74
3
Selection and application
3-9 MCCBs for semiconductor circuit
Fig. 3-22 Circuit configurations and component current of thyristor converters
Circuit configuration
Element (arm)
Current waveformCurrent Ia
DC side
Current waveformCurrent Id
AC side
Current waveform
I
=Ia=Id
I
=
I
a
Id
Ia
I
Id
Ia
I
Id
1/f
1/2f
ip
1/f
1/2f
ip
1/f
1/2f
ip
1/3f
1/f
ip
S
1
Average: Ia (av)
ip
2
1
Effective: Ia (eff)
ip
S
1
Average: Ia (av)
ip
2
1
Effective: Ia (eff)
ip
S
1
Average: Ia (av)
ip
2
1
Effective: Ia (eff)
ip
S
1
Average: Ia (av)
ip
Effective: Ia (eff)
6
1
+
4S
3
ip | 0.553ip
1/f
ip
1/f
ip
1/f
ip
1/f
ip
S
1
Average: Id (av)
ip
2
1
Effective: Id (eff)
ip
S
2
Average: Id (av)
ip
Effective: Id (eff)
1
2
ip
S
2
Average: Id (av)
ip
Effective: Id (eff)
1
2
ip
S
3
Average: Id (av)
ip
Effective: Id (eff)
6
3
+
4S
3 3
ip | 0.956ip
1/f
ip
1/f
ip
1/f
ip
1/f
ip
Current I
S
1
Average: I (av)
ip
2
1
Effective: I (eff)
ip
S
1
Average: I (av)
ip
2
1
Effective: I (eff)
ip
Effective: I (eff)
1
2
ip
Effective: I (eff)
6
2
+
4S
2 3
ip | 0.78ip
Note: The loads are resistive, and the conduction angle of the device is 180°.
3-9-2 MCCB rated current
When an MCCB is used as a protective device, it is installed
on either the AC or DC side. The current that flows through the
MCCB may differ depending on the side in which it is installed.
Remember this point when selecting an MCCB rated current
rating.
With three-phase bridge circuits, installing an MCCB on the
AC side may be more economical because an MCCB with
a smaller current rating can be used in this setup. The type
of failure may dictate, however, that the MCCB be installed
on the DC side. Hence, the location of the MCCB should be
determined with the importance of the load equipment and
economy taken into consideration.
Table 3-22 indicates the circuit configurations and component
current values of thyristor converters. Select an MCCB with
a current rating higher than the effective circuit current,
depending on its installation location. A 20% margin is
recommended.
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