ABB RES670 Applications Manual page 156

Section 8
Current protection
150
In a high impedance system the fault current is assumed to be limited by the system
zero sequence shunt impedance to earth and the fault resistance only. All the series
impedances in the system are assumed to be zero.
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-US
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-US
The impedance Z is dependent on the system earthing. In an isolated system
0
(without 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-US
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:
0
1MRK 511 407-UEN C
Phasor measurement unit RES670 2.2 IEC
Application manual
(Equation 40)
(Equation 41)
(Equation 42)