Thyristor Protection MCQ Quiz - Objective Question with Answer for Thyristor Protection - Download Free PDF

dv/dt Protection of SCR

• When the SCR is forward biased, junctions J₁ and J₃ are forward biased and junction J₂ is reverse biased. This reverse biased junction J2 exhibits the characteristics of a
  capacitor.
• If the rate of forward voltage applied is very high across the SCR, the charging current flows through the junction J₂ is high. This current is enough to turn ON the SCR even without any gate signal.
• This is called as dv/dt triggering of the SCR. This can be reduced by using RC snubber network across the SCR.
• False turn - ON of an SCR by large dv/dt, even without application of gate signal, can be prevented by using a snubber circuit.
• False turn-on of a thyristor by large \(\rm \frac{dv}{dt}\) can be prevented by using a snubber circuit in parallel with the device.

Explanation:

Effect of Over-Voltages on Thyristors

Introduction: Thyristors are semiconductor devices used for switching and rectification in various electronic circuits. However, they are sensitive to over-voltages, which can damage the device or affect its performance. Over-voltages can occur due to various reasons, such as lightning strikes, switching operations, or faults in the electrical system. Therefore, it is crucial to protect thyristors from over-voltages to ensure their reliable operation.

Correct Option Analysis:

The correct option for minimizing the effect of over-voltages on thyristors is:

Option 3: Metal Oxide Varistors

Metal oxide varistors (MOVs) are widely used for protecting electronic devices, including thyristors, from over-voltage transients. MOVs are voltage-dependent resistors that exhibit high resistance at normal operating voltages and low resistance when exposed to over-voltage conditions. This property allows them to effectively clamp the voltage to a safe level, thereby protecting the thyristor from damage.

Working Principle of MOVs:

MOVs are made from a mixture of zinc oxide (ZnO) and other metal oxides, which are sintered together to form a polycrystalline ceramic material. The grain boundaries in the ceramic structure act as potential barriers. Under normal operating conditions, these barriers exhibit high resistance, preventing current flow. However, when an over-voltage transient occurs, the barriers break down, and the resistance drops significantly, allowing the MOV to conduct current and clamp the voltage.

Advantages of Using MOVs:

• High Energy Absorption: MOVs can absorb a large amount of energy from over-voltage transients, providing effective protection for thyristors.
• Fast Response Time: MOVs respond quickly to over-voltage transients, ensuring immediate protection for the thyristor.
• Compact Size: MOVs are available in compact sizes, making them suitable for integration into various electronic circuits.
• Cost-Effective: MOVs are relatively inexpensive compared to other over-voltage protection devices, making them a cost-effective solution.

Applications:

MOVs are used in a wide range of applications to protect electronic devices from over-voltages, including:

• Power Supplies: MOVs are used to protect power supplies and other sensitive components from voltage spikes and transients.
• Communication Systems: MOVs protect communication equipment from over-voltages caused by lightning strikes and other sources.
• Industrial Equipment: MOVs are used in industrial applications to protect thyristors and other semiconductor devices from over-voltage conditions.

Analysis of Other Options:

Option 1: Semiconductor Varistor

Semiconductor varistors are also used for over-voltage protection, but they are not as effective as MOVs for protecting thyristors. Semiconductor varistors have a different construction and may not provide the same level of energy absorption and fast response time as MOVs. Therefore, they are not the preferred choice for this application.

Option 2: Insulator Varistor

Insulator varistors are not commonly used for over-voltage protection in electronic circuits. They are primarily used for high-voltage insulation purposes and do not have the same clamping properties as MOVs. Therefore, they are not suitable for protecting thyristors from over-voltages.

Option 4: Capacitor

Capacitors are used for various purposes in electronic circuits, including filtering and energy storage. However, they are not specifically designed for over-voltage protection. While capacitors can absorb some energy from voltage transients, they do not provide the same level of protection as MOVs. Therefore, capacitors are not the best choice for protecting thyristors from over-voltages.

Conclusion:

Protecting thyristors from over-voltages is crucial for their reliable operation. Among the options provided, metal oxide varistors (MOVs) are the most effective solution for minimizing the effect of over-voltages on thyristors. MOVs offer high energy absorption, fast response time, compact size, and cost-effectiveness, making them the preferred choice for this application. Understanding the properties and advantages of MOVs helps in selecting the appropriate protection device for thyristors and ensuring their reliable performance in electronic circuits.

