Exercise 7. Thyristor Three-Phase Rectifier/Inverter EXERCISE OBJECTIVE DISCUSSION OUTLINE DISCUSSION. Thyristor three-phase rectifier/inverter

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1 Exercise 7 Thyristor Three-Phase Rectifier/Inverter EXERCISE OBJECTIVE When you have completed this exercise, you will know what a thyristor threephase rectifier/limiter (thyristor three-phase bridge) is, and how it operates. You will be familiar with the waveforms of voltages and currents present in a thyristor three-phase bridge. You will be able to explain how a thyristor three-phase bridge can operate as a rectifier or an inverter. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Thyristor three-phase rectifier/inverter Firing signals in a thyristor three-phase bridge Average voltage and current at the dc side of a thyristor three-phase bridge as a function of the firing angle Purely resistive load. Resistive-inductive load. Operation as a rectifier or an inverter Applications of thyristor three-phase bridges DISCUSSION Thyristor three-phase rectifier/inverter Figure 103 shows the diagram of a thyristor three-phase rectifier/inverter. Observe that the circuit topology is the same as that of a power diode threephase full-wave rectifier, except that all diodes are replaced with thyristors. Using thyristors instead of diodes in a three-phase full-wave rectifier allows the beginning of the conduction interval of each thyristor to be delayed, and thereby, the values of the average (dc) voltage and current at the rectifier output to be varied. The operation of the thyristor three-phase rectifier/inverter is studied in detail in this exercise. Notice that the thyristor three-phase rectifier/inverter is usually referred to as a thyristor three-phase bridge, or Graetz bridge. Festo Didactic

2 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Discussion Thyristor three-phase bridge L1 Three-phase ac power source L2 Load L3 Firing signals (to gates of to ) Thyristor firing control circuit Sync. input Figure 103. Thyristor three-phase rectifier/inverter. Firing signals in a thyristor three-phase bridge In Exercise 2, you learned that in a three-phase full-wave rectifier made of power diodes, the diodes naturally enter into conduction sequentially as the ac power source voltages vary. Whenever a diode stops conducting, another diode immediately starts conducting (see Figure 104). Each diode conducts current during an interval of 120. At any instant, there are always two diodes in conduction, thereby ensuring uninterrupted current flow at the rectifier output. The value of the average (dc) voltage at the rectifier output is equal to 192 Festo Didactic

3 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Discussion Phase voltages ( ) ( ) Line-to-line voltages ( ) ( ) Order of conduction of the diodes ( ) Rectifier output current ( ) Rectifier output voltage ( ) Figure 104. Waveforms of voltages and current in a power diode three-phase full-wave rectifier. Festo Didactic

4 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Discussion The operation of a power diode three-phase full-wave rectifier can be reproduced in a thyristor three-phase bridge by firing each thyristor at the same instant as the corresponding diode in a three-phase full-wave rectifier naturally enters into conduction. This is achieved by using a firing angle of 0 (i.e., without delaying the conduction of the thyristors). In that case, the thyristors firing signals are as shown in Figure 105. This figure also shows the waveforms of the current and voltage at the dc side of the thyristor bridge for a purely resistive load. To generate firing signals that are properly synchronized with the ac power source voltages, the thyristor firing circuit samples one of the line-to-line voltages (e.g., line-to-line voltage in Figure 105). Since the firing angle is 0, thyristor is fired at phase angle 60 of line-to-line voltage (or phase angle 30 of phase voltage ). This turns thyristor off. 120 later, thyristor is fired and thyristor turns off. 120 later, thyristor is fired and thyristor turns off. Also, the complementary thyristors,, and are fired 180 later than thyristors,, and, respectively. Consequently, thyristor is fired at phase angle 240 of line-to-line voltage (or phase angle 210 of phase voltage ) and thyristor turns off. 120 later, thyristor is fired and thyristor turns off. 120 later, thyristor is fired and thyristor turns off. The firing sequence described above repeats over and over. The pulses in each firing signal have a duration of 120, which corresponds to the conduction interval of each diode in a power diode three-phase full-wave rectifier. Consequently, the thyristors conduct current by pairs, one after the other, during equal intervals of 60 and in the same order as the diodes in a power diode three-phase full-wave rectifier, as indicated in Table 5. Table 5. Conducting thyristors for each 60 interval (firing angle set to 0 ) when the load is purely resistive. Angular interval (Phase voltage ) (Line-to-line voltage ) Conducting thyristors and and and and and and 194 Festo Didactic

