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

Explanation:

Subcycle Surge Current Rating of a Thyristor

Definition: The subcycle surge current rating of a thyristor refers to the maximum permissible current the thyristor can safely conduct during a short-duration (subcycle) surge event. This rating is crucial for ensuring the device's reliability and protecting it from damage during transient overcurrent conditions. The subcycle surge current occurs over a fraction of the supply frequency cycle, and its magnitude is directly related to the duration of the surge and the thermal capacity of the thyristor.

Mathematical Expression:

The subcycle surge current rating \(I_{sb}\) is mathematically expressed as:

\(I_{sb} = I \sqrt{\frac{T}{t}}\)

Where:

• \(I_{sb}\): Subcycle surge current rating
• \(I\): Normal current rating of the thyristor
• \(T\): Time for one half-cycle of the supply frequency (e.g., for a 50 Hz supply, \(T = \frac{1}{2 \times 50} = 0.01 \, \text{seconds}\))
• \(t\): Duration of the subcycle surge

Derivation and Explanation:

The subcycle surge current rating is derived from the thermal energy considerations of the thyristor. The heat generated in the thyristor during a surge event is proportional to the square of the current (\(I^2\)) and the duration of the surge (\(t\)). To ensure the thyristor does not overheat, the total energy dissipated during the surge must not exceed the thyristor's thermal capacity. The equation \(I_{sb} = I \sqrt{\frac{T}{t}}\) ensures this condition is met by relating the surge current to the normal current rating and the duration of the surge.

Key Insight: The subcycle surge current rating increases as the duration of the surge decreases because the thyristor has less time to accumulate excessive heat. The square root relationship between \(T/t\) and \(I_{sb}\) reflects this thermal behavior.

Correct Option Analysis:

The correct option is:

Option 1: \(I_{sb} = I \sqrt{\frac{T}{t}}\)

This equation correctly represents the subcycle surge current rating of a thyristor, considering the thermal capacity and the energy dissipation relationship. The surge current is proportional to the square root of the ratio of the half-cycle duration (\(T\)) to the subcycle surge duration (\(t\)), ensuring the device's safety and reliability.

Important Information

To further understand the analysis, let’s evaluate the other options:

Option 2: \(I_{sb} = I \sqrt{\frac{t}{T}}\)

This option is incorrect because it reverses the relationship between \(T\) and \(t\). In this formulation, the surge current would decrease as the surge duration (\(t\)) decreases, which contradicts the thermal behavior of the thyristor. The correct equation shows that the surge current increases as the surge duration decreases.

Option 3: \(I_{sb} = t \sqrt{\frac{I}{T}}\)

This option is incorrect as it introduces a linear dependence on \(t\), which is not valid in the context of the thyristor's thermal behavior. The surge current rating is not directly proportional to the surge duration; instead, it depends on the square root of the ratio of \(T\) to \(t\).

Option 4: \(I_{sb} = T \sqrt{\frac{I}{t}}\)

This option is also incorrect because it incorrectly introduces a direct proportionality to \(T\) and inversely to \(t\). The correct relationship involves the square root of the ratio of \(T\) to \(t\), and \(I\) is the normal current rating, not under a square root.

Conclusion:

The subcycle surge current rating of a thyristor is a critical parameter for ensuring the device's safe operation during transient overcurrent events. The correct equation, \(I_{sb} = I \sqrt{\frac{T}{t}}\), accurately captures the thermal constraints of the thyristor and provides a reliable basis for its design and application. A proper understanding of this relationship is essential for selecting and operating thyristors in power electronic circuits.

Surge current rating

Surge current rating specifies the maximum non-repetitive or surge current that the SCR can withstand for a limited number of times during its life span.

This rating is specified in terms of the number of surge cycles with corresponding surge current peaks.

The surge current is always calculated for a half cycle of the total surge period.

The surge current in one complete cycle is given by:

Isb = I\(\rm\sqrt{\frac{T}{ t}}\)

It is usual to measure the surge duration in terms of the number of cycles of normal power frequency of 50 or 60 Hz.

