Setting Guidelines - ABB REL650 Applications Manual

Section 7
Current protection
7.7.3
216

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:
V
= phase
V
×
0 3 R
+
1 f
Z
0
EQUATION2020-ANSI V1 EN
Where
V is the phase voltage in the fault point before the fault,
phase
R is the resistance to ground in the fault point and
f
Z is the system zero sequence impedance to ground
0
The fault current, in the fault point, can be calculated as:
×
3 V
phase
= =
I 3I
j 0
+ ×
Z 3 R
0 f
EQUATION2021 V1 EN
The impedance Z is dependent on the system grounding. In an isolated system (without
0
neutral point apparatus) the impedance is equal to the capacitive coupling between the
phase conductors and ground:
×
3 V
= - = - phase
Z jX j
0 c
I
j
EQUATION2022-ANSI V1 EN
1MRK 506 334-UUS A
(Equation 126)
(Equation 127)
(Equation 128)
Application manual