5 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Discussion Phase voltages ( ) ( ) Line-to-line voltages ( ) ( ) Firing angle Thyristor firing signals ( ) Current at the dc side of the bridge ( ) Voltage at the dc side of the bridge ( ) Figure 105. Waveforms of voltages and current in a thyristor three-phase bridge (firing angle set to 0 ) when the load is purely resistive. Festo Didactic

6 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Discussion Average voltage and current at the dc side of a thyristor three-phase bridge as a function of the firing angle A major advantage of the thyristor three-phase bridge over the power diode three-phase full-wave rectifier is that the average values of the dc current and voltage at the dc side of the thyristor bridge, and thus, the amount of power supplied to the load, can be varied by changing the firing angle of the thyristors. The values of the dc current and voltage, and thus the power supplied to the load, are maximum when the firing angle is 0. When the firing angle is increased, the firing pulses for each thyristor are delayed, which reduces the average values of the dc current and voltage at the dc side of the bridge, and thus, the amount of power supplied to the load. For example, Figure 107 shows the thyristor firing signals and waveforms of voltages and current in a thyristor three-phase bridge for a firing angle of 30 and a purely resistive load. Since each thyristor enters into conduction later with respect to the beginning (phase angle 0 ) of line-to-line voltage, the values of the dc current and voltage at the dc side of the bridge, and thus the power supplied to the load, are lower than the maximum values. Figure 106. Thyristor three-phase bridges are used in power supplies for welding machines such as metal-arc inert gas (MIG) welders. 196 Festo Didactic

7 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Discussion Phase voltages ( ) ( ) Line-to-line voltages ( ) ( ) Firing angle Thyristor firing signals ( ) Current at the dc side of the bridge ( ) Voltage at the dc side of the bridge ( ) Figure 107. Waveforms of voltages and current in the thyristor three-phase bridge (firing angle set to 30 ) when the load is purely resistive. Festo Didactic

8 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Discussion Current flow at the dc side of the thyristor three-phase bridge remains continuous as long as the firing angle is lower than 60. When the firing angle is higher than 60, current flow at the dc side of the bridge is discontinuous, i.e., the current is null (zero) during part of the ac power source cycle. This is because when the pair of conducting thyristors turns off, the next thyristor due to conduct is not fired immediately, but only after a certain time. This results in a time interval when all thyristors are off and there is no current flow in the thyristor bridge. Figure 109 shows an example in which the firing angle is 90. At phase angle, the two thyristors that are conducting current (i.e., thyristors and ) turn off but the next thyristor due to conduct (thyristor ) is not fired immediately; it is fired only 30 later (i.e., at phase angle ). Consequently, all thyristors are off between phase angles 120 and 150, and thus, the current and voltage at the dc side of the bridge are null during this interval. The interval during which all thyristors are off increases as the firing angle approaches 120. At firing angles of 120 or higher, all thyristors stay off during the entire cycle of the ac power source. Therefore, the current and voltage at the dc side of the thyristor bridge are null and, thus, the amount of power supplied to the load is null. Figure 108. Thyristor three-phase bridges are used to supply dc power to the excitation circuit of synchronous generators used in large power plants, such as hydropower electric plants. The photo shows generators of the hydropower electric plant of the Hoover Dam on the Colorado River in the United States of America. 198 Festo Didactic

9 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Discussion Phase voltages ( ) ( ) Line-to-line voltages ( ) ( ) Firing angle Thyristor firing signals ( ) Pair of conducting thyristors ( and ) turns off Next thyristor ( ) is fired Current at the dc side of the bridge ( ) Voltage at the dc side of the bridge ( ) Figure 109. Waveforms of voltages and current in the thyristor three-phase bridge (firing angle set to 90 ) when the load is purely resistive. Festo Didactic

10 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Discussion The average value ( ) of the voltage at the dc side of the bridge can be calculated from the rms value ( ) of the line-to-line voltage of the ac power source and firing angle using the following equation: (8) where: is the average value of the voltage at the dc side of the bridge (V). is the rms value of the line-to-line voltage of the ac power source (V). is the firing angle ( ). Equation (8) is valid as long as current flow in the thyristor three-phase bridge is continuous (i.e., as long as there are no time intervals during which the current flow is null). This equation is represented by the curve in dashed lines shown in Figure 110. This curve shows the average voltage at the dc side of a thyristor three-phase bridge versus the firing angle, when current flow in the bridge is continuous Purely resistive load Continuous current flow Resistive-inductive load Average voltage at the dc side of the bridge (V) Firing angle ( ) Curve area for resistiveinductive loads Figure 110. Average voltage at the dc side of a thyristor three-phase bridge as a function of the firing angle. 200 Festo Didactic

