Thyristor Protection or SCR Protection
Protection of a device is an important aspect for its reliable and efficient operation. 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 of the device like
- Over voltage protection.
- Over current protection.
- High dv/dt protection.
- High di/dt protection.
- Thermal protection.
Over Voltage ProtectionIt is the most important protection scheme w. r. t. others as thyristors are very sensitive to over voltages. Maximum time thyristor failures happen due to over-voltage transients. 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.
Over Current ProtectionOver current mainly occurs due to different types of faults in the circuit. Due to over current i2R loss will increase and high generation of heat may take place that can exceed the permissible limit and burn the device.
Protective Measure : SCR can be protected from over current by using CB and fast acting current limiting fuses (FACLF). CB are used for protection of thyristor against continuous overloads or against surge currents of long duration as a CB has long tripping time. But fast-acting fuses is used for protecting SCR against high surge current of very short duration.
High dv/dt ProtectionWhen a thyristor is in forward blocking state then only J2 junction is reverse biased which acts as a capacitor having constant capacitance value Cj (junction capacitance). As we know that current through capacitor follows the relation
Hence leakage current through the J2 junction which is nothing but the leakage current through the device will increase with the increase in dva/dt i.e. rate of change of applied voltage across the thyristor. This current can turn-on the device even when the gate signal is absent. This is called dv/dt triggering and must be avoided which can be achieved by using Snubber circuit in parallel with the device. Protective Measure : Snubber Circuit : It consists of a capacitor connected in series with a resistor which is applied parallel with the thyristor, when S is closed then voltage Vs is applied across the device as well as Cs suddenly. At first Snubber circuit behaves like a short circuit. Therefore voltage across the device is zero. Gradually voltage across Cs builds up at a slow rate. So dv/dt across the thyristor will stay in allowable range. Before turning on of thyristor Cs is fully charged and after turning on of thyristor it discharges through the SCR. This discharging current can be limited with the help of a resistance (Rs) connected in series with the capacitor (Cs) to keep the value of current and rate of change of current in a safe limit.