Thyristor Commutation Techniques

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Thyristor Commutation Techniques

  1. 1. Thyristor Commutation Techniques Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  2. 2. Introduction <ul><li>Commutation </li></ul><ul><ul><li>Process of turning off a conducting thyristor. </li></ul></ul><ul><li>Current Commutation </li></ul><ul><li>Voltage Commutation </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  3. 3. Methods of Commutation <ul><li>Natural Commutation </li></ul><ul><li>Forced Commutation </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  4. 4. Natural Commutation <ul><li>Occurs in AC circuits </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  5. 5. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  6. 6. <ul><li>Natural Commutation of Thyristors takes place in </li></ul><ul><ul><li>AC voltage controllers. </li></ul></ul><ul><ul><li>Phase controlled rectifiers. </li></ul></ul><ul><ul><li>Cyclo converters. </li></ul></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  7. 7. Forced Commutation <ul><li>Applied to dc circuits </li></ul><ul><li>Commutation achieved by reverse biasing the SCR or by reducing the SCR current below holding current value. </li></ul><ul><li>Commutating elements such as inductance and capacitance are used for commutation purpose. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  8. 8. Methods of Forced Commutation <ul><li>Self commutation. </li></ul><ul><li>Resonant pulse commutation. </li></ul><ul><li>Complementary commutation. </li></ul><ul><li>Impulse commutation. </li></ul><ul><li>External pulse commutation. </li></ul><ul><li>Load Commutation. </li></ul><ul><li>Line Commutation. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  9. 9. Forced Commutation is applied to <ul><li>Choppers. </li></ul><ul><li>Inverters. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  10. 10. Self Commutation Or Load Commutation Or Class A Commutation (Commutation By Resonating The Load) Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  11. 11. <ul><li>Circuit is under damped by including suitable values of L & C in series with load. </li></ul><ul><li>Oscillating current flows. </li></ul><ul><li>SCR is turned off when current is zero. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  12. 12. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  13. 13. Expression for Current <ul><li>Fig. shows a transformed network </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  14. 14. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  15. 15. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  16. 16. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  17. 17. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  18. 18. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  19. 19. Expression For Voltage Across Capacitor At The Time Of Turn Off Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  20. 20. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  21. 21. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  22. 22. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  23. 23. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  24. 24. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  25. 25. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  26. 26. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  27. 27. Problem <ul><li>Calculate the conduction time of SCR and the peak SCR current that flows in the circuit employing series resonant commutation (self commutation or class A commutation), if the supply voltage is 300 V, C = 1  F, L = 5 mH and R L = 100  . Assume that the circuit is initially relaxed. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  28. 28. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  29. 29. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  30. 30. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  31. 31. Problem <ul><li>Figure shows a self commutating circuit. The inductance carries an initial current of 200 A and the initial voltage across the capacitor is V, the supply voltage. Determine the conduction time of the SCR and the capacitor voltage at turn off. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  32. 32. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  33. 33. The transformed circuit of the previous figure is shown in figure below Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  34. 34. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  35. 35. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  36. 36. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  37. 37. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  38. 38. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  39. 39. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  40. 40. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  41. 41. <ul><li>In the circuit shown in figure V = 600 volts, initial capacitor voltage is zero, L = 20  H, C=50  F and the current through the inductance at the time of SCR triggering is I O = 350 A. Determine </li></ul><ul><ul><li>Peak values of capacitor voltage and current </li></ul></ul><ul><ul><li>Conduction time of T 1 . </li></ul></ul>Problem Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  42. 42. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  43. 43. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  44. 44. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  45. 45. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  46. 46. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  47. 47. To calculate conduction time of T 1 <ul><li>The waveform of capacitor current is shown in figure. </li></ul><ul><li>When the capacitor current becomes zero the SCR turns off. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  48. 48. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  49. 49. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  50. 50. Resonant Pulse Commutation (Class B Commutation) Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  51. 