11 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Discussion The curve in dashed lines representing Equation (8) indicates that: when the firing angle passes from 0 to 90, the average voltage at the dc side of the bridge decreases from the maximum value ( ) to zero. a when the firing angle passes from 90 to 180, the average voltage reverses polarity and increases from zero to the maximum value ( ). Note that in practice, the maximum allowed firing angle for the thyristor is 165 or lower to prevent short-circuit currents that otherwise would impair circuit operation. In actual three-phase thyristor bridges, the maximum firing angle allowed is generally limited to 165 to avoid short-circuit currents in the bridge. The explanations for this limitation are complex and beyond the scope of this manual. Purely resistive load The green solid curve in Figure 110 shows how the average voltage at the dc side of the bridge varies as a function of the firing angle for a purely resistive load. For firing angles between 0 and 60, the curve has a cosine shape, i.e., it is identical to the curve in dashed lines representing Equation (8). For firing angles higher than 60, however, the curve diverges from the cosine shape because current flow in the bridge is no longer continuous. Resistive-inductive load The range of firing angles over which current flow in the bridge stays continuous can be increased by connecting an inductor in series with the resistive load. The energy stored in the inductor maintains current flow at the dc side of the bridge for a certain interval, thereby delaying the instant when the conducting thyristors turn off. The greater the time constant ( ratio) of the resistive-inductive load, the higher the firing angle at which current flow interruptions occur. The red solid curve in Figure 110 shows an example of how the average voltage at the dc side of the bridge varies as a function of the firing angle for a resistive-inductive load. This curve has a cosine shape between firing angle 0 and the maximum firing angle (about 75 in this example) for which current flow in the bridge remains continuous, then the curve diverges from the cosine shape. The greater the ratio of the load, the longer the interval of firing angles over which the curve follows the cosine shape. The diverging portion of the curve passes in the shaded area of Figure 110. The exact shape of the curve depends on the ratio of the load. Operation as a rectifier or an inverter When a passive load such as a purely resistive load or a resistive-inductive load is connected to the dc side of a thyristor three-phase bridge (Figure 111), the polarity of the average voltage is always positive. Consequently, the polarity of the average output current is positive, and the polarity of the power on the dc side of the bridge is also positive. In this case, the ac power Festo Didactic

12 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Discussion source supplies power to the load via the thyristor three-phase bridge and therefore, power flows from the ac power source to the load. Since power is converted from ac to dc as it flows through the thyristor three-phase bridge, the bridge acts as a rectifier. Notice that the thyristor three-phase bridge is represented by a rectangular box containing the symbol for a thyristor. Power flow (+) L1 (+) Three-phase ac power source L2 (+) Passive load L3 Figure 111. When a passive load is connected to the dc side of a thyristor three-phase bridge, power flow is from the ac power source to the load. Since power is converted from ac to dc as it flows through the bridge, the bridge acts as a rectifier. When an active load that behaves like a source of current is connected to the dc side of a thyristor three-phase bridge, current flow in the thyristor bridge is continuous, no matter the value of the firing angle. Consequently, the average voltage ( ) at the dc side of the thyristor three-phase bridge can theoretically be varied over the full range, i.e., from to by varying the firing angle from 0 to 180. When the firing angle varies between 0 and 90 (Figure 112), the polarity of the average voltage, average current, and power on the dc side of the bridge is positive. Power flow is from the ac power source to the active load and the thyristor three-phase bridge acts as a rectifier. Power flow (+) L1 (+) Active load operating as a current source Three-phase ac power source L2 (+) L3 Figure 112. When an active load that behaves like a source of current is connected to the dc side of a thyristor three-phase bridge, and the firing angle varies between 0 and 90, the bridge acts as a rectifier. 202 Festo Didactic

