 103)
(Equation 104)
can be
0
Application manual