51. <ul><li>Series LC circuit connected across thyristor ‘T’. </li></ul><ul><li>Initially ‘C’ is charged to ‘V’ volts with plate ‘a’ as positive. </li></ul><ul><li>Current in LC oscillates when SCR is ON. </li></ul><ul><li>‘ T’ turns off when capacitor discharges through thyristor in a direction opposite to I L </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  52. 52. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  53. 53. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  54. 54. Expression For t C , the Circuit Turn Off Time Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  55. 55. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  56. 56. The LC Circuit During The Commutation Period <ul><li>LC Circuit Transformed Circuit </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  57. 57. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  58. 58. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  59. 59. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  60. 60. Expression For Conduction Time Of SCR Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  61. 61. Alternate Circuit For Resonant Pulse Commutation Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  62. 62. <ul><li>Initially C charged with polarity as shown in figure. </li></ul><ul><li>T 1 is conducting & I L is constant. </li></ul><ul><li>To turn off T 1 , T 2 is fired. </li></ul><ul><li>i C (t) flows opposite to I L & T 1 turns off at i C (t) = I L </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  63. 63. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  64. 64. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  65. 65. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  66. 66. Expression For t C Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  67. 67. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  68. 68. Resonant Pulse Commutation With Accelerating Diode Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  69. 69. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  70. 70. <ul><li>Diode D 2 connected as shown to accelerate discharge. </li></ul><ul><li>T 2 turned on to turn off T 1 . </li></ul><ul><li>Once T 1 is off at t 1 . i C (t) flows through D 2 until current reduces to I L at time t 2 . </li></ul><ul><li>From t = t 2 , ‘C’ charges through load, T 2 self commutates. </li></ul><ul><li>But thyristor recovery process low hence longer reverse bias time. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  71. 71. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  72. 72. Problem <ul><li>The circuit in figure shows a resonant pulse commutation circuit. The initial capacitor voltage V C(O) =200V, C = 30  F and L = 3  H. Determine the circuit turn off time t C , if the load current I L is 200 A and 50 A. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  73. 73. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  74. 74. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  75. 75. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  76. 76. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  77. 77. Problem Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  78. 78. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  79. 79. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  80. 80. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  81. 81. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  82. 82. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  83. 83. Problem <ul><li>For the circuit shown in figure calculate the value of L for proper commutation of SCR. Also find the conduction time of SCR . </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  84. 84. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  85. 85. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  86. 86. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  87. 87. Problem <ul><li>For the circuit shown in figure given that the load current to be commutated is 10 A, turn off time required is 40  sec and the supply voltage is 100 V. Obtain the proper values of commutating elements. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  88. 88. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  89. 89. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  90. 90. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  91. 91. Problem <ul><li>In a resonant commutation circuit supply voltage is 200 V. Load current is 10 A and the device turn off time is 20  s. The ratio of peak resonant current to load current is 1.5. Determine the value of L and C of the commutation circuit. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  92. 92. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  93. 93. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  94. 94. Complementary Commutation (Class C Commutation, Parallel Capacitor Commutation) Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  95. 95. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  96. 96. <ul><li>Two SCRs are used, turning ON one SCR turns off the other. </li></ul><ul><li>T 1 is fired, I L flows through R 1 . </li></ul><ul><li>At same time ‘C’ charges towards ‘V’ through R 2 with plate ‘b’ positive. </li></ul><ul><li>To turn off T 1 , T 2 is fired resulting in capacitor voltage reverse biasing T 1 and turning it off. </li></ul><ul><li>When T 2 is fired current through load shoots up as voltage across load is V+V C </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  97. 97. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  98. 98. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  99. 99. Expression For Circuit Turn Off Time t C Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  100. 100. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  101. 101. <ul><li>In the circuit shown in figure, the load resistances R 1 = R 2 = R = 5  & the capacitance C = 7.5  F, V = 100 volts. Determine the circuit turn off time t C </li></ul>Problem Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  102. 102. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  103. 