13 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Discussion a When the firing angle varies between 90 and 180 (Figure 113), the polarity of the average voltage is negative. Since the polarity of the average current remains positive, the polarity of the power at the dc side of the bridge is thus negative. This indicates that power flow is from the dc side of the bridge to the ac power source. Since power is converted from dc to ac as it passes through the thyristor three-phase bridge, the bridge acts as an inverter. In actual three-phase thyristor bridges, the maximum firing angle allowed is generally limited to 165 to avoid short-circuit currents in the bridge that could damage the thyristors. The explanations for this limitation are complex and beyond the scope of this manual. Power flow (-) L1 (+) Active load operating as a current source Three-phase ac power source L2 (-) L3 Figure 113. When an active load that behaves like a source of current is connected to the dc side of a thyristor three-phase bridge, and the firing angle varies between 90 and 180, the bridge acts as an inverter. Applications of thyristor three-phase bridges Thyristor three-phase bridges are used to supply dc power to the excitation circuit of synchronous generators used in large power plants, such as synchronous generators in hydropower electric plants (see Figure 108). In this application, each thyristor bridge operates as a rectifier to supply dc power to the excitation circuit of a synchronous generator. The amount of excitation is set to the exact value required by adjusting the firing angle of the thyristor bridge. This is achieved via closed-loop control. Thyristor three-phase bridges are also used in high-voltage, direct-current (HVDC) power transmission lines. Before transmission over the line, power is converted from ac to dc by thyristor three-phase bridges set to operate as rectifiers. At the other end of the HVDC power transmission line, dc power is converted back to ac power by thyristor three-phase bridges set to operate as inverters. HVDC power transmission lines permit the interconnection of electric power grids or networks with different frequency or voltage, so that power can be exchanged between them. Festo Didactic

14 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Discussion Figure 114. Thyristor three-phase bridges are used in converter stations at the ends of high-voltage, direct current (HVDC) power transmission lines. Figure 115. HVDC converter station in the province of Manitoba, Canada. Two long distance HVDC transmission lines carry dc power generated in the north of Manitoba to this station. The station converts dc power back to ac power using thyristor valves for connection to the electric power grid. 204 Festo Didactic

15 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections Thyristor three-phase bridge Observation of the thyristor firing signals. Observation of the load voltage and current waveforms. Average voltage at the dc side of the thyristor bridge as a function of the firing angle (for a purely resistive load). Average voltage at the dc side of the thyristor bridge as a function of the firing angle (for a resistive-inductive load). Operation of a thyristor three-phase bridge as a rectifier/inverter PROCEDURE High voltages are present in this laboratory exercise. Do not make or modify any banana jack connections with the power on unless otherwise specified. Set up and connections In this part of the exercise, you will set up and connect the equipment. 1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform the exercise. Install the equipment in the Workstation. 2. Connect the Power Input of the Data Acquisition and Control Interface to a 24 V ac power supply. Connect the Low Power Input of the Power Thyristors module to the Power Input of the Data Acquisition and Control Interface. Turn the 24 V ac power supply on. 3. Connect the USB port of the Data Acquisition and Control Interface to a USB port of the host computer. Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to a USB port of the host computer. 4. Make sure that the ac and dc power switches on the Power Supply are set to the O (off) position, then connect the Power Supply to a three-phase ac power outlet. Make sure that the main power switch of the Four-Quadrant Dynamometer/Power Supply is set to O (off), then connect its Power Input to an ac power outlet. Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power Supply to Power Supply. This connects the internal power supply of the module to the Power Supply terminals on the front panel. Festo Didactic

16 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Procedure Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main power switch to I (on). 5. Turn the host computer on, then start the LVDAC-EMS software. In the LVDAMEMS Start-Up window, make sure that the Data Acquisition and Control Interface and the Four-Quadrant Dynamometer/Power Supply are detected. Make sure that the Computer-Based Instrumentation and Thyristor Bridge Control functions for the Data Acquisition and Control Interface are available. Select the network voltage and frequency that correspond to the voltage and frequency of your local ac power network, then click the OK button to close the LVDAMEMS Start-Up window. Thyristor three-phase bridge In this part of the exercise, you will study the operation of a thyristor three-phase bridge. You will observe the effect that varying the firing angle of the thyristors has on the average voltage at the dc side of the bridge when the load is purely resistive and when it is resistive-inductive. Observation of the thyristor firing signals 6. On the Power Thyristors module, set switches and to the I (on) position. This interconnects thyristors through of the Power Thyristors module in a thyristor three-phase bridge. Set up the circuit shown in Figure 116. In this circuit, the Three-Phase Power Transformer (Model ) is used to reduce the voltage at the ac side of the thyristor three-phase bridge. This reduces the maximum average voltage at the dc side of the bridge too avoid exceeding the maximum power rating of the Resistive Load module. E1, E2, E3, E4, and I3 are inputs of the Data Acquisition and Control Interface (DACI). The load resistor is implemented with the Resistive Load module. The resistance value to be used for load resistor depends on your local ac power network voltage (see table in the diagram). a Input E4 of the DACI is used for synchronization of the firing signals of the thyristors in the Power Thyristors module. This input must be connected as shown in Figure Connect the Digital Outputs of the Data Acquisition and Control Interface to the Firing Control Inputs of the Power Thyristors module using the provided cable with DB9 connectors. Also, perform the following connections to observe the control signals applied to thyristors through : connect Firing Control Inputs 1 through 6 of the Power Thyristors module to Analog Inputs 1 through 6, respectively, of the DACI, using 2 mm leads. Connect the common (white) terminal of the Firing Control Inputs on the Power Thyristors module to one of the two analog common (white) terminals of the DACI using a 2 mm lead. 206 Festo Didactic

