operation must be both very fast and selective, which is usually achieved by using
coarse current settings.
The purpose is also to protect the transformer from short circuits occurring
outside the protection zone, that is, from through-faults. Transformer overcurrent
protection also provides protection for the LV-side busbars. In this case, the
magnitude of the fault current is typically lower than 12xIn, depending on the fault
location and transformer impedance. Consequently, the protection must operate
as fast as possible, taking into account the selectivity requirements, switching-
in currents and the thermal and mechanical withstand of the transformer and
outgoing feeders.
Traditionally, overcurrent protection of the transformer has been arranged as
shown in
. The low-set stage PH3LPTOC operates time-selectively both
in transformer and LV-side busbar faults. The high-set stage PH3HPTOC operates
instantaneously, making use of current selectivity only in the transformer HV-side
faults. If there is a possibility that the fault current can also be fed from the LV-
side up to the HV-side, the transformer must also be equipped with an LV-side
overcurrent protection. Inrush current detectors are used in startup situations to
multiply the current start value setting in each particular IED where the inrush
current can occur. The overcurrent- and contact-based circuit breaker failure
protection CCBRBRF is used to confirm the protection scheme in case of circuit
breaker malfunction.
MF
MF
MF
MF
MF
MF
MF
MF
MF
MEASUREMENT
INCOMING
G
N
I
O
G
T
U
O
G
N
I
O
G
T
U
O
E
I
T
_
S
U
B
E
I
T
-
S
U
B
G
N
I
O
G
T
U
O
G
N
I
O
G
T
U
O
INCOMING
MEASUREMENT
PH3LPTOC
PH3HPTOC
INRPHAR
PH3LPTOC
PH3HPTOC
CCBRBRF
PH3LPTOC
PH3HPTOC
CCBRBRF
INRPHAR
PH3LPTOC
PH3HPTOC
INRPHAR
PH3LPTOC
PH3HPTOC
CCBRBRF
INRPHAR
Figure 129: Example of traditional time selective transformer overcurrent protection
The operating times of the main and backup overcurrent protection of the above
scheme become quite long. This applies especially in the busbar faults and
also in the transformer LV-terminal faults. To improve the performance of the
above scheme, a multiple-stage overcurrent protection with a reverse blocking is
proposed.
shows this arrangement.
1MRS757644 H
Protection functions
620 series
Technical Manual
263
