76
3
Selection and application
3-9 MCCBs for semiconductor circuit
Any examination of the scheme of protection coordination
between MCCBs and devices should allow conversion of
the device overcurrent immunity into effective values for
comparison. For example, in a three-phase bridge like that
shown in Fig. 3-32, the currents through the MCCBs differ
from that in devices and they must be compared on the same
current base.
Fig. 3-33 Typical protection coordination curves
t
i
1. Thyristor overcurrent immunity characteristics
2. Instantaneous trip type circuit breaker operating characteristics
3. Semiconductor protection fuse operating characteristics
4. Overcurrent relay operating characteristics
5. Motor breaker operating characteristics
(a)
Region A
Region B Region C
4
5
2
3
1
(b)
i
t
1. Thyristor overcurrent immunity characteristics
2. Instantaneous trip type circuit breaker operating characteristics
3. Semiconductor protection fuse operating characteristics
Region B Region C
2
1
3
(4) Use of MCCBs on the DC side of thyristors
When MCCBs are installed on the DC side of a converter Fig.
3-34, their primary duty will be interrupting the fault current
that flows through the circuit when commutation fails during
inversion in a thyristor Ward-Leonard or similar configuration.
Typically, an instantaneous trip type circuit breaker is used with
the instantaneous trip current set to about two or three times
its rating. The scheme of protection coordination is considered
in terms of I
2
t.
Fig. 3-34 Using an MCCB in a DC circuit
AC
MCCB
+
Faulty device
Since, in the circuit configuration shown in Fig. 3-34, the fault
currents flowing through the MCCB and the devices are equal,
it is necessary to meet the relation: allowable thyristor I
2
t >
MCCB maximum interrupting I
2
t of the MCCB.
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