Transcript AHMEDABAD INSTITUTE OF TECHNOLOGY

AHMEDABAD INSTITUTE OF TECHNOLOGY
SUB : POWER ELECTRONICS.
TOPIC : PROTECTION OF SCR.
ENROLLMENT NO :
130020109030 : Ankit
 130020109031 : Dhruvil
 130020109032 : Jigar
 130020109033 : Jinal
 130020109034 : Nikunj
 130020109035 : Parth
 130020109036 : Suresh
 130020109037 : Nirav
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In the converter circuits voltage transient get generated
due to reverse recovery process of the power devices and
switching taking place in presence of supply and load
inductance.
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The short circuit fault condition take place in converter
will result in heavy fault currents flowing through the
devices.
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The following protection circuit is use for the protection
of SCR.
1. fuse for over current protection.
 2. crowbar circuit for over voltage protection.
 3. snubber circuit for dv/dv and di/dt protection.
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They are briefly explain as follows:
 (1)
Over current protection using fuse:
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In the power converter fault may take place which result
in large fault currents.
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These fault current must be cleared in order to protect the
power devices.
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It is possible to protect the power diodes and thyristors by
connecting fuses in the circuits as shown in figure (a) & (b).
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The glass fuses are slow blowing type fuses.
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Hence they are not use in the power electronic circuits.
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Instead the fast acting semiconductor fuses are used.
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Fast acting fuses are normally used to protect the
semiconductor devices.
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As the fault current increases the fuse connected in the
series with the devices is open circuited to protect the
devices.
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It is better to use that a fuse should be placed in series
with each devices.
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For successful protection of a devices the clearing time
should be shorter than the time required for the power
devices to get damage .
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Advantages of such individual fusing are as follows:
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It ensures better co-ordination between each devices and
its fuse.
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Better utilization of the devices becomes possible.
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Provides protection against the cross conduction .
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In case of devices get damage then that device can be
isolated from the rest of circuit by opening the
corresponding fuse.
 (2)
Crowbar circuit for over voltage protection:
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Fig show the circuit diagram of a crowbar circuit.
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This circuit is used in order to protect circuit where a large
amount of energy is involved.
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The SCR used in the crowbar circuit is used in the normally in
the off state.
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It has a voltage or current sensitive firing circuit.
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If the current through transistor Q goes above a pre-decided
value then the current sensitive firing circuit will turn on the
SCR.
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The SCR will act as a closed switch and will short circuit the
points A and B.
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So in the fault condition the SCR turns on and creates a
virtual short circuit which will blow the fuse link and the
transistor is protected.
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The crowbar circuit can be placed across any converter
circuit which is to be protected.
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Snubber circuit for dv/dt and di/dt protection :
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Function: Protect semiconductor devices by:
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Limiting device voltages during turn-off transients
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Limiting device currents during turn-on transients
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Limiting the rate-of-rise (di/dt) of currents through the
semiconductor device at device turn-on
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Limiting the rate-of-rise (dv/dt) of voltages across the
semiconductor device at device turn-off
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Shaping the switching trajectory of the device as it turns on/off
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Types of Snubber Circuits
1.Unpolarized series R-C snubbers
Used to protect diodes and thyristors
2.Polarized R-C snubbers
Used as turn-off snubbers to shape the turn-on switching
trajectory of controlled switches.
Used as overvoltage snubbers to clamp voltages applied to
controlled switches to safe values.
Limit dv/dt during device turn-off
3.Polarized L-R snubbers
Used as turn-on snubbers to shape the turn-off switching
trajectory of controlled switches.
Limit di/dt during device turn-on
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di/dt protection(turn on snubber) :
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SCR should be protected against high di/dt.
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The rate of change of anode current should not be
allowed to exceed the di/dt rating specified by the
manufacturer.
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Due to high rate of increase of SCR current there is a
possibility of SCR damage.
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To avoid this we must decrease the rate of change of SCR
current.
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This can be achieved by connecting an inductance in series with the
SCR.
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The external inductor protect the SCR against the damage due to
high di/dt.
 dv/dt
protection (turn off snubber) :
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When switch SW in fig(a) is turned on at t=0 a step voltage is across
SCR.
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This voltage increase suddenly and therefore will have a high dv/dt.
(a) snubber circuit.
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Due to high rate of change of supply voltage , accidental
turn on of SCR can take place.
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To avoid this malfunctioning we will have to reduce the rate
of increase of voltage across SCR.
This can be avoid by using a snubber circuit as shown in
figure (a).
When SCR is in the blocking state , the capacitor will charge
through diode D.
Therefore voltage across SCR will increase gradually.
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Thus the rate of change of voltage across SCR is reduced.
When the capacitor is fully charged to the supply voltage ,
the diode is reverse biased and stop conducting.
When SCR is turned on , the fully charged capacitor will
discharge through resistance R and SCR.
Thus R will limit the discharge current of the capacitor at the
time of turn on and prevents damage of SCR due to overcurrent.
THANK YOU.