ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. B
4-6
The influence of external phase faults on the circulating current
circuit is limited, especially if the connections between the c.t.
cores can be kept short. Phase faults are therefore neglected
when designing a scheme for a solidly grounded system. They
may not be neglected, however, where a system is impedance
grounded.
The value of the stabilising resistor is chosen such that the volt-
age drop caused by the highest external E/F and possibly phase
fault current across the secondary winding and leads of the satu-
rated c.t. cannot reach the pick-up setting of the protection (see
Fig. 4.3).
The knee-point voltage of the c.t’s is specified such that the they
can supply sufficient current during an internal fault to enable the
protection to trip. The knee-point voltage U
k
of the c.t’s must
therefore be appreciably higher than the voltage drop DU
a
.
Symbols used:
I
E
primary star-point current (AC component) for a
through-fault
I
2
secondary current of the non-saturated c.t’s
I
2N
c.t. secondary rated current
I
1N
c.t. primary rated current
I
N
protection rated current
R
2
secondary resistance of the saturated c.t. at 75°C
R
L
lead resistance according to the Figure
DU
a
, DU
i
voltage drops across the circulating current circuit
for external and internal faults
U
k
c.t. knee-point voltage
I current setting
R
S
stabilising resistor
I
F
highest primary fault current (AC component) for an
internal E/F.
To Next Page
Loading...
