ABB REG670 - Page 365

In the setting of earth fault protection, in a high impedance earthed system, the neutral
point voltage (zero sequence voltage) and the earth 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
U
U
3 R
1
Z
=
×
+
EQUATION1943 V1 EN (Equation 195)
Where
U
phase
is the phase voltage in the fault point before the fault,
R
f
is the resistance to earth in the fault point and
Z
0
is the system zero sequence impedance to earth
The fault current, in the fault point, can be calculated as:
phase
j 0
0 f
3 U
I 3I
Z 3 R
×
= =
+ ×
EQUATION1944 V1 EN (Equation 196)
The impedance Z
0
is dependent on the system earthing. In an isolated system (without
neutral point apparatus) the impedance is equal to the capacitive coupling between the
phase conductors and earth:
phase
0 c
j
3 U
Z jX j
I
×
= - = -
EQUATION1945 V1 EN (Equation 197)
Where
I
j
is the capacitive earth fault current at a non-resistive phase-to-earth fault
X
c
is the capacitive reactance to earth
In a system with a neutral point resistor (resistance earthed system) the impedance Z
0
can be calculated as:
1MRK 502 071-UEN - Section 9
Current protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 359
Application manual