48
3
Selection and application
3-3 Selective trip applications
Fig. 3-7 PF·S high-voltage power receiving facility
SC
1øT
3øT
3øT
MCCB
MCCB
MCCB
VCT
LBS with power fuse (PF)
Fig. 3-8 PF high-voltage power receiving facility
MCCB
MCCB
MCCB
MCCB
T
PF
DS
Fig. 3-9 MCCB – PF selective trip coordination
Current
MCCB trip
characteristic
Power fuse
Short-circuit current
(transformer
secondary side)
(Converted to transformer primary side)
Oper
ating time
(allowable current-time
characteristic)
3-3-2
Selective trip coordination between
MCCBs and high-voltage side protective
devices
(1) Coordination between MCCBs and power fuses
In type PF·S high-voltage power receiving facilities like
those shown in Fig. 3-7, power fuses (PF) are often used as
protective devices. Power fuses are also used to protect the
primary circuit of a transformer as shown in Fig. 3-8.
In these types of facility, selective trip coordination must
be maintained between the PF and the MCCB installed
on the transformer secondary circuit. Without selective trip
coordination between the PF and the MCCB, faults occurring
on the load side of the MCCB will trip the PF, causing a total
system shutdown.
To establish selective trip coordination between the PF
and MCCB, the following condition must be satisfied: when
the allowable current-time characteristic curve of the PF is
superimposed on the operating characteristic curve of the
MCCB as shown in Fig. 3-9 (by converting the current of the
PF to the low voltage side, or the current of the MCCB to the
high-voltage side), these curves do not cross.
Fig. 3-9 shows the operating characteristics of the MCCB
converted to the high-voltage side (transformer primary side).
Conversion to the high-voltage side is done by dividing the
current in the operating characteristic curve of the MCCB by
the voltage ratio of the transformer. (50, if 20kV/400V)
Conversion to the low-voltage side is done by multiplying the
current value in the allowable current time characteristic curve
of the PF by the same voltage ratio.
Because the maximum rated current of a PF is limited by the
conditions stated below, to achieve selective trip coordination,
it is necessary to reduce the current rating of the MCCB, or to
select an MCCB with an adjustable instantaneous trip current
feature.
(a) Conditions for selecting PF current rating:
s
Selective tripping can be coordinated with upstream power
fuse protective devices.
s !SHORTCIRCUITCURRENTTIMESHIGHERTHANTHETRANSFORMER
current rating can be interrupted within 2 seconds to protect
the transformer. Sometimes, an MCCB may be substituted
for the PF to provide this function.
s $EGRADATIONOFTHE0&DUETOTRANSFORMEREXCITATIONINRUSH
current can be prevented.
Table 3-7 lists the applicable combinations of FUJI MCCBs
and FUJI high-voltage current-limiting fuses from the
standpoint of selective trip coordination.