17 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Procedure Power Thyristors module L1 Three-phase transformer module (8348-4) 1 2 L1 4 AC power source (8823) L L2 L L3 Firing control signals from the digital outputs of the DACI Local ac power network Voltage (V) Frequency (Hz) () Figure 116. Thyristor three-phase bridge with a purely resistive load. 8. In LVDAC-EMS, open the Thyristor Control window, and make the following settings: Set the Function parameter to Thyristor Three-Phase Bridge. Make sure that the Firing Angle Control parameter is set to Knob. This allows the Firing Angle parameter to be controlled manually. Set the Firing Angle parameter to 0 by entering 0 in the field next to this parameter or by using the control knob in the lower left corner of the window. This sets the firing angle to 0. Festo Didactic

18 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Procedure Make sure that the parameters through are all set to Active. This makes the firing signals of these thyristors depend on the Firing Angle Control and Firing Angle parameters. Leave the other parameters set to their default values. Start the Thyristor Three-Phase Bridge function by clicking the Start/Stop button or by setting the Status parameter to Started. 9. On the Power Supply, turn the three-phase ac power source on by setting the corresponding switch to I (on). 10. Start the Oscilloscope. In the Data Acquisition and Control Settings window of LVDAC-EMS, set the Range of voltage inputs E1, E2, and E3 to High. On the Oscilloscope, display line-to-line voltages and (E1, E2) and the firing signals of thyristors through (Analog Inputs 1 through 6 of the DACI) on channels 1, 2, 3, 4, 5, 6, 7, and 8, respectively. Set the Oscilloscope in the continuous refresh mode. Set the time base to display at least two cycles of the source voltage waveform. Observe the relationship between the pulses in each thyristor firing signal and line-to-line voltage. Notice that thyristors through are fired at phase angles 60, 180, 300, 240, 0, and 120 of voltage, respectively. This is because the firing angle is set to 0 (i.e., the conduction of the thyristors is not delayed.) Record below the firing sequence of the thyristors. Is the width of the pulses in the firing signals of thyristors through the same as the conduction interval (i.e., 120 ) of the diodes in a power diode, three-phase full-wave rectifier? Yes No From your observations, are the thyristors fired in the same order and at the same phase angles as the diodes enter into conduction in a power diode, three-phase full-wave rectifier? Yes No 11. By using the Firing Angle control knob in the Thyristor Control window, slowly vary the firing angle between 0 and 90. Describe what happens. 208 Festo Didactic

19 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Procedure Observation of the load voltage and current waveforms 12. In the Thyristor Control window, set the Firing Angle parameter back to 0. Set the Oscilloscope to display line-to-line voltages and (E1, E2), the firing signals of thyristors,,, and (Analog Inputs 5, 1, 6, and 2 of the DACI), and the current (I3) and voltage (E3) at the dc side of the thyristor three-phase bridge. 13. Open the Metering window. Set a meter to measure the rms value of voltage (E1). Set two meters to measure the average (dc) current (I3) and voltage (E3) at the dc side of the thyristor three-phase bridge. Finally, set meter PQS3 to measure the active load power from inputs E3 and I3. Disable meter E4. Select the Continuous Refresh mode by clicking the Continuous Refresh button. 14. By using the Firing Angle control knob in the Thyristor Control window, slowly vary the firing angle between 0 and 60 while observing the signals on the Oscilloscope and the values indicated by the meters. Notice that the average current and voltage at the dc side of the thyristor bridge and, thus, the power supplied to the load decrease when the firing angle increases, and vice versa. Explain why. 15. By using the Firing Angle control knob in the Thyristor Control window, slowly vary the firing angle between 30 and 90 while observing the signals on the Oscilloscope. Notice that when the firing angle is set to a value higher than 60, current flow in the thyristor bridge becomes discontinuous (i.e., the current is null during part of the ac power source cycle). Explain why. Festo Didactic

