ABB RET670 - Page 460

phase
0
f
0
V
V
3 R
1
Z
=
×
+
EQUATION2020-ANSI V1 EN (Equation 353)
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 354)
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 355)
Where
I
j
is the capacitive ground-fault current at a non-resistive phase to ground-fault
X
c
is the capacitive reactance to ground
In a system with a neutral point resistor (resistance grounded system) the impedance Z
0
can be calculated as:
Section 3 1MRK504116-UUS C
IED application
454
Application manual