75
3
Selection and application
3-9 MCCBs for semiconductor circuit
3-9-3 Protecting thyristors from overcurrent
The following methods are commonly used to protect
semiconductor devices such as thyristors and diodes from
overcurrent:
Direct protection
Current-limiting fuses
Circuit
breakers
Circuit
protectors
High-speed DC circuit breakers
Indirect protection
DC current limiting control
Gate
control
These combinations can protect devices from all types of
overcurrent, but they are a very costly method. It is best to
achieve a balanced system that considers the importance of
the equipment, the desired reliability, the cost performance,
the potential faults and the probability of those faults when
designing a protective system for semiconductor equipment.
When devices must be fully protected in large-capacity
replacement equipment (in which devices are expensive) and
critical equipment, for example, it may be quite expensive,
but the protective combination described above is sometimes
needed for added assurance. In equipment where cost is
critical on the other hand, every effort must be made to at least
protect against the most likely faults.
(1) Protection in the overload current region
The overcurrent immunity of a thyristor, as represented in
Fig. 3-30, is expressed with the period of time over which the
thyristor can tolerate the peak value of positive half cycles of a
sinusoidal current flowing through it.
The overload characteristics indicated by the solid lines
suggest that the junction temperature remains within tolerable
limits even when an overcurrent flows. The limit characteristic
curves indicated by the dotted lines, generally known as
allowable surge-on current limits, indicate limits of the thermal
immunity of the device. Hence, the appropriate protective
device to be selected must be capable of interrupting the
current within the limits of time shown in Fig. 3-30. When
making this selection, however, remember that the operating
characteristics of MCCBs (including current-limiting fuses)
are generally expressed using effective values of sinusoidal
current, but in the case shown in Fig. 3-30, characteristics are
expressed using the peak value of sinusoidal current.
It is therefore necessary to convert the overcurrent immunity
characteristics expressed on the effective value base to
compare with the characteristics of the protective device. Fig.
3-31 shows an example of a coefficient curve for converting to
effective values.
Fig. 3-30 Overcurrent immunity characteristics of semiconductor
devices
i
t
Rated load
Overload characteristics
Limit characteristics
No load
Rated load
From no load
From a 50% load
From a 100% load
Fig. 3-31 Coefficient for converting to effective values
2.0
1.9
1.8
1.7
1.6
1.5
1.4
Time (seconds)
0.01 0.02 0.03 0.05 0.07 0.1 0.2 0.3 0.5 0.7 1 2 3 5 7 10
Ieff=
Ip
K
Ip
K=
2(2n-1)
n
Con
v
ersion coefficient (K)
0
2
3
4
5
(2) Protection in the short-circuit region
If a short circuit in the load occurs during forward conversion
(rectification) or if an arm short circuit results from device
breakdown, the overcurrent must be interrupted in an
extremely short period of time to protect the normal devices
against the resulting large current.
In such a region, a protective device should be selected to
meet the following relation with respect to the allowable limit
value of I
2
t of the devices:
Allowable I
2
t of device > I
2
t flowing through device when the
protective device trips
Fuses for protecting semiconductors provide better current-
limiting performance than MCCBs, that is, fuses are better
suited for protecting thyristors against overcurrent caused by
short circuits.
(3) Use of MCCBs on the AC side of thyristors
When MCCBs are installed on the AC side of a converter
as shown in Fig. 3-32, their primary duty will be interrupting
the fault current during forward conversion on rectification.
From the standpoint of protection coordination with devices,
instantaneous trip type MCCBs will be more suitable than
MCCBs for line protection.
An instantaneous trip type circuit breaker is tripped within
one cycle of any current exceeding its preset trip current Ii.
Accordingly, if MCCB preset currents are specified as shown
in (a) and (b) in Fig. 3-33, overcurrent protection is available in
region B.
If the instantaneous trip characteristics of an MCCB are preset
as indicated by 2 in (a), Fig. 3-33, an additional protective
relay such as an overcurrent relay will be needed to provide
protection in region A.
There will be no problem as long as the maximum current
flowing through the circuit does not enter region C. Circuits
in which fault currents are likely to flow in region C, however,
would benefit by installation of reactors to suppress the fault
current, or fuses for protecting semiconductors.
Fig. 3-32 MCCBs for AC applications
MCCB