ABB Relion 670 series - Setting Guidelines

9.6.3 Setting guidelines
The sensitive ground-fault protection is intended to be used in high impedance grounded
systems, or in systems with resistive grounding where the neutral point resistor gives an
ground-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 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:
phase
0
f
0
V
V
3 R
1
Z
=
×
+
EQUATION2020-ANSI V1 EN (Equation 468)
Where
V
phase
is the phase voltage in the fault point before the fault,
R
f
is the resistance to ground in the fault point and
Z
0
is the system zero sequence impedance to ground
The fault current, in the fault point, can be calculated as:
phase
j 0
0 f
3 V
I 3I
Z 3 R
×
= =
+ ×
EQUATION2021 V1 EN (Equation 469)
The impedance Z
0
is dependent on the system grounding. In an isolated system (without
neutral point apparatus) the impedance is equal to the capacitive coupling between the
phase conductors and ground:
phase
0 c
j
3 V
Z jX j
I
×
= - = -
EQUATION2022-ANSI V1 EN (Equation 470)
1MRK 506 369-UUS - Section 9
Current protection
Line distance protection REL670 2.2 ANSI 531
Application manual