ABB RET670 Applications Manual page 460

Section 3
IED application
454
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
0
(without 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
Where
I is the capacitive ground-fault current at a non-resistive phase to ground-fault
j
X is the capacitive reactance to ground
c
In a system with a neutral point resistor (resistance grounded system) the impedance Z
can be calculated as:
1MRK504116-UUS C
(Equation 353)
(Equation 354)
(Equation 355)
0
Application manual