For a 50 Hz supply,

 \(T={1\over f}={1\over 50}\)

T = 20 ms

But for surge, half of 20 ms = 10 ms will be the value of the surge period.

Isb = \(\sqrt{10\times 10^{-3}\over t}\)

Isb = I\(\rm\sqrt{\frac{1}{100 t}}\)

where, I_sb = Subtransient surge current

I = Surge current in complete one cycle

Peak Working Revere Voltage (VRWM):

• It is the maximum reverse voltage (anode is negative with respect to cathode) which can be withstood by the thyristor repeatedly or periodically.
• It is nothing but the peak negative value of the AC sinusoidal voltage.
   

Peak Repetitive Revere Voltage (VRRM):

• It is the value of transient voltage that can be withstood by SCR in reverse bias at maximum allowable temperature.
• The reason behind the appearance of this voltage is also the same as VDRM.
   

Anode Voltage Rating:

• This rating gives us a brief idea about withstanding power of a thyristor in forward blocking made in the absence of gate current.
   

Finger Voltage of SCR (VFV):

• The minimum value of voltage which must be applied between anode and cathode for turning off the device by gate triggering.
• Generally, this voltage value is a little more than the normal ON state voltage drop.

Concept:

String Efficiency of SCR: String Efficiency of SCR is the degree of capacity utilization of individual SCRs in a string of series / parallel connected SCRs. String efficiency is always less than 1.

String Efficiency \(= {{V} \over V_1 \times N} = {{I} \over I_1 \times N}\)

Where,

• V = Actual voltage of whole string
• I = Actual current of whole string
• V₁ = Voltage rating of one SCR
• I1 = Current rating of one SCR
• N = Total number of SCRs in a string
   

Derating Factor (DFR) of SCR: Derating Factor is the amount by which the string efficiency deviates from unity or 100%.

DFR = [1 – String Efficiency]

DFR = [100 – % String Efficiency]

• Derating Factor gives an idea of unused capacity available in a sting of SCRs.
• That's why it is a measure of reliability of string.
• The lesser the value of sting efficiency, the lesser will be voltage / current sharing by the individual SCRs.
• It means, more unused capacity will be available with the string and hence more DFR.
• But lesser string efficiency increases the cost of string.
• So that a compromise is made in between economy and reliability by properly designing a value of string efficiency.

Calculation :

Given, String efficiency = 80% = 0.8

∴ Derating factor (DFR) = 1 - 0.8

∴ DFR = 0.2

Current Ratings of SCR:

• SCR is made of a semiconductor that is very much thermal sensitive.
• Even due to a short time over current, the temperature of the device may rise to such a high value that it may cross its maximum allowable limit.
• Hence there will be a high chance of permanent destruction of the device.
• For this reason, the current ratings of SCR are a very essential part to protect the SCR.
• Some of the current ratings used in the industry to specify the SCR device are given below.
   

Forward Current Rating:

• The maximum value of anode current, that an SCR can handle safely (without any damage), is called the forward current rating.
• The usual current rating of SCRs is from about 30 A to 100 A.
• In case the current exceeds the forward current rating, the SCR may get damaged due to intensive heating at the junctions.
   

On-state Current:

• When the device is in conduction, it carries a load current determined by the supply voltage and the load.
• On-state current is defined in terms of average and RMS values.
• I_av is the average value of maximum continuous sinusoidal on-state current (frequency 40-60 Hz, conduction angle 180°) which should not be exceeded even with intensive cooling.
• The temperature at which the current is permissible has to be mentioned.
• It is this current that determines the application of the device.
• I_rms is the RMS value of maximum continuous sinusoidal on-state current (frequency 40-60 Hz, conduction angle 180°) which should not be exceeded even with intensive cooling.
   

Latching Current:

• This is the rating of current below which the SCR can’t be turned on even if the gate signal is applied.
• That means this much anode current must rise to turn on the device.
• The gate pulse must be continuous until the anode current is greater or equal to the latching current of the thyristor otherwise the device will fail to be turned on.
   

Holding Current:

• It is the minimum on-state current required to keep the SCR in conducting state without any gate signal.
• Its usual value is 5 mA.
   

