ABB Relion REC670 Applications Manual page 142

Section 7
Current protection
The fault current, in the fault point, can be calculated as:
=
I 3I
j 0
EQUATION1944 V1 EN-US
The impedance Z
point apparatus) the impedance is equal to the capacitive coupling between the phase
conductors and earth:
= -
Z jX
0
EQUATION1945 V1 EN-US
Where
I
j
X
c
In a system with a neutral point resistor (resistance earthed system) the impedance Z
calculated as:
-
=
Z
0
-
EQUATION1946 V1 EN-US
Where
R
n
In many systems there is also a neutral point reactor (Petersen coil) connected to one or more
transformer neutral points. In such a system the impedance Z
= -
Z jX // 3R // j3X
0
EQUATION1947 V1 EN-US
Where
X
n
Now consider a system with an earthing via a resistor giving higher earth fault current than the
high impedance earthing. The series impedances in the system can no longer be neglected.
The system with a single phase to earth fault can be described as in Figure 64.
136
×
3 U
= phase
+ ×
Z 3 R
0 f
is dependent on the system earthing. In an isolated system (without neutral
0
×
3 U
= - phase
j
c
I
j
is the capacitive earth fault current at a non-resistive phase-to-earth fault
is the capacitive reactance to earth
×
jX 3R
c n
+
jX 3R
c n
is the resistance of the neutral point resistor
=
c n n
3X X
n
is the reactance of the Petersen coil. If the Petersen coil is well tuned we have 3X
impedance Z will be: Z = 3R
0 0
9R X X
n n c
( )
+ × -
j3R 3X X
c n n c
n
1MRK 511 358-UEN A
(Equation 47)
(Equation 48)
(Equation 49)
can be calculated as:
0
(Equation 50)
= X In this case the
n c
Application manual
can be
0