# HVDC Transmission Systems, 5 Harmonic Reduction using - LabVolt

Exercise

5

Harmonic Reduction using Thyristor 12-Pulse Converters EXERCISE OBJECTIVE

When you have completed this exercise, you will understand what a thyristor 12pulse converter is and how it operates. You will learn the advantages of thyristor 12-pulse converters over thyristor 6-pulse converters.

DISCUSSION OUTLINE

The Discussion of this exercise covers the following points:

    

DISCUSSION

Waveforms and harmonic contents of the ac-side line currents and dc-side voltage in a thyristor 6-pulse converter connected to a wye-wye (Y-Y) transformer Introduction to the thyristor 12-pulse converter Waveforms and harmonic contents of the ac-side line currents and dc-side voltage in a thyristor 6-pulse converter connected to a wye-delta (Y-ǻ) transformer Comparison of the ac-side line currents and dc-side voltage in thyristor 6-pulse converters and thyristor 12-pulse converters Monopolar HVDC transmission system implemented with a thyristor 12-pulse converter at each converter station

Waveforms and harmonic contents of the ac-side line currents and dc-side voltage in a thyristor 6-pulse converter connected to a wye-wye (Y-Y) transformer As you have learned in Exercise 1, the line currents at the ac side of a thyristor three-phase bridge contain harmonics, i.e., sinusoidal current components at frequencies equal to integer multiples of the fundamental frequency (i.e., the ac power network frequency). For example, consider the thyristor 6-pulse converter shown in Figure 74a. This converter contains a thyristor three-phase bridge connected to the ac power network through a wye-wye (Y-Y) transformer. The firing angle Ƚof the thyristor three-phase bridge is set to 0° in this example.

Figure 74b shows the waveforms of one of the line currents ( ͳ) at the ac side of the converter, the corresponding phase voltage (ͳǦ) of the ac power network, and the voltage  at the dc side of the converter. As Figure 74b shows, the waveform of line current ͳ consists of rectangular pulses. Because of this, harmonics are present in line current ͳ. These harmonics are of the following orders: 5, 7, 11, 13, 17, 19, 23, 25, 29, 31, 35, 37, 41, 43, etc., as Figure 74c shows. They are all of positive polarity. Figure 74b also shows that the converter dc voltage  contains ripple. This ripple consists of six pulses of equal duration per cycle of the ac power network, hence the name thyristor 6-pulse converter. The voltage ripple frequency is therefore 6 times the ac power network frequency, resulting in harmonic components of the following orders: 6, 12, 18, 24, etc. To summarize, a thyristor 6-pulse converter produces line current harmonics at the ac side and voltage harmonics at the dc side.

163

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Discussion

ͳ

L1

L2

AC power network



L3

(a) 6-pulse converter connected to the ac power network through a wye-wye (Y-Y) transformer

AC power network voltage ͳǦ (fundamental frequency, f) Converter dc voltage 

210

0 30

60

90

120

150

240

270

300

Line current ͳ

330

180

0

30

60

90

120

150 180

Phase angle ͳǦ (°)

(b) Waveforms of line current ͳ, ac power network voltage ͳǦ, and converter dc voltage 

Level (+)

11

13

17

19

23

25

29

31

35

37

41

43

Order (nf)

(-) (c) Harmonic spectrum of line current ͳ Figure 74. Thyristor 6-pulse converter connected to the ac power network through a wyewye (Y-Y) transformer.

164

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Discussion

Introduction to the thyristor 12-pulse converter The harmonics present in the line currents at the ac side of a thyristor 6-pulse converter produce undesirable effects in the ac power network, such as overheating due to increased power losses, voltage distortion, and interference. These undesirable effects can be significantly mitigated by using a thyristor 12-pulse converter instead of a thyristor 6-pulse converter, as is explained later in the discussion. Figure 75 shows the diagram of a thyristor 12-pulse converter. This converter consists of two thyristor 6-pulse converters (i.e., two thyristor three-phase bridges): Ͳ

The ac sides of the two thyristor 6-pulse converters are connected to the same ac power network. One converter is connected to the ac power network through a wye-wye (Y-Y) transformer. The other converter is connected to the ac power network through a wye-delta (Y-ǻ) transformer.

Ͳ

Both thyristor 6-pulse converters operate with the same firing angle.

Ͳ

The turns ratio of each of the two transformers is selected so that the two thyristor 6-pulse converters produce the same dc voltage value to ensure balanced operation of the two converters.

Ͳ

The dc sides of the two thyristor 6-pulse converters are connected in series, so that the dc voltage produced by one converter adds to the dc voltage produced by the other converter. Consequently, the voltage at the dc side of the thyristor 12-pulse converter is twice the voltage at the dc side of each thyristor 6-pulse converter. Bridge 1 L1 Wye-wye transformer

AC power network

L2

L3

Bridge 2



Wye-delta transformer

Figure 75. Simplified diagram of a thyristor 12-pulse converter.

