ABB Relion REC670, Applications Manual

Page: 199 / 658

8.6.3 Setting guidelines
SEMOD171961-4 v10
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-US
(Equation 46)
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-US
(Equation 47)
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-US
(Equation 48)
1MRK 511 401-UUS A
Section 8
Current protection
Bay control REC670 2.2 ANSI 193
Application manual