54
Fig. 6.23 Coordinated PF and MCCB Characteristics
6.8.2 Electronic MCCBs and HV PF
A basic requirement is that the deteriorated short-de-
lay curve of the PF, and the short-delay trip curve of
Electronic MCCB, which is s10% along the
current axis, do not overlap.
To facilitate matching, the rated current of the PF
should be as large as possible; however, there is an
upper limit, as seen from the following criteria:
1. The rated current should be 1.5~2 times the load
current.
2. To ensure protection in the event of a short circuit,
the PF must interrupt a current of 25 times the trans-
former rating within 2 seconds.
3. To ensure that the PF neither deteriorates nor fuses
as a result of the transformer excitation surge
current, the short-delay deterioration curve of the
PF must be more than 0.1 seconds, at a current of
10 times the transformer rating. The “10 times”
factor becomes “15 times” in the case of a single-
phase transformer.
Thus, if normal starting current is assumed as 600%
of full-load current, the peak inrush becomes 1200%
in Y-delta restarting and 1600% in direct restarting.
The MCCB instantaneous-trip setting must be selected
at larger than these values.
Fig. 6.21 shows test date with respect to four condi-
tions of transient inrush current, expressed as magni-
fications of full-load current, measured on motors rated
from 0.2~30kW. The MCCB was used for line-start-
ing switching, and the contactor for the other switch-
ing duties. Phase matching between the line and re-
sidual voltages was uncontrolled.
The oscillographs taken showed that the peak inrush
currents persist for about one-half cycle, followed by
a rapid decrease to normal starting-current level. From
the curves it can be concluded that peak inrush mag-
nifications vary greatly depending on the duty involved;
for reversing duty, the MCCB instantaneous trip set-
tings must be selected from 1600 ~ 3400% of full-
load current. For line starting and Y-delta starting, the
range spans from 1000~2000%.
6.8 Coordination with Devices on the High-
Voltage Circuit.
6.8.1
High-Voltage Power Fuse
The MCCB on the secondary (low-voltage) side of a
power transformer must have tripping characteristics
that provide protective coordination with the power
fuse (PF) on the high-voltage side (Fig. 6.22). The
MCCB must always trip in response to overcurrent,
to ensure that the PF does not fuse or deteriorate by
elevated temperature aging.
Fig. 6.23 shows the MCCB curve in relationship to
the deteriorated PF curve (if this is unavailable, the
average fusing curve reduced by 20% can usually be
assumed). The PF characteristic can be converted to
the secondary side, or the MCCB characteristic to the
primary side; the curves must not overlap in the
overcurrent region.
Where the MCCB instantaneous-tripping current of
the MCCB is adjustable, difficulties in matching the
curves can be overcome as shown, but a 10% mar-
gin must be included to allow for the tolerance of the
MCCB tripping setting.
The shaded area in Fig. 6.23 belong to overcurrent
region, the overcurrent generally occur at the lower
circuit of MCCB
2
.
Thus, it may in some cases be better to accept a co-
ordination between the PF and MCCB
2
, permitting a
mismatch between the PF and MCCB
1
.
Fig. 6.22 Protective Coordination of MCCBs and HV-Side PF
PF
Tr
MCCB
1
MCCB
2
Time
MCCB tripping
curve
Minimum
setting of
inst,-trip
current
Short-delay fusing
of PF (deteriorated)
Overcurrent