Surge Current Rating:

• The peak anode current an SCR can handle for a short duration is known as surge current rating.
• It is the maximum admissible peak value of a sinusoidal half cycle of 10 ms duration at a frequency of 50 Hz.
• The value is specified at a given junction temperature.
• During maximum surge on-state current the junction temperature is exceeded though temporarily and forward blocking capabilities are lost for a short period.
• The maximum surge on-state current should only occur occasionally.
• If a thyristor operates under its repetitive voltage and current ratings, its maximum allowable temperature is never exceeded.
• But an SCR may fall into an abnormal operating condition due to a fault in the circuit.
• To overcome this problem, a maximum allowable surge current rating is also specified by the manufacturer.
• This rating specifies the maximum non-repetitive surge current, that the device can withstand.
• This rating is specified depending on the number of surge cycles.
• I²t Value: I²t value is the time integral of the square of the maximum sinusoidal on-state current.

Concept:

String Efficiency of SCR: String Efficiency of SCR is the degree of capacity utilization of individual SCRs in a string of series / parallel connected SCRs. String efficiency is always less than 1.

String Efficiency \(= {{V} \over V_1 \times N} = {{I} \over I_1 \times N}\)

Where,

• V = Actual voltage of whole string
• I = Actual current of whole string
• V₁ = Voltage rating of one SCR
• I1 = Current rating of one SCR
• N = Total number of SCRs in a string
   

Derating Factor (DFR) of SCR: Derating Factor is the amount by which the string efficiency deviates from unity or 100%.

DFR = [1 – String Efficiency]

DFR = [100 – % String Efficiency]

• Derating Factor gives an idea of unused capacity available in a sting of SCRs.
• That's why it is a measure of reliability of string.
• The lesser the value of sting efficiency, the lesser will be voltage / current sharing by the individual SCRs.
• It means, more unused capacity will be available with the string and hence more DFR.
• But lesser string efficiency increases the cost of string.
• So that a compromise is made in between economy and reliability by properly designing a value of string efficiency.

Calculation :

Given, String efficiency = 80% = 0.8

∴ Derating factor (DFR) = 1 - 0.8

∴ DFR = 0.2

Peak Working Revere Voltage (VRWM):

• It is the maximum reverse voltage (anode is negative with respect to cathode) which can be withstood by the thyristor repeatedly or periodically.
• It is nothing but the peak negative value of the AC sinusoidal voltage.
   

Peak Repetitive Revere Voltage (VRRM):

• It is the value of transient voltage that can be withstood by SCR in reverse bias at maximum allowable temperature.
• The reason behind the appearance of this voltage is also the same as VDRM.
   

Anode Voltage Rating:

• This rating gives us a brief idea about withstanding power of a thyristor in forward blocking made in the absence of gate current.
   

Finger Voltage of SCR (VFV):

• The minimum value of voltage which must be applied between anode and cathode for turning off the device by gate triggering.
• Generally, this voltage value is a little more than the normal ON state voltage drop.

Surge current rating

Surge current rating specifies the maximum non-repetitive or surge current that the SCR can withstand for a limited number of times during its life span.

This rating is specified in terms of the number of surge cycles with corresponding surge current peaks.

The surge current is always calculated for a half cycle of the total surge period.

The surge current in one complete cycle is given by:

Isb = I\(\rm\sqrt{\frac{T}{ t}}\)

It is usual to measure the surge duration in terms of the number of cycles of normal power frequency of 50 or 60 Hz.

For a 50 Hz supply,

 \(T={1\over f}={1\over 50}\)

T = 20 ms

But for surge, half of 20 ms = 10 ms will be the value of the surge period.

Isb = \(\sqrt{10\times 10^{-3}\over t}\)

Isb = I\(\rm\sqrt{\frac{1}{100 t}}\)

where, I_sb = Subtransient surge current

I = Surge current in complete one cycle

Current Ratings of SCR:

• SCR is made of a semiconductor that is very much thermal sensitive.
• Even due to a short time over current, the temperature of the device may rise to such a high value that it may cross its maximum allowable limit.
• Hence there will be a high chance of permanent destruction of the device.
• For this reason, the current ratings of SCR are a very essential part to protect the SCR.
• Some of the current ratings used in the industry to specify the SCR device are given below.
   

