POWER ELECTRONICS- l TOPIC- PROTECTION CIRCUIT OF SCR Group member Bhimani Viral 130070109006 Suthar Brijesh 130070109007 Chauhan Chirag 130070109008 Chauhan Nimit 130070109009 Chauhan Shailesh 130070109010

WHY PROTECTION OF SCR NEEDED? SCR is a very delicate semiconductor device. So we have to use it in its specified ratings to get desired output. SCR may face different types of threats during its operation due to over voltages, over currents etc. There are different types of thyristor protection schemes available for satisfactory operation in market. Under Power Electronics Protection of a device is an important aspect for its reliable and efficient operation.

TYPE OF PROTECTION FOR SCR Overvoltage protection. Overcurrent protection. High di/dt protection. High dv/dt protection. And some circuit like snubber circuit ,crowbar circuit etc. are also used for scr protection.

OVERVOLTAGE PROTECTION A thyristor may be subjected to internal or external over- voltages. Internal Over-Voltages : After commutation of a thyristor reverse recovery current decays abruptly with high di/dt which causes a high reverse voltage [as, V = L(di/dt) so if di/dt is high then V will be large] that can exceed the rated break-over voltage and the device may be damaged. External Over-Voltages : These are caused due to various reasons in the supply line like lightning, surge conditions (abnormal voltage spike) etc. External over voltage may cause different types of problem in thyristor operation like increase in leakage current, permanent breakdown of junctions, unwanted turn-on of devices etc. So, we have to suppress the over-voltages.

Protective Measure : The effect of over-voltages can be minimized by using non-linear resistors called voltage clamping devices like metal oxide like metal oxide varistor. At the time of normal operation it offers high impedance and acts as it is not present in the circuit. But when the voltage exceeds the rated voltage then it serves as a low impedance path to protect SCR.

Voltage Clamping Device: It is a non-linear resistor called as VARISTOR (VARIable resiSTOR) connected across the SCR. The resistance of varistor will decrease with increase in voltage. During normal operation, varistor has high Resistance and draws only small leakage current. When high voltage appears, it operates in low resistance region and the surge energy is dissipated across the resistance by producing a virtual short- circuit across the SCR.

SCR Over Voltage Protection

OVERCURRENT PROTECTION In an SCR due to over-current, the junction temperature exceeds the rated value and the device gets damaged. Over-current is interrupted by conventional fuses and circuit breakers. The fault current must be interrupted before the SCR gets damaged and only the faulty branches of the network should be isolated. Circuit breaker has long tripping time. So it is used for protecting SCR against continuous over loads (or) against surge currents of long duration.ast acting current limiting fuse is used to protect SCR against large surge currents of very short duration.

SCR Over Current Protection

CROWBAR CIRCUIT

SCR has high surge current ability SCR has high surge current ability.SCR is used in electronic crowbar circuit for overcurrent protection of power converter. In this protection, an additional SCR is connected across the supply which is known as ‘Crowbar SCR’. Current sensing resistor detects the value of converter current.If it exceeds preset value, then gate trigger circuits turn ON the crowbar SCR. So the input terminals are short-circuit by SCR and thus it bypass the converter over current. After some time the main fuse interrupts the fault current.

HIGH di/dt PROTECTION When a thyristor is forward biased and is turned on by a gate pulse, conduction of anode current begins in the immediate neighbourhood of the gate-cathode junction. Thereafter, the current spreads across the whole area of junction. The thyristor design permits the spread of conduction to the whole junction area as rapidly as possible. However, if the rate of rise of anode current, i.e. di/dt, is large as compared to the spread velocity of carriers, local hot spots will be formed near the gate connection on account of high current density.

applying a gate current nearer to (but never greater)This localized heating may destroy the thyristor. Therefore, the rate of rise of anode current at the time of turn-on must be kept below the specified limiting value. The value of di/dt can be maintained below acceptable limit by using a small inductor, called di/dt inductor, in series with the anode circuit. Typical di/dt limit values of SCRs are 20-500 A/µ sec. Local spot heating can also be avoided by ensuring that the conduction spreads to the whole area as rapidly as possible. This can be achieved by than) the maximum specified gate current.

A thyristor requires a minimum time to spread the current conduction uniformly throughout the junctions Otherwise, a localized “hot-spot” heating may occur due to high current density.

High dv/dt protection Protection against high rate of change of voltage i.e. dV/dt also needed for satisfactory operation of SCR. Effect of High Voltage Rise Rate Protection against high rate of voltage rise is necessary because if SCR is not in conduction mode and is forward biased mode then high dV/dt may trigger the SCR, and SCR will not be able to serve it purpose.

Protection Method As we all know capacitor is a good charge storing option and provide less resistance for high frequency voltage so a capacitor may be connected in parallel to the SCR to protect it from high rate of change of voltage i.e. dV/dt. These were the methods of protection of SCR against high di/dt and high dV/dt. Typical values of dv/dt are 20 – 500 V/µsec. False turn-on of a thyristor by large dv/dt can be prevented by using a snubber circuit in parallel with the device.

Snubber circuit A snubber circuit consists of a series combination of resistance Rs and capacitance Cs in parallel with the thyristor as shown in Fig. Strictly speaking, a capacitor Cs in parallel with the device is sufficient to prevent unwanted dv/dt triggering of the SCR. When switch S is closed, a sudden voltage appears across the circuit. Capacitor Cs behaves like a short circuit, therefore voltage across SCR is zero. With the passage of time, voltage across Cs builds up at a slow rate such that dv/dt across Cs and therefore across SCR is less than the specified maximum dv/dt rating of the device. Here the question arises that if Cs is enough to prevent accidental turn-on of the device by dv/dt, what is the need of putting Rs in series with Cs ? The answer to this is as under.

Before SCR is fired by gate pulse, Cs charges to full voltage Vs Before SCR is fired by gate pulse, Cs charges to full voltage Vs. When the SCR is turned on, capacitor discharges through the SCR and sends a current equal to Vs / (resistance of local path formed by Cs and SCR). As this resistance is quite low, the turn-on di/dt will tend to be excessive and as a result, SCR may be destroyed. In order to limit the magnitude of discharge current, a resistance Rs is inserted in series with Cs as shown in Fig. Now when SCR is turned on, initial discharge current Vs/Rs is relatively small and turn-on di/dt is reduced.

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