165

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Discussion

Waveforms and harmonic contents of the ac-side line currents and dc-side voltage in a thyristor 6-pulse converter connected to a wye-delta (Y-ƣ) transformer Figure 76 shows a thyristor 6-pulse converter similar to the one described at the beginning of the discussion, except that the thyristor three-phase bridge is connected to the ac power network through a wye-delta (Y-ǻ) transformer (like one of the two 6-pulse converters in a 12-pulse converter) instead of a wyewye (Y-Y) transformer. This causes the voltage at the ac side of the thyristor bridge in this 6-pulse converter to lag the ac power network voltage by 30° (instead of being in phase with the ac power network voltage when a Y-Y transformer is used). The firing angle Ƚof the thyristor bridge is also set to 0° in this example. Figure 76b shows the waveforms of one of the line currents ( ͳ) at the ac side of the converter, the corresponding phase voltage (‫ܧ‬ଵିே ) of the ac power network, and the voltage  at the dc side of the converter. The line current ͳ has a staircase-step waveform. Because of this, harmonics are present in the line current ͳ. These harmonics are of the following orders: 5, 7, 11, 13, 17, 19, 23, 25, 29, 31, 35, 37, 41, 43, etc., as Figure 76c shows. These harmonics have the same orders and levels as the harmonics present in the line currents of the 6-pulse converter connected to a wye-wye transformer. However, the 5th, 7th, 17th, 19th, 29th, 31st, 41st, and 43rd harmonics in the line currents of the 6-pulse converter connected to a wye-delta transformer are of negative polarity, while these harmonics are of positive polarity in the case of the line currents of the 6-pulse converter connected to a wye-wye transformer. Harmonic components of the same order but of opposite polarity represent sine waves (e.g., sinusoidal currents in the present case) having the same frequency but which are 180° out of phase. Figure 76b also shows that the converter dc voltage  contains ripple. This ripple consists of six pulses of equal duration per cycle of the ac power network, like the ripple in the dc voltage of a thyristor 6-pulse converter connected to a wye-wye transformer, thereby resulting in harmonic components of the following orders: 6, 12, 18, 24, etc. The level of the ripple in the dc voltage  is the same for both 6-pulse converters. However, the ripple in the dc voltage  of the converter connected to a wye-delta transformer is phase shifted by 1/12 of the ac power network cycle (i.e., by 30°) with respect to the ripple in the dc voltage  of the converter connected to a wye-wye transformer. This is due to the fact that the voltage at the ac side of the converter with a wye-delta transformer is phase shifted by 30° with respect to the ac power network voltage.

166

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Discussion

ͳ

L1

L2

AC power network



L3

(a) 6-pulse converter connected to the ac power network through a wye-delta (Y-ǻ) transformer AC power network voltage ͳǦ (fundamental frequency, f) Converter dc voltage 

210

0 30

60

90

120

240

270

300

Line current ͳ

330

150 180

0

30

60

90

120

150 180

Phase angle

ͳǦ (°)

(b) Waveforms of line current ͳ, ac power network voltage ͳǦ, and converter dc voltage 

Level (+)

(-)

11

13

17

19

23

25

29

31

35

37

41

43

Order (nf)

(c) Harmonic spectrum of line current ͳ Figure 76. Thyristor 6-pulse converter connected to the ac power network through a wyedelta (Y-ǻ) transformer.

167

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Discussion

Comparison of the ac-side line currents and dc-side voltage in thyristor 6-pulse converters and thyristor 12-pulse converters Figure 77 shows the waveforms of the line current ͳ, ac power network voltage ͳǦ, and converter dc voltage  for the two thyristor 6-pulse converters and the thyristor 12-pulse converter seen thus far. The major advantage of using the thyristor 12-pulse converter is that the line current ͳ in this converter (Figure 77c), which is the sum of the line currents ͳ in each individual 6-pulse converter (Figure 77a and Figure 77b), tends towards a sinusoidal waveform, and thus contains much less harmonics than the line currents ͳ in each 6-pulse converter. To illustrate this, Figure 78 shows the harmonics contained in the line current ͳ of the two 6-pulse converters and the 12-pulse converter. Figure 78a and Figure 78b show that the 5th, 7th, 17th, 19th, 29th, 31st, 41st, and 43rd harmonics in the line currents of the 6-pulse converters are of opposite polarity (i.e., these harmonics are positive for the 6-pulse converter connected to a wye-wye transformer, while they are negative for the 6-pulse converter connected to a wye-delta transformer). Consequently, these harmonics are phase shifted by 180° with respect to each other, so that they cancel each other out in the 12-pulse converter. Therefore, the line current ͳ of the 12-pulse converter (Figure 78c) contains only the 11th, 13th, 23rd, 25th, 35th, and 37th harmonics. This is the reason why line current ͳ in the 12-pulse converter tends towards a sine wave at the frequency of the fundamental component. In conclusion, using 12-pulse converters instead of 6-pulse converters reduces the harmonics in the ac line currents, and thus greatly helps in mitigating the undesirable effects of ac line current harmonics. Also, since the dc voltages  produced by the thyristor 6-pulse converters in Figure 77a and Figure 77b are added together in the thyristor 12-pulse converter (Figure 77c), the dc voltage of the 12-pulse converter is twice the dc voltage produced by each separate 6-pulse converter. Therefore, the power transfer efficiency obtained with the 12-pulse converter is higher than that obtained when using either of the 6-pulse converters individually; this is because the amount of power transferred (at any dc line current value) with the 12-pulse converter is twice that transferred with either 6-pulse converter, while the ܴ‫ܫ‬ଶ losses remain virtually the same. Furthermore, the ripple in the dc voltage of the 12-pulse converter contains 12 pulses per cycle of the ac power network instead of six, and its amplitude is lower than that of the ripple in the 6-pulse converters. This is because the 30° phase shift between the ripple in the dc voltages of the 6-pulse converters causes partial ripple cancellation when the dc voltages are added together in the 12-pulse converter. The lower ripple amplitude and higher ripple frequency of the 12-pulse converter make ripple easier to filter and result in a smoother voltage on the dc transmission line.