20 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Procedure Average voltage at the dc side of the thyristor bridge as a function of the firing angle (for a purely resistive load) 16. In the Thyristor Control window, temporarily stop the Thyristor Three-Phase Bridge function by clicking the Start/Stop button or by setting the Status parameter to Stopped. Note and record below the rms value of voltage indicated by meter E1 in the Metering window. V Using the equation below, calculate the theoretical value of voltage for each of the firing angles listed in Table 6, using the value of voltage measured above. Record your results in Table 6. Table 6. Average voltage at the dc side of the bridge ( ) as a function of the firing angle. Firing angle ( ) Purely resistive load Measured voltage (V) Resistive-inductive load Theoretical voltage (V) 210 Festo Didactic

21 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Procedure 17. In the Thyristor Control window, start the Thyristor Three-Phase Bridge function by clicking the Start/Stop button or by setting the Status parameter to Started. Set the Firing Angle to each of the values listed in Table 6. For each setting, note and record the average voltage at the dc side of the bridge (indicated by meter E3) in the Purely resistive load column of the table. 18. From the values recorded in Table 6, plot a curve of the average voltage measured at the dc side of the thyristor bridge versus the firing angle for a purely resistive load. On the same graph, plot the theoretical curve of the average voltage at the dc side of the bridge versus the firing angle. Does the measured curve retain a cosine shape (i.e., follow the theoretical curve) for firing angles up to 60, but diverge markedly for firing angles higher than 60? Explain why. 19. In the Thyristor Control window, stop the Thyristor Three-Phase Bridge function by clicking the Start/Stop button or by setting the Status parameter to Stopped. On the Power Supply, turn the three-phase ac power source off. Average voltage at the dc side of the thyristor bridge as a function of the firing angle (for a resistive-inductive load) 20. Set the resistance of the load resistor to 57, if it is not already set to this value. Then, connect a load inductor in series with load resistor, as Figure 117 shows. The load inductor is implemented with one of the inductors in the Filtering Inductors/Capacitors module. a If your local ac power network voltage is either 220 V or 240 V, use the Resistive Load module with a low (120 V) voltage rating (Model or 8311-A0) to implement the 57 load resistor. Festo Didactic

22 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Procedure 21. On the Power Supply, turn the three-phase ac power source on. In the Thyristor Control window, start the Thyristor Three-Phase Bridge. Power Thyristors module L1 Three-phase transformer module (8348-4) 1 2 L1 AC power source (8823) L L L L3 Firing control signals from the digital outputs of the DACI Figure 117. Thyristor three-phase bridge with a resistive-inductive load. 22. Using the buttons of the Firing Angle control knob in the Thyristor Control window, slowly vary the firing angle between 30 and 90 while observing the signals on the Oscilloscope. Notice that the current (I3) at the dc side of the thyristor bridge is still continuous even when the firing angle exceeds 60. Explain why. If your local ac power network voltage is 220 V or 240 V, vary the firing angle between 45 and 90 to avoid excessive voltage across the resistor in the resistive-inductive load. 212 Festo Didactic

23 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Procedure In the space provided, record the maximum firing angle for which current flow in the thyristor bridge is still continuous. 23. Set the Firing Angle to each of the values listed in Table 6. For each setting, note and record the average voltage at the dc side of the thyristor bridge (indicated by meter E3) in the Resistive-inductive load column of this table. a If your local ac power network voltage is 220 V or 240 V, start with a firing angle of 45 and increase this angle by steps using the values listed in Table On the same graph used in the previous subsection (step 18), plot a curve of the average voltage measured at the dc side of the thyristor bridge versus the firing angle for a resistive-inductive load, using the values measured in the previous step. Compare the curve for a resistive-inductive load with the theoretical curve. Does the curve for a resistive-inductive load retain a cosine shape (i.e., follows the theoretical curve) up to the maximum firing angle ensuring continuous current flow you measured in step 22? Compare the curve for a resistive-inductive load with the curve for a purely resistive load. What is the effect of connecting an inductor in series with the load resistor? Explain. 25. In the Thyristor Control window, stop the Thyristor Three-Phase Bridge function by clicking the Start/Stop button or by setting the Status parameter to Stopped. On the Power Supply, turn the three-phase ac power source off. Festo Didactic