Forward Current Rating:

• The maximum value of anode current, that an SCR can handle safely (without any damage), is called the forward current rating.
• The usual current rating of SCRs is from about 30 A to 100 A.
• In case the current exceeds the forward current rating, the SCR may get damaged due to intensive heating at the junctions.
   

On-state Current:

• When the device is in conduction, it carries a load current determined by the supply voltage and the load.
• On-state current is defined in terms of average and RMS values.
• I_av is the average value of maximum continuous sinusoidal on-state current (frequency 40-60 Hz, conduction angle 180°) which should not be exceeded even with intensive cooling.
• The temperature at which the current is permissible has to be mentioned.
• It is this current that determines the application of the device.
• I_rms is the RMS value of maximum continuous sinusoidal on-state current (frequency 40-60 Hz, conduction angle 180°) which should not be exceeded even with intensive cooling.
   

Latching Current:

• This is the rating of current below which the SCR can’t be turned on even if the gate signal is applied.
• That means this much anode current must rise to turn on the device.
• The gate pulse must be continuous until the anode current is greater or equal to the latching current of the thyristor otherwise the device will fail to be turned on.
   

Holding Current:

• It is the minimum on-state current required to keep the SCR in conducting state without any gate signal.
• Its usual value is 5 mA.
   

Surge Current Rating:

• The peak anode current an SCR can handle for a short duration is known as surge current rating.
• It is the maximum admissible peak value of a sinusoidal half cycle of 10 ms duration at a frequency of 50 Hz.
• The value is specified at a given junction temperature.
• During maximum surge on-state current the junction temperature is exceeded though temporarily and forward blocking capabilities are lost for a short period.
• The maximum surge on-state current should only occur occasionally.
• If a thyristor operates under its repetitive voltage and current ratings, its maximum allowable temperature is never exceeded.
• But an SCR may fall into an abnormal operating condition due to a fault in the circuit.
• To overcome this problem, a maximum allowable surge current rating is also specified by the manufacturer.
• This rating specifies the maximum non-repetitive surge current, that the device can withstand.
• This rating is specified depending on the number of surge cycles.
• I²t Value: I²t value is the time integral of the square of the maximum sinusoidal on-state current.

Explanation:

Subcycle Surge Current Rating of a Thyristor

Definition: The subcycle surge current rating of a thyristor refers to the maximum permissible current the thyristor can safely conduct during a short-duration (subcycle) surge event. This rating is crucial for ensuring the device's reliability and protecting it from damage during transient overcurrent conditions. The subcycle surge current occurs over a fraction of the supply frequency cycle, and its magnitude is directly related to the duration of the surge and the thermal capacity of the thyristor.

Mathematical Expression:

The subcycle surge current rating \(I_{sb}\) is mathematically expressed as:

\(I_{sb} = I \sqrt{\frac{T}{t}}\)

Where:

• \(I_{sb}\): Subcycle surge current rating
• \(I\): Normal current rating of the thyristor
• \(T\): Time for one half-cycle of the supply frequency (e.g., for a 50 Hz supply, \(T = \frac{1}{2 \times 50} = 0.01 \, \text{seconds}\))
• \(t\): Duration of the subcycle surge

Derivation and Explanation:

The subcycle surge current rating is derived from the thermal energy considerations of the thyristor. The heat generated in the thyristor during a surge event is proportional to the square of the current (\(I^2\)) and the duration of the surge (\(t\)). To ensure the thyristor does not overheat, the total energy dissipated during the surge must not exceed the thyristor's thermal capacity. The equation \(I_{sb} = I \sqrt{\frac{T}{t}}\) ensures this condition is met by relating the surge current to the normal current rating and the duration of the surge.

Key Insight: The subcycle surge current rating increases as the duration of the surge decreases because the thyristor has less time to accumulate excessive heat. The square root relationship between \(T/t\) and \(I_{sb}\) reflects this thermal behavior.