168

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Discussion

AC power network voltage ͳǦ

ͳ

Converter dc voltage

L1 AC power network

L2

L3



Line current ͳ

210 240 270 300 330

0

0

30 60 90 120 150 180

30 60 90 120 150 180 Phase angle ͳǦ (°)

(a) 6-pulse converter (wye-wye transformer)

AC power network voltage ͳǦ

ͳ

Converter dc voltage

L1 AC power network

L2

L3



0

Line current ͳ

210 240 270 300 330 0

30 60 90 120 150 180

30 60 90 120 150 180 Phase angle ͳǦ (°)

(b) 6-pulse converter (wye-delta transformer)

AC power network

ͳ

AC power network voltage ͳǦ

L1

Converter dc voltage

L2

Line current ͳ

L3



0

210 240 270 300 330 30 60 90 120 150 180

0

30 60 90 120 150 180

Phase angle ͳǦ (°)

(c) 12-pulse converter

Figure 77. Waveforms of the line current ͳ, the corresponding phase voltage ͳǦ of the ac power network, and dc voltage  for the two 6-pulse converters and 12-pulse converter.

169

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Discussion Level (+)

11

13

17

19

23

25

29

31

35

37

35

37

35

37

41

43

41

43

Order (nf)

(-) (a) 6-pulse converter (wye-wye transformer)

Level (+)

11

13

17

19

23

25

29

31

Order (nf)

(-) (b) 6-pulse converter (wye-delta transformer)

Level (+)

(-)

11

13

23

25

Order (nf)

(c) 12-pulse converter

Figure 78. Harmonics contained in the line current ͳ at the ac side of the two 6-pulse converters and the 12-pulse converter.

170

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Discussion

Monopolar HVDC transmission system implemented with a thyristor 12-pulse converter at each converter station Figure 79 shows a simple diagram of a monopolar HVDC transmission system implemented with a thyristor 12-pulse converter at each converter station. To obtain the desired dc line voltage, the dc sides of several thyristor 12-pulse converters are connected in series in each of the two stations. Converter station 1 12-pulse converter

AC power network 1

Bridge 1 Wye-wye transformer

Bridge 2 Wye-delta transformer

Converter station 2

Transmission line



12-pulse converter Bridge 1 Wye-wye transformer

AC power network 2

Bridge 2 Wye-delta transformer

Figure 79. Simplified diagram showing a monopolar HVDC transmission system implemented with a thyristor 12-pulse converter at each converter station.

171

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Discussion

Figure 80. HVDC converter station in Longquan, China, for the Three Gorges hydropower plant (photo courtesy of ABB).

Figure 81. Project: offshore wind farm connected to the electric power grid via an HVDC transmission line (© Siemens AG 2012, all rights reserved).

172

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Procedure Outline

PROCEDURE OUTLINE

The Procedure is divided into the following sections:

   PROCEDURE

Set up and connections Waveforms and harmonic contents related to the two 6-pulse converters in a 12-pulse converter 6-pulse converter 1 (converter connected to the wye-wye transformer). 6pulse converter 2 (converter connected to the wye-delta transformer).

Waveforms and harmonic contents related to the 12-pulse converter

Set up and connections In this part of the exercise, you will set up a thyristor 12-pulse converter and the equipment required to measure the circuit parameters.

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 as indicated below to simplify the connections required to implement the thyristor 12-pulse converter shown in Figure 82. 





Install the modules required to implement 6-pulse converter 1 (i.e., a Three-Phase Transformer Bank, a Power Thyristors module, and a Data Acquisition and Control Interface) in the left-hand side openings of the Workstation. Similarly, install the modules required to implement 6-pulse converter 2 (i.e., a Three-Phase Transformer Bank, a Power Thyristors module, and a Data Acquisition and Control Interