Thyristor Power Electronics, 7 Thyristor Three-Phase ... · Thyristor three-phase...

© Festo Didactic 86363-00 191

When you have completed this exercise, you will know what a thyristor three-phase 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.

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

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 three-phase 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.

Thyristor Three-Phase Rectifier/Inverter

Exercise 7

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

Exercise 7 – Thyristor Three-Phase Rectifier/Inverter Discussion

192 © Festo Didactic 86363-00

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

L1

L2

L3

Thyristor three-phase bridge

Firing signals

(to gates of to )

Thyristor firing control circuit

Sync. input

LoadThree-phase ac

power source



Figure 104. Waveforms of voltages and current in a power diode three-phase full-wave rectifier.

Phase angle

(°)

Phase angle

(°)

Phase voltages

( ) 30

90 210

150 270

330

30

90 210

150 270

330

Order of conduction of

the diodes

60 120 240180 300 0 60 120 240 180 300 0

60 120 240180 300 0 60 120 240 180 300 0

Rectifier output

current

Rectifier output

voltage

60

90 210

150 270

330

30

90 210

150 270

330

Line-to-line voltages

( )

Phase angle

(°)

Phase angle

(°)

Phase angle

(°)



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.


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.



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 Conducting thyristors

(Phase voltage ) (Line-to-line voltage )

30° - 90° 60° - 120° and

90° - 150° 120° - 180° and

150° - 210° 180° - 240° and

210° - 270° 240° - 300° and

270° - 330° 300° - 0° and

330° - 30° 0° - 60° and



Figure 105. Waveforms of voltages and current in a thyristor three-phase bridge (firing angle set to 0°) when the load is purely resistive.

Phase angle

(°)

Phase angle

(°)

Phase voltages

( )

60

90 210

150 270

330

30

90 210

150 270

330

Thyristor firing signals

60 120 240180 300 0 60 120 240 180 300 0

60 120 240180 300 0 60 120 240 180 300 0

Current at the dc side of the

bridge

Voltage at the dc side of the

bridge


( )

Phase angle

(°)

Phase angle

(°)

30

90 210

150 270

330

30

90 210

150 270

330

60 120 240180 300 0 60 120 240 180 300 0

Firing angle

Phase angle

(°)



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.



Figure 107. Waveforms of voltages and current in the thyristor three-phase bridge (firing angle set to 30°) when the load is purely resistive.

Phase angle

(°)

Phase angle

(°) Phase voltages

( )

60

90 210

150 270

330

30

90 180

150 270

330


60 150 270210 330 30 90 150 270 210 330

Current at the dc side

of the bridge


( )

Phase angle

(°)

Phase angle

(°)

30

90 210

150 270

330

30

90 210

150 270

330

Firing angle

90

60 150 270210 330 30 90 150 270 210 33090

Voltage at the dc side

of the bridge

Phase angle

(°) 60 150 270210 330 30 90 150 270 210 33090



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.



Figure 109. Waveforms of voltages and current in the thyristor three-phase bridge (firing

angle set to 90°) when the load is purely resistive.

Phase angle

(°)

Phase angle

(°) Phase voltages

( )

60

90 210

150 270

330

30

90 180

150 270

330


60 150 240 300 0 30 120 240 150 270

Current at the dc side

of the bridge


( )

Phase angle

(°)

Phase angle

(°)

30

90 210

150 270

330

30

90 210

150 270

330

Firing angle

120

Voltage at the dc side

of the bridge

Phase angle

(°)

0

60 150 300 0 30 120 240 150 270 0

60 210150 270 330 30 90 180 150 270 330120

Pair of conducting thyristors ( and ) turns off

Next thyristor ( ) is fired

240120



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.

Figure 110. Average voltage at the dc side of a thyristor three-phase bridge as a

function of the firing angle .

Purely resistive load

Resistive-inductive load

Curve area for resistive-inductive loads

Continuouscurrent flow

Firing angle (°)

Average voltage at the dc side of the

bridge (V)

-1.35

1.35

30 60 90 120 150 180 0



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.

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.

a 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



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.

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.

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.

Power flow (+)

(+)

(+) Passive

load

L1

L2

L3

Power flow (+)

(+)

(+)

Active load operating as a current source

L1

L2

L3

Three-phase acpower source

Three-phase ac power source



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.

a 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.

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.

Power flow (-)

(+)

(-)

Active load operating as a current source

L1

L2

L3

Three-phase acpower source



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.

Exercise 7 – Thyristor Three-Phase Rectifier/Inverter Procedure Outline


The Procedure is divided into the following sections:

Set up and connections

Thyristor three-phase bridgeObservation 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

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.

PROCEDURE OUTLINE

PROCEDURE

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 LVDAM EMS 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 LVDAM EMS 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 8348-4) 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 116.

7. 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.



Local ac power network

( ) Voltage

(V)

Frequency

(Hz)

120 60 57

220 50 210

240 50 229

220 60 210

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.

Three-phase transformer module (8348-4)

AC power source (8823)

Power Thyristors module

L1

L2

L3

1 2

6 7

L1

L2

L3 11 12

15

14

4

5

9

10

Firing control signals from the digital outputs of the DACI



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.



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.



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 (°)

Measured voltage (V) Theoretical

voltage (V) Purely resistive load Resistive-inductive load

0

15

30

45

55

60

65

70

75

80

85

88

90

105

120

135

150

165



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 8311-00 or 8311-A0) to implement the 57 load resistor.



21. On the Power Supply, turn the three-phase ac power source on. In the Thyristor Control window, start the Thyristor Three-Phase Bridge.

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.





1 2

6 7

11 12

15

14

4

5

9

10

L1

L2

L3

L1

L2

L3



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 6.

24. 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.



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 three-phase 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.



Local ac power network Source current

(A) Voltage

(V)

Frequency

(Hz)

120 60 -2.0

220 50 -1.0

240 50 -1.0

220 60 -1.0

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°.



N


L1

L2

L3

1 2

6 7

L1

L2

L3 11 12

15

14

4

5

9

10


Negativecurrentsource



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 thesystem. You can use the up and down Firing Angle control buttons in the Thyristor Controlwindow 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.



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)

0

15

30

45

55

60

65

70

75

80

85

88

90

105

120

135

150

165

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.

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

CONCLUSION

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°.

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 three-phase 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

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.



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.

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