103. <ul><li>Calculate the values of R L and C to be used for commutating the main SCR in the circuit shown in figure. When it is conducting a full load current of 25 A flows. The minimum time for which the SCR has to be reverse biased for proper commutation is 40  sec. Also find R 1 , given that the auxiliary SCR will undergo natural commutation when its forward current falls below the holding current value of 2 mA. </li></ul>Problem Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  104. 104. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  105. 105. <ul><li>In this circuit only the main SCR carries the load and the auxiliary SCR is used to turn off the main SCR. Once the main SCR turns off the current through the auxiliary SCR is the sum of the capacitor charging current i C and the current i 1 through R 1 , i C reduces to zero after a time t C and hence the auxiliary SCR turns off automatically after a time t C , i 1 should be less than the holding current. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  106. 106. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  107. 107. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  108. 108. Impulse Commutation (Class D Commutation) Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  109. 109. <ul><li>‘ C’ charged to a voltage V C (O) with polarity as shown. </li></ul><ul><li>T 1 is conducting and carries load current I L . </li></ul><ul><li>To turn off T 1 , T 2 is fired. </li></ul><ul><li>Capacitor voltage reverse biases T 1 and turns it off. </li></ul><ul><li>‘ C’ Charges through load. </li></ul><ul><li>T 2 self commutates. </li></ul><ul><li>To reverse capacitor voltage T 3 is turned ON. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  110. 110. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  111. 111. Expression For Circuit Turn Off Time t C Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  112. 112. <ul><li>T 1 is turned off by applying a negative voltage across its terminals. Hence this is voltage commutation. </li></ul><ul><li>t C depends on load current. For higher load currents t C is small. This is a disadvantage of this circuit. </li></ul><ul><li>When T 2 is fired, voltage across the load is V+V C ; hence the current through load shoots up and then decays as the capacitor starts charging. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  113. 113. An Alternative Circuit For Impulse Commutation Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  114. 114. <ul><li>Initially ‘C’ is charged to V C with top plate positive. </li></ul><ul><li>T 1 is fired, load current I L flows. </li></ul><ul><li>‘ C’ discharges at the same time & reverses its polarity. </li></ul><ul><li>‘ D’ ensures bottom plate remains positive. </li></ul><ul><li>To turn off T 1 , T 2 is fired. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  115. 115. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  116. 116. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  117. 117. Problem <ul><li>An impulse commutated thyristor circuit is shown in figure. Determine the available turn off time of the circuit if V = 100 V, R = 10  and C = 10  F. Voltage across capacitor before T 2 is fired is V volts with polarity as shown. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  118. 118. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  119. 119. When T 2 is triggered the circuit is as shown in figure Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  120. 120. Writing the transform circuit, we obtain Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  121. 121. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  122. 122. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  123. 123. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  124. 124. The waveform of v C (t) is shown in figure Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  125. 125. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  126. 126. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  127. 127. Problem <ul><li>In the commutation circuit shown in figure C = 20  F, the input voltage V varies between 180 and 220 V and the load current varies between 50 and 200 A. Determine the minimum and maximum values of available turn off time t C . </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  128. 128. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  129. 129. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  130. 130. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  131. 131. External Pulse Commutation (Class E Commutation) Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  132. 132. <ul><li>T 1 is conducting & R L is connected across supply. </li></ul><ul><li>T 3 is fired & ‘C’ is charged to 2V AUX with upper plate positive. </li></ul><ul><li>T 3 is self commutated. </li></ul><ul><li>To turn off T 1 , T 2 is fired. </li></ul><ul><li>T 2 ON results in a reverse voltage V S – 2V AUX appearing across T 1. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  133. 133. Load Side Commutation <ul><li>In load side commutation the discharging and recharging of capacitor takes place through the load. Hence to test the commutation circuit the load has to be connected. Examples of load side commutation are Resonant Pulse Commutation and Impulse Commutation. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  134. 134. Line Side Commutation <ul><li>In this type of commutation the discharging and recharging of capacitor takes place through the supply. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  135. 135. Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  136. 136. <ul><li>Thyristor T 2 is fired to charge the capacitor ‘C’. When ‘C’ charges to a voltage of 2V, T 2 is self commutated. To reverse the voltage of capacitor to -2V, thyristor T 3 is fired and T 3 commutates by itself. Assuming that T 1 is conducting and carries a load current I L thyristor T 2 is fired to turn off T 1 . </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
  137. 137. <ul><li>The turning ON of T 2 will result in forward biasing the diode (FWD) and applying a reverse voltage of 2V acrossT 1 . This turns off T 1 , thus the discharging and recharging of capacitor is done through the supply and the commutation circuit can be tested without load. </li></ul>Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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