Secondary current is given by

= 5 × 0.25 = 1.25 A

Secondary Burden of CT = VI

⇒ 5 = 1.25 × V

dv/dt protection:

• When the SCR is forward biased, junctions J1 and J3 are forward biased and junction J2 is reverse biased. This reverse-biased junction J2 exhibits the characteristics of a capacitor.
• If the rate of forward voltage applied is very high across the SCR, charging current flows through the junction J2 is high. This current is enough to turn ON the SCR even without any gate signal.
• This is called as dv/dt triggering of the SCR. This can be reduced by using the RC snubber network across the SCR.
• A snubber circuit consists of a series combination of resistance Rs and capacitance Cs in parallel with the thyristor.
• False turn – ON of an SCR by large dv/dt, even without application of the gate signal can be prevented by using a snubber circuit.

di/dt protection:

• The anode current starts flowing through the SCR when it is turned ON by the application of the gate signal. This anode current takes some finite time to spread across the junctions of an SCR.
• If the rate of rising of anode current (di/dt) is high results a non-uniform spreading of current over the junction. Due to the high current density, this further leads to form local hot spots near the gate-cathode junction. This effect may damage the SCR due to overheating.
• Hence, during the turn ON process of SCR, the di/dt must be kept below the specified limits. To prevent the high rate of change of current, an inductor is connected in series with a thyristor.

I2t rating is used to determine the thermal energy absorption of the device. This rating is required in the choice of a fuse or other protective equipment employed for the SCR. This is the measure of the thermal energy that the SCR can absorb for a short period of time before clearing the fault by the fuse.

Explanation:

The fuse is the only safety device connected in series to the circuit. So the answer is 4).

Wall bracket: This is a support structure for mounting electrical equipment and is not directly involved in the electrical circuit.

Reflectors: These are designed to redirect light and are not part of the electrical circuit.

Ceiling rose: This is a terminal block used to connect wires in a lighting circuit but does not function as a safety device.

Fuse: A fuse is a safety device designed to melt and break the circuit if the current exceeds a safe level. This protects the circuit and connected equipment from damage.

Therefore, only the fuse is a safety device connected in series to the circuit.

Key Points

Holding Current: It is the minimum anode current to maintain the thyristor in the on-state. 

Latching current is always greater than holding current.

Additional Information The thyristor or SCR is a power semiconductor device which is used in power electronic circuits.

They work like a bistable switch and it operates from nonconducting to conducting.

The designing of thyristors can be done with 3-PN junctions and 4 layers.

It includes three terminals namely anode, gate, and cathode. 

dv/dt protection:

• When the SCR is forward biased, junctions J1 and J3 are forward biased and junction J2 is reverse biased. This reverse-biased junction J2 exhibits the characteristics of a capacitor.
• If the rate of the forward voltage applied is very high across the SCR, charging current flows through the junction J2 is high. This current is enough to turn ON the SCR even without any gate signal.
• This is called as dv/dt triggering of the SCR. 
• dv/dt rating of thyristor indicates the maximum rate of rise of anode voltage that will not trigger the device without any gate signal. We use a snubber circuit to control this limit.
• A snubber circuit consists of a series combination of resistance Rs and capacitance Cs in parallel with the thyristor.
• False turn – ON of an SCR by large dv/dt, even without application of a gate signal can be prevented by using a snubber circuit.
• Snubber limits the dv/dt across the switching device during the turnoff of the device.

SCRs are constructed from silicon and are most commonly used for converting AC current to DC current (rectification), hence the name Silicon controlled rectifier.

They are also used in other applications such as regulation of power, inversion, etc.

The SCRs have an ability to handle high value of current and Voltage hence they are used in most of the industrial applications.

SCR has three terminals namely Anode (A), Cathode (K) and gate (G), and it can be turned ON or OFF by controlling the biasing conditions or the gate input.

SCR can operate in three different modes

• Forward Blocking mode
• Forward Conduction mode
• Reverse Blocking mode

Among all of the options, high rate of rise of voltage does not cause permanent damage to an SCR

dv/dt Protection:

• When the SCR is forward biased, junctions J1 and J3 are forward biased and junction J2 is reverse biased. This reverse biased junction J2 exhibits the characteristics of a capacitor.
• If the rate of forward voltage applied is very high across the SCR, charging current flows through the junction J2 is high. This current is enough to turn ON the SCR even without any gate signal.
• This is called as dv/dt triggering of the SCR. This can be reduced by using RC snubber network across the SCR.
• A snubber circuit consists of a series combination of resistance Rs and capacitance Cs in parallel with the thyristor.
• False turn – ON of an SCR by large dv/dt, even without application of gate signal can be prevented by using a snubber circuit.

dv/dt protection:

• When the SCR is forward biased, junctions J1 and J3 are forward biased and junction J2 is reverse biased. This reverse-biased junction J2 exhibits the characteristics of a capacitor.
• If the rate of forward voltage applied is very high across the SCR, charging current flows through the junction J2 is high. This current is enough to turn ON the SCR even without any gate signal.
• This is called as dv/dt triggering of the SCR. This can be reduced by using the RC snubber network across the SCR.
• A snubber circuit consists of a series combination of resistance Rs and capacitance Cs in parallel with the thyristor.
• False turn – ON of an SCR by large dv/dt, even without application of the gate signal can be prevented by using a snubber circuit.

di/dt protection:

• The anode current starts flowing through the SCR when it is turned ON by the application of the gate signal. This anode current takes some finite time to spread across the junctions of an SCR.
• If the rate of rising of anode current (di/dt) is high results a non-uniform spreading of current over the junction. Due to the high current density, this further leads to form local hot spots near the gate-cathode junction. This effect may damage the SCR due to overheating.
• Hence, during the turn ON process of SCR, the di/dt must be kept below the specified limits. To prevent the high rate of change of current, an inductor is connected in series with a thyristor.

