ABB RELION RET670 Applications Manual page 565

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
Where
I is the capacitive ground fault current at a non-resistive phase-to-ground fault
j
X is the capacitive reactance to ground
c
In a system with a neutral point resistor (resistance grounded system) the impedance Z
can be calculated as:
- ×
jX 3R
=
Z c n
0
- +
jX 3R
c n
EQUATION1946 V1 EN-US
Where
R is the resistance of the neutral point resistor
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
as:
= - =
Z jX // 3R // j3X
0 c n n
EQUATION1947 V1 EN-US
Where
X is the reactance of the Petersen coil. If the Petersen coil is well tuned we have 3X
n
case the impedance Z
Now consider a system with an grounding via a resistor giving higher ground fault
current than the high impedance grounding. The series impedances in the system can
no longer be neglected. The system with a single phase to ground fault can be
described as in Figure 278.
9R X X
n n c
(
+ × -
3X X j3R 3X X
n c n n c
will be: Z = 3R
0 0 n
Section 9
Current protection
can be calculated
0
)
n
0
(Equation 438)
(Equation 439)
= X In this
c
559