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Selection and application
3-9 MCCBs for semiconductor circuit
Fig. 3-29 Example of the path of overcurrent flow in thyristor converters
M
Commutation failure due to loss of
the power source
Commutation failure due to
faulty ignition
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Device breakdown
Short-circuit
Commutation failure
Table 3-21 and Fig. 3-29 show that, to protect normal devices,
an overcurrent protection device must be installed, for each
element (arm) in the conversion or on the AC side, for each
element in an inversion or on the DC side.
The Ward-Leonard thyristor configuration in which the speed
of the DC motor is controlled by thyristor phase control
provides two modes: one in which the thyristor converter is run
as a conversion (driving the DC motor), and one in which the
thyristor converter is run as an inversion (regenerative braking
of the DC motor). Installation of protective devices should be
examined by considering possible failures in these two modes.
Table 3-21 Possible causes of overcurrents in line-commutated thyristor converters
Causes of overcurrent
Overcurrent conditions
Category
Phenomena
Possible cause
During conversion
During inversion
Internal
faults
Misfiring
Thyristors fail to fire. Suspect a
broken wire in the gate circuit
or a fault in the controller.
Decreased output voltage. If some of
the SCRs connected in parallel misfire,
the remaining SCRs in that arm carry an
overcurrent.
When all SCRs in one arm misfire, com-
mutation fails, resulting in a short circuit
on the DC side.
If some SCRs in one arm misfire, the nor-
mal SCRs in that arm carry an overcur-
rent.
Faulty ignition
SCRs fire when they should
maintain forward blocking.
Suspect an excessive forward
voltage, excessive dv/dt, or
gate noise.
If SCRs are connected in parallel, current
concentrates in the SCRs that misfire,
resulting in an overcurrent.
Commutation fails, resulting in a short
circuit occurring on the DC side.
Device break-
down
A short circuit has resulted
from the loss of SCR forward
blocking capability. Suspect an
excessive junction tempera-
ture or overvoltage input.
A short circuit has occurred in the input
AC source.
An AC interphase short circuit generated
a backward current that caused a transi-
tion to commutation failure, resulting in a
short circuit on the DC side.
External
faults
Commutation
failure
Suspect power failure or a
broken wire in the power sup-
ply circuit.
With an inductive load, current flows
through the arm that had been turned on
until loss of the power source for a rela-
tively long period of time raises junction
temperature.
The loss of the commutating power
source causes a commutation failure,
resulting in a short circuit on the DC side.
Short circuit in
load side
Suspect a short circuit in the
DC circuit or flashover in the
DC motor.
A short circuit in the AC input source. The
overcurrent flowing through the SCRs
varies with the short-circuit point, or the
presence or absence of a DC reactor.
Commutation fails as the AC voltage
required for commutation is lost, but no
overcurrent flows through the SCRs.
3-9-1
Faults and overcurrents in thyristor
converters
The possible causes of overcurrents in thyristor converters
can be broadly classified into two categories: internal faults
in the converters, and those external to the converters. Table
3-21 lists the typical possible causes of overcurrents in
line- commutated thyristor converters and their associated
conditions. Fig. 3-29 shows examples of the path of
overcurrent flow.
3-9 MCCBs for semiconductor circuit
Circuits containing semiconductor devices such as thyristors
and diodes differ in the following respects:
s 4HECURRENTmOWINGTHROUGHTHE-##"DEPENDSONWHERE
the MCCB is installed in the circuit.
s 4HEFAULTCURRENTDEPENDSONFAULTMODES
s 4HEOVERCURRENTCAPACITYOFSEMICONDUCTORDEVICESISLOWER
than that of other electrical apparatus.
Allowance should be made for these characteristics when
selecting an MCCB.
