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

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. The short circuit fault condition take place in converter will result in heavy fault currents flowing through the devices. The following protection circuit is use for the protection of SCR.

(1) Over current protection using fuse: 1. fuse for over current protection. 2. crowbar circuit for over voltage protection. 3. snubber circuit for dv/dv and di/dt protection. They are briefly explain as follows: (1) Over current protection using fuse: In the power converter fault may take place which result in large fault currents. These fault current must be cleared in order to protect the power devices.

It is possible to protect the power diodes and thyristors by connecting fuses in the circuits as shown in figure (a) & (b).

The glass fuses are slow blowing type fuses. Hence they are not use in the power electronic circuits. Instead the fast acting semiconductor fuses are used.

Fast acting fuses are normally used to protect the semiconductor devices. As the fault current increases the fuse connected in the series with the devices is open circuited to protect the devices. It is better to use that a fuse should be placed in series with each devices. For successful protection of a devices the clearing time should be shorter than the time required for the power devices to get damage .

Advantages of such individual fusing are as follows: It ensures better co-ordination between each devices and its fuse. Better utilization of the devices becomes possible. Provides protection against the cross conduction . 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: Fig show the circuit diagram of a crowbar circuit. This circuit is used in order to protect circuit where a large amount of energy is involved. The SCR used in the crowbar circuit is used in the normally in the off state. It has a voltage or current sensitive firing circuit. If the current through transistor Q goes above a pre-decided value then the current sensitive firing circuit will turn on the SCR. The SCR will act as a closed switch and will short circuit the points A and B.

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. The crowbar circuit can be placed across any converter circuit which is to be protected.

Snubber circuit for dv/dt and di/dt protection : Function: Protect semiconductor devices by: Limiting device voltages during turn-off transients Limiting device currents during turn-on transients Limiting the rate-of-rise (di/dt) of currents through the semiconductor device at device turn-on Limiting the rate-of-rise (dv/dt) of voltages across the semiconductor device at device turn-off Shaping the switching trajectory of the device as it turns on/off

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

di/dt protection(turn on snubber) : SCR should be protected against high di/dt. The rate of change of anode current should not be allowed to exceed the di/dt rating specified by the manufacturer. Due to high rate of increase of SCR current there is a possibility of SCR damage. To avoid this we must decrease the rate of change of SCR current.

dv/dt protection (turn off snubber) : This can be achieved by connecting an inductance in series with the SCR. The external inductor protect the SCR against the damage due to high di/dt. dv/dt protection (turn off snubber) : When switch SW in fig(a) is turned on at t=0 a step voltage is across SCR. This voltage increase suddenly and therefore will have a high dv/dt.

(a) snubber circuit. Due to high rate of change of supply voltage , accidental turn on of SCR can take place.

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.

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 over- current.

Thank you.