24 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Procedure Operation of a thyristor three-phase bridge as a rectifier/inverter In this part of the exercise, you will study the operation of a thyristor three-phase bridge acting as a rectifier/inverter. To do this, you will connect an active load to the dc side of the thyristor bridge and observe what happens to the load current, voltage, and power when the firing angle is varied. The active load you will use is a current source implemented with the Four-Quadrant Dynamometer/Power Supply. 26. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply window and make the following settings: Set the Function parameter to Current Source (-). This setting makes the internal power source operate as a negative current source. Make sure that the Current Control parameter is set to Knob. This allows the Current parameter to be controlled manually. Set the Current parameter to the value indicated in the table of Figure 118. This value depends on your local ac power network voltage. DO NOT start the Negative Current Source (-) function yet. This will be done in another step. 27. Disconnect the resistive-inductive load from the thyristor three-phase bridge. As Figure 118 shows, connect the dc side of the thyristor three-phase bridge to the negative current source implemented using the Four-Quadrant Dynamometer/Power Supply, via the 50-mH inductor in the Filtering Inductors/Capacitors module. The inductor stabilizes the operation of the circuit. 28. In the Thyristor Control window, set the Firing Angle of the thyristor threephase bridge to 90. Start the Thyristor Three-Phase Bridge function. On the Power Supply, turn the three-phase ac power source on. 29. In the Four-Quadrant Dynamometer/Power Supply, start the Negative Current Source (-) function by clicking the Start/Stop button or by setting the Status parameter to Started. 214 Festo Didactic

25 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Procedure Power Thyristors module L1 Three-phase transformer module (8348-4) 1 2 L1 AC power source (8823) L L L L3 Negative current source N Firing control signals from the digital outputs of the DACI Local ac power network Voltage (V) Frequency (Hz) Source current (A) Figure 118. Operation of a thyristor three-phase bridge as a rectifier/inverter. 30. In the Metering window, notice that the average voltage (E3) at the dc side of the thyristor three-phase bridge is nearly 0 V. Also, notice that the average current (I3) at the dc side of the thyristor three-phase bridge is close to the current setting of the negative current source but of opposite polarity (i.e., it has a positive polarity). Therefore, the active load power (PQS3) is 0 W approximately. This is because the firing angle is currently set to 90. Festo Didactic

26 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Procedure 31. By using the Firing Angle control knob in the Thyristor Control window, slowly vary the firing angle between 0 and 90 while observing the signals on the Oscilloscope and the average voltage, current, and power at the dc side of the thyristor bridge indicated by the meters: Do not vary the firing angle suddenly as this can cause overcurrents to occur in the system. You can use the up and down Firing Angle control buttons in the Thyristor Control window to slowly vary the firing angle. Notice that the current flow is continuous no matter the firing angle, and that the average current at the dc side of the thyristor bridge remains constant (because its value is imposed by the current source) and has a positive polarity. Also, notice that the average voltage at the dc side of the thyristor bridge decreases as the firing angle increases, but that its polarity remains positive. Therefore, the polarity of the load power is always positive since the polarity of the average voltage and current at the dc side of the thyristor three-phase bridge are both positive. From your observation, what is the direction of power flow when the firing angle is between 0 and 90 approximately? Does the thyristor bridge operate as a rectifier or an inverter over this firing angle range? Explain. 32. In the Thyristor Control window, slowly vary the firing angle between 90 and 165 (do not exceed 165 ) while observing the signals on the Oscilloscope and the average voltage, current, and power at the dc side of the thyristor bridge indicated by the meters. Do not exceed a firing angle of 165 as this will cause an overcurrent condition to occur. Notice that the current flow is still continuous no matter the firing angle, and that the average current at the dc side of the thyristor bridge remains constant and has a positive polarity because its value is imposed by the current source. Also, notice that the polarity of the average voltage at the dc side of the thyristor bridge is negative, and that this voltage increases as the firing angle increases. Therefore, the polarity of the load power is always negative since the polarity of the average current at the dc side of the thyristor bridge can only be positive. 216 Festo Didactic

