ABB REG670 Applications Manual page 365

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