ABB RELION 620 Series Technical Manual page 470

Section 4
Protection functions
464
s
A
o
u
B
r
c
C
e
Ifault
Io
For internal fault
GUID-D5D712D4-2291-4C49-93DE-363F9F10801C V2 EN
Figure 215: Current flow in all the CTs for an internal fault
87LOZREF does not respond to phase-to-phase faults either, as in this case the fault
current flows between the two line CTs and so the neutral CT does not experience this fault
current.
87LOZREF is normally applied when the transformer is solidly grounded because in this
case the fault current is high enough and the ground fault can be detected easily.
Blocking based on the second harmonic of the neutral current
The transformer magnetizing inrush currents occur when the transformer is energized
after a period of de-energization. The inrush current can be many times the rated current,
and the halving time can be up to several seconds. For the differential protection relay, the
inrush current represents the differential current, which causes the protection relay to trip
almost always when the transformer is connected to the network. Typically, the inrush
current contains a large amount of second harmonics.
The blocking also prevents unwanted operation at the recovery and sympathetic
magnetizing inrushes. At the recovery inrush, the magnetizing current of the transformer
to be protected increases momentarily when the voltage returns to normal after the
clearance of a fault outside the protected area. The sympathetic inrush is caused by the
energization of a transformer running in parallel with the protected transformer already
connected to the network.
zone of
protection
Uzs
Io
Restrain for
Operate for
external fault
internal fault
1MAC504801-IB E
I I
a = 0
I I
b = 0
I
c = 0
I
N
I
N
Reference is Neutral
Current
620 series ANSI
Technical Manual
a
b
c