Correct Option Analysis:

The correct option is:

Option 1: \(I_{sb} = I \sqrt{\frac{T}{t}}\)

This equation correctly represents the subcycle surge current rating of a thyristor, considering the thermal capacity and the energy dissipation relationship. The surge current is proportional to the square root of the ratio of the half-cycle duration (\(T\)) to the subcycle surge duration (\(t\)), ensuring the device's safety and reliability.

Important Information

To further understand the analysis, let’s evaluate the other options:

Option 2: \(I_{sb} = I \sqrt{\frac{t}{T}}\)

This option is incorrect because it reverses the relationship between \(T\) and \(t\). In this formulation, the surge current would decrease as the surge duration (\(t\)) decreases, which contradicts the thermal behavior of the thyristor. The correct equation shows that the surge current increases as the surge duration decreases.

Option 3: \(I_{sb} = t \sqrt{\frac{I}{T}}\)

This option is incorrect as it introduces a linear dependence on \(t\), which is not valid in the context of the thyristor's thermal behavior. The surge current rating is not directly proportional to the surge duration; instead, it depends on the square root of the ratio of \(T\) to \(t\).

Option 4: \(I_{sb} = T \sqrt{\frac{I}{t}}\)

This option is also incorrect because it incorrectly introduces a direct proportionality to \(T\) and inversely to \(t\). The correct relationship involves the square root of the ratio of \(T\) to \(t\), and \(I\) is the normal current rating, not under a square root.

Conclusion:

The subcycle surge current rating of a thyristor is a critical parameter for ensuring the device's safe operation during transient overcurrent events. The correct equation, \(I_{sb} = I \sqrt{\frac{T}{t}}\), accurately captures the thermal constraints of the thyristor and provides a reliable basis for its design and application. A proper understanding of this relationship is essential for selecting and operating thyristors in power electronic circuits.

Concept:

String Efficiency of SCR: String Efficiency of SCR is the degree of capacity utilization of individual SCRs in a string of series / parallel connected SCRs. String efficiency is always less than 1.

String Efficiency \(= {{V} \over V_1 \times N} = {{I} \over I_1 \times N}\)

Where,

• V = Actual voltage of whole string
• I = Actual current of whole string
• V₁ = Voltage rating of one SCR
• I1 = Current rating of one SCR
• N = Total number of SCRs in a string
   

Derating Factor (DFR) of SCR: Derating Factor is the amount by which the string efficiency deviates from unity or 100%.

DFR = [1 – String Efficiency]

DFR = [100 – % String Efficiency]

• Derating Factor gives an idea of unused capacity available in a sting of SCRs.
• That's why it is a measure of reliability of string.
• The lesser the value of sting efficiency, the lesser will be voltage / current sharing by the individual SCRs.
• It means, more unused capacity will be available with the string and hence more DFR.
• But lesser string efficiency increases the cost of string.
• So that a compromise is made in between economy and reliability by properly designing a value of string efficiency.

Calculation :

Given, String efficiency = 80% = 0.8

∴ Derating factor (DFR) = 1 - 0.8

∴ DFR = 0.2

Explanation:

Gate current is used to turn-on the thyristor. Once the thyristor is ON, no gate current is required for the device to remain in on-state. A small gate current controls the large anode current.

Anode current depends on the load connected through the thyristor. It does not depend on the gate current.

Important Points

SCR has three operating modes.

Forward blocking mode:

• Anode voltage is made positive with respect to the cathode, with the gate circuit open
• Both outer junctions are forward biased
• The inner junction is reversed biased
• SCR remains in the off state

Forward conduction mode:

• Anode voltage is greater than the forward break over voltage
• All three junction is forward biased
• The device is in conduction mode

Reverse blocking mode:

• The cathode is at a higher potential than the anode
• Both outer junctions are reverse biased (high resistance)
• The inner (middle) junction is forward biased
• It is reversed biased and non-conducting

Peak Working Revere Voltage (VRWM):

• It is the maximum reverse voltage (anode is negative with respect to cathode) which can be withstood by the thyristor repeatedly or periodically.
• It is nothing but the peak negative value of the AC sinusoidal voltage.
   

