ABB RELION 670 Series Applications Manual page 134

Section 8
Current protection
In a high impedance system the fault current is assumed to be limited by the system zero
sequence shunt impedance to ground and the fault resistance only. All the series impedances in
the system are assumed to be zero.
In the setting of ground-fault protection, in a high impedance grounded system, the neutral point
voltage (zero sequence voltage) and the ground-fault current will be calculated at the desired
sensitivity (fault resistance). The complex neutral point voltage (zero sequence) can be calculated
as:
V
=
V
0
+
1
EQUATION2020-ANSI V1 EN-US
Where
V
phase
R
f
Z
0
The fault current, in the fault point, can be calculated as:
=
I 3I
j 0
EQUATION2021 V1 EN-US
The impedance Z
point apparatus) the impedance is equal to the capacitive coupling between the phase conductors
and ground:
= -
Z
0
EQUATION2022-ANSI V1 EN-US
Where
I
j
X
c
In a system with a neutral point resistor (resistance grounded system) the impedance Z
calculated as:
128
phase
×
3 R
f
Z
0
is the phase voltage in the fault point before the fault,
is the resistance to ground in the fault point and
is the system zero sequence impedance to ground
×
3 V
= phase
+ ×
Z 3 R
0 f
is dependent on the system grounding. In an isolated system (without neutral
0
×
3 V
= - phase
jX j
c
I
j
is the capacitive ground fault current at a non-resistive phase-to-ground fault
is the capacitive reactance to ground
1MRK 511 364-UUS A
(Equation 39)
(Equation 40)
(Equation 41)
can be
0
Application manual