SCR: Operation and Protection | Power Electronics - Electrical Engineering (EE) PDF Download

Series  Parallel Operation of SCR

• When single SCR is not sufficient to provide required voltage or current rating, then more SCR's, are connected in series (to meet higher voltage demand) or in parallel (to meet higher current demand). 
• In series or parallel operation, SCRs are not utilized fully (as characteristics of SCR are not matched exactly). To measure the degree of utilization of thyristors string efficiency (x) is used and defined as

* (Number of SCRs in string)

When η = 1 each SCR in string has 100% identical characteristic or utilization of thyristor will be maximum. But it is not possible practically. 

• Derating factor : A measure of the reliability of string is given by a factor called derating factor. DRF = 1 – η

Series Operation

• Series connected SCRs (1) and (2) carry same current I_O but due to unmatched static characteristic as shown in figure voltages are different say V₁ and V₂. So, h = V₁ + V₂/2V₁

•  This shows that even though SCRs have identical ratings, voltage shared by each is not the same and string efficiency is therefore less than one. 

• A uniform voltage distribution in steady state can be achieved by connecting suitable resistance across each SCR. A more practical way of obtaining a reasonably uniform voltage distribution during steady state of series connected SCRs to connect the same value of shunt resistance R across each SCR as shown in figure below. 
• For the sake of simplicity, let us consider out of the string of n SCRs, only 1^st is different. Let leakage current for SCR be I_bmn and for remaining (n-1) SCRs be I_bmx such as I_bmx > I_bmn
  Let string current be I
  Hence I₁ = I – I_bmn and I₂ I – I_bms
  Voltage across SCRI is V_bm = I₁R

Voltage across each of remaining (n – 1) SCRs = (n – 1) I₂R

For a string voltage V_s, the voltage equation for the series circuit will be 

V_s = I₁ R + (n –1)I₂ R

= V_bm + (n – 1) R [I₁ – (I_bmx – I_bmn)]

let (I_bmx – I_bmn) = ΔIb

RI₁ = Vbm

• Not only the static characteristics need to be identical but also the transient or dynamic characteristic.
  Identical dynamic characteristic means equalization of potential is needed during turning on and tuning off periods. These periods may not be exactly equal for all the thyristors connected in series. 

• To equalize these transient voltages, a dynamic equalizing circuit consisting of a capacitor is connected across each SCR is used. 
• The R_c connected in parallel with Diode D in dynamic equalizing circuit are provided to limit discharging current of capacitor through SCR.

Parallel Operation 

When current required by the load is more than the rated current of a single thyristor, SCRs are connected in parallel in string. For equal sharing of currents V-I characteristics of each SCRs during forward conduction must be identical as far as possible.

For parallel-connected SCRs, voltage V_T across them must be equal. Figure (b) show that for a same voltage drop V_T, SCR₁ shares a rated current I₁ whereas SCR₂ carries current I₂ much less than rated current I₁. The total current carried by the unit is I₁ + I₂ and not the rated current 2I₁ as required. Therefore string efficiency is given by:

• When SCRs are connected in parallel, all units must operate at the same temperature as far as possible. This is done by having a common heat sink. 
• Unequal current distribution in a parallel unit is also caused by the inductive effect of current carrying conductors. This unequal current distribution can be avoided by mounting the SCRs symmetrically on the heat sink.

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.

Over Current Protection

• 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.

High dv/dt Protection

• 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.

High di/dt Protection

• 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.

High Temperature Protection

• 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.