Thyristor Protection - Notes | Study Power Electronics - Electrical Engineering (EE)

Protection of a device is an important aspect for its reliable and efficient operation. Silicon Controlled Rectifier (SCR) are a very delicate semiconductor device. 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

• It 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 varistors. 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.

• Overcurrent mainly occurs due to different types of faults in the circuit. Due to overcurrent 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 overcurrent by using Circuit Breaker (CB) and fast acting current limiting fuses (FACLF). CBs 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.

• When a thyristor is in forward blocking state then only J₂ junction is reverse biased which acts as a capacitor having constant capacitance value C_j (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.
• 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.

• When a thyristor is turned on by gate pulse then charge carriers spread through its junction rapidly. But if rate of rise of anode current, i.e. di/dt is greater than the spreading of charge carriers then localized heat generation will take place which is known as local hot spots. This may damage the thyristor.
• Protective Measure: To avoid local hot spots we use an inductor in series with the device as it prevents high rate of change of current through it.

• With the increase in the temperature of the junction, insulation may get failed. So we have to take proper measures to limit the temperature rise.
• Protective Measure: We can achieve this by mounting the thyristor on heat sink which is mainly made by high thermal conductivity metals like aluminum (Al), Copper (Cu) etc. Mainly aluminum (Al) is used due to its low cost. There are several types of mounting techniques for SCR such as – Lead-mounting, stud-mounting, Bolt-down mounting, press-fit mounting, press-pack mounting etc.

Gate Protection of Thyristor

• Like a thyristor, Gate circuit should also be protected from overvoltages and overcurrents. Overvoltages in the gate circuit can cause false triggering and overcurrent can cause high junction temperature.
• Protective Measure: Overvoltages thyristor protection is achieved by using a zener diode and a resistor can be used to protect the gate circuit from overcurrent. Noise in gate circuit can also cause false triggering which can be avoided by using a resistor and a capacitor in parallel. A diode (D) may be connected in series or in parallel with the gate to protect it from high reverse voltage.

Overall Protection of a Thyristor

• Lead mounting: In such mounting technique housing of SCR itself is used as heat radiator. Hence no need of additional heat zink arrangement. Hence, this technique of thyristor Protection is generally used for low current application, normally less than one ampere.
• Stud mounting: The anode of the thyristor is in the form of threaded stud which is screwed to a metalling heat sink block.
• Bolt-down mounting: Here the device is connected to the heat sink with the help of nut-bolt mechanism. It is mainly used in small and medium rating circuit.
• Press fit mounting: This kind of mounting is obtained by inserting the whole SCR into the metallic block. It is used in high rating circuit.
• Press-Pack mounting: This kind of mounting for thyristor protection is obtained by sandwiching the thyristor between to heat sink with the help of clamps. It is used for very high rating circuit.