Peak Repetitive Revere Voltage (VRRM):

• It is the value of transient voltage that can be withstood by SCR in reverse bias at maximum allowable temperature.
• The reason behind the appearance of this voltage is also the same as VDRM.
   

Anode Voltage Rating:

• This rating gives us a brief idea about withstanding power of a thyristor in forward blocking made in the absence of gate current.
   

Finger Voltage of SCR (VFV):

• The minimum value of voltage which must be applied between anode and cathode for turning off the device by gate triggering.
• Generally, this voltage value is a little more than the normal ON state voltage drop.

Surge current rating

Surge current rating specifies the maximum non-repetitive or surge current that the SCR can withstand for a limited number of times during its life span.

This rating is specified in terms of the number of surge cycles with corresponding surge current peaks.

The surge current is always calculated for a half cycle of the total surge period.

The surge current in one complete cycle is given by:

Isb = I\(\rm\sqrt{\frac{T}{ t}}\)

It is usual to measure the surge duration in terms of the number of cycles of normal power frequency of 50 or 60 Hz.

For a 50 Hz supply,

 \(T={1\over f}={1\over 50}\)

T = 20 ms

But for surge, half of 20 ms = 10 ms will be the value of the surge period.

Isb = \(\sqrt{10\times 10^{-3}\over t}\)

Isb = I\(\rm\sqrt{\frac{1}{100 t}}\)

where, I_sb = Subtransient surge current

I = Surge current in complete one cycle

Current Ratings of SCR:

• SCR is made of a semiconductor that is very much thermal sensitive.
• Even due to a short time over current, the temperature of the device may rise to such a high value that it may cross its maximum allowable limit.
• Hence there will be a high chance of permanent destruction of the device.
• For this reason, the current ratings of SCR are a very essential part to protect the SCR.
• Some of the current ratings used in the industry to specify the SCR device are given below.
   

Forward Current Rating:

• The maximum value of anode current, that an SCR can handle safely (without any damage), is called the forward current rating.
• The usual current rating of SCRs is from about 30 A to 100 A.
• In case the current exceeds the forward current rating, the SCR may get damaged due to intensive heating at the junctions.
   

On-state Current:

• When the device is in conduction, it carries a load current determined by the supply voltage and the load.
• On-state current is defined in terms of average and RMS values.
• I_av is the average value of maximum continuous sinusoidal on-state current (frequency 40-60 Hz, conduction angle 180°) which should not be exceeded even with intensive cooling.
• The temperature at which the current is permissible has to be mentioned.
• It is this current that determines the application of the device.
• I_rms is the RMS value of maximum continuous sinusoidal on-state current (frequency 40-60 Hz, conduction angle 180°) which should not be exceeded even with intensive cooling.
   

Latching Current:

• This is the rating of current below which the SCR can’t be turned on even if the gate signal is applied.
• That means this much anode current must rise to turn on the device.
• The gate pulse must be continuous until the anode current is greater or equal to the latching current of the thyristor otherwise the device will fail to be turned on.
   

Holding Current:

• It is the minimum on-state current required to keep the SCR in conducting state without any gate signal.
• Its usual value is 5 mA.
   

Surge Current Rating:

• The peak anode current an SCR can handle for a short duration is known as surge current rating.
• It is the maximum admissible peak value of a sinusoidal half cycle of 10 ms duration at a frequency of 50 Hz.
• The value is specified at a given junction temperature.
• During maximum surge on-state current the junction temperature is exceeded though temporarily and forward blocking capabilities are lost for a short period.
• The maximum surge on-state current should only occur occasionally.
• If a thyristor operates under its repetitive voltage and current ratings, its maximum allowable temperature is never exceeded.
• But an SCR may fall into an abnormal operating condition due to a fault in the circuit.
• To overcome this problem, a maximum allowable surge current rating is also specified by the manufacturer.
• This rating specifies the maximum non-repetitive surge current, that the device can withstand.
• This rating is specified depending on the number of surge cycles.
• I²t Value: I²t value is the time integral of the square of the maximum sinusoidal on-state current.