ABB Relion REQ650, Руководство по применению

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earth-fault current larger than what normal high impedance gives but smaller than the
phase-to-phase short circuit current.
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:
phase
0
f
0
U
U
3 R
1
Z
=
×
+
EQUATION1943 V1 EN
(Equation 29)
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 30)
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 31)
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:
Section 6
1MRK 505 291-UEN A
Current protection
124
Application manual