I2t rating is used to determine the thermal energy absorption of the device. This rating is required in the choice of a fuse or other protective equipment employed for the SCR. This is the measure of the thermal energy that the SCR can absorb for a short period of time before clearing the fault by the fuse.

Concept:

\(\frac{{di}}{{dt}}\) protection of SCR or TRAIC:

• When a thyristor is turned on by gate pulse then charge carriers spread through its junction rapidly.
• But if the rate of rise of anode current, that is 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.
• To avoid local hot spots we use an inductor in series with the device as it prevents a high rate of change of current through it.

​

Let's consider load as a purely resistive load of R Ω

The expression of current  for series RL circuit is Given by

i = \(\frac{V_s}{R}\left[ {1 - {e^{\frac{{ - t}}{τ }}}} \right]\)

Where

Vs = source voltage

R = load resistance

τ = time constant = L/R

By differentiating the above equation with respect to time, we get

\(\frac{{di}}{{dt}} = \frac{V_s}{L}\;{e^{\frac{{ - t}}{\tau }}}\)

We can get the maximum value of di/dt by taking t = 0 then,

\({\left( {\frac{{di}}{{dt}}} \right)_{max}} = \frac{{{V_s}}}{L}\;\)

Calculation:

Given that

Supply voltage Vs = 120V

The maximum value of di/dt = 50 A / μ sec

We know that \({\left( {\frac{{di}}{{dt}}} \right)_{max}} = \frac{{{V_s}}}{L}\;\)

⇒ 50 = 120 / L

⇒ L = 2.4 μH

∴ The value of the Snubber inductor required for protection against di/dt is equal to 2.4 μH.

The correct option is Option 2.
Concept:

• When to gate signal is applied across the SCR for sufficient time, it is turned on.
• After this, the Anode current in SCR starts spreading across the Device's Junctions. This should be a uniform process.
• If this is not th case, the rate of rise of anode current (di/dt) will be high enough and so this spread will be uniform 
• And SCR gets overheated due to the formation of local hotspots(local points of charge carriers concentration) .

So, we need to limit this di/dt under certain specified limits.

This is achieved by putting an inductor in series with SCR. 

Additional Information

Other protections of thyristor:

• The characteristic VI- curve for Varistor is as follows:-

1. Overcurrent protection: We must connect either fuse or Circuit breaker in series with the SCR for overcurrent protection

2. Overvoltage protection: We should use Varistor ( a device that behaves as a variable resistor) for its overvoltage protection.

3. dV/dt protection:

 \(Ic = C{dV\over dt}\)

Where I_c = Current through the SCR junction capacitance developed

• At high dV/dt, SCR may Turn ON before the gate pulse is given.
• It is an accidental Turn-On.
• Snubber circuit is used for dV/dt protection.

The different types of Snubber circuits are given below:

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-off 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-on switching trajectory of controlled switches.
• Limit di/dt during device turn-on

dv/dt protection:

• When the SCR is forward biased, junctions J1 and J3 are forward biased and junction J2 is reverse biased. This reverse biased junction J2 exhibits the characteristics of a capacitor.
• If the rate of forward voltage applied is very high across the SCR, charging current flows through the junction J2 is high. This current is enough to turn ON the SCR even without any gate signal.
• This is called as dv/dt triggering of the SCR. This can be reduced by using RC snubber network across the SCR.
• A snubber circuit consists of a series combination of resistance Rs and capacitance Cs in parallel with the thyristor.
• False turn – ON of an SCR by large dv/dt, even without application of gate signal can be prevented by using a snubber circuit.

dv/dt protection:

When the SCR is forward biased, junctions J1 and J3 are forward biased and junction J2 is reverse biased. This reverse-biased junction J2 exhibits the characteristics of a capacitor.

If the rate of a forward voltage applied is very high across the SCR, the charging current flows through the junction J2 is high. This current is enough to turn ON the SCR even without any gate signal.

This is called as dv/dt triggering of the SCR.

This can be reduced by using the RC snubber network across the SCR.

Additional Information

di/dt protection:

The anode current starts flowing through the SCR when it is turned ON by the application of the gate signal. This anode current takes some finite time to spread across the junctions of an SCR.

If the rate of rising of anode current (di/dt) is high results a non-uniform spreading of current over the junction. Due to the high current density, this further leads to form local hot spots near the gate-cathode junction. This effect may damage the SCR due to overheating.

Hence, during turn ON process of SCR, the di/dt must be kept below the specified limits. To prevent the high rate of change of current, an inductor is connected in series with a thyristor.