27 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Procedure From your observation, what is the direction of power flow when the firing angle is between 90 and 165? Does the thyristor bridge operate as a rectifier or an inverter over this firing angle range? Explain. 33. By using the Firing Angle control knob in the Thyristor Control window, set the firing angle to each of the values listed in Table 7 and, for each setting, note and record the average voltage and power at the dc side of the thyristor bridge in this table. a If your ac power network voltage and frequency are equal to 240 V and 50 Hz, respectively, do not set the firing angle to 165 (i.e., stop recording data at 150 ). This is because the negative current source cannot maintain the current at -1.0 A when the firing angle is higher than 150. Table 7. Average voltage and power at the dc side of the thyristor bridge as a function of the firing angle with current source as the load. Firing angle ( ) Measured voltage at the dc side of the thyristor bridge (V) Measured power at the dc side of the bridge (W) Festo Didactic

28 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Conclusion 34. In the Four-Quadrant Dynamometer/Power Supply, stop the Negative Current Source (-) function by clicking the Start/Stop button or by setting the Status parameter to Stopped. In the Thyristor Control window, stop the Thyristor Three-Phase Bridge function by clicking the Start/Stop button or by setting the Status parameter to Stopped. 35. From the values recorded in Table 7, plot a curve of the average voltage measured at the dc side of the thyristor bridge versus the firing angle on the same graph used in this exercise. Compare the curve of voltage versus obtained with the negative current source as a load with the theoretical curve. Does the curve of voltage versus obtained with the negative current source retain a cosine shape (i.e., follows the theoretical curve)? Explain. 36. From the values recorded in Table 7, plot a curve of the power measured at the dc side of the thyristor bridge versus the firing angle. Is the polarity of power positive for firing angles between 0 and 90 approximately? What does this indicate about the direction of power flow and the operation of the thyristor three-phase bridge? Is the polarity of power negative for firing angles between 90 and 165? What does this indicate about the direction of power flow and the operation of the thyristor three-phase bridge? 37. On the Power Supply, turn the three-phase ac power source off. Close LVDAC-EMS. Disconnect all leads and return them to their storage location. CONCLUSION In this exercise, you studied the operation of a thyristor three-phase bridge. You learned that the circuit topology is the same as that of a power diode three-phase full-wave rectifier, except that all diodes are replaced with thyristors. You learned that a major advantage of the thyristor three-phase bridge over the power diode 218 Festo Didactic

29 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Review Questions three-phase full-wave rectifier is that the average values of the dc current and voltage at the dc side of the thyristor bridge, and thus, the amount of power supplied to the load, can be varied by changing the firing angle of the thyristors. You learned that when the firing angle is higher than 60, current flow at the dc side of the bridge becomes discontinuous when the load is purely resistive. The firing angle for which the current flow becomes discontinuous is higher than 60 when an inductor is connected with the resistive load. At firing angles of 120 or higher, all thyristors are off during the entire cycle of the ac power source, and thus, the amount of power supplied to the load is null. You learned that when a passive load such as a purely resistive load or a resistive-inductive load is connected to the dc side of a thyristor three-phase bridge, the bridge acts as a rectifier, and power flow is always from the ac power source to the load. However, when an active load like a current source is connected to the dc side of the thyristor bridge, current flow is continuous no matter the firing range. Consequently, the firing angle can be varied between 0 and 165 (theoretically up to 180 ) so that the thyristor bridge operates as a rectifier (power flow is from the ac power source to the load) for firing angles between 0 and 90, and as an inverter (power flow is from the load to the ac power source) for firing angles between 90 and 165. REVIEW QUESTIONS 1. How can the operation of a power diode three-phase full-wave rectifier be reproduced in a thyristor three-phase bridge? 2. When a purely resistive load is connected to the dc side of a thyristor threephase bridge, what is the maximum firing angle at which current flow at the dc side of the bridge remains continuous? Why is current flow discontinuous at higher firing angles? 3. Describe the curve of the average voltage at the dc side of a thyristor three-phase bridge versus the firing angle when current flow through the bridge is continuous (theoretical curve). Then, compare the actual curves of Festo Didactic

30 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Review Questions voltage versus the firing angle obtained when the load is purely resistive and when it is resistive-inductive to the theoretical curve. 4. Explain why a thyristor three-phase bridge operates as a rectifier when a passive load such as a purely resistive load or a resistive load is connected to the dc side of this bridge. 220 Festo Didactic

31 Exercise 7 Thyristor Three-Phase Rectifier/Inverter Review Questions 5. Explain why a thyristor three-phase bridge can operate as a rectifier or an inverter, depending of the firing angle, when an active load (current source) is connected to the dc side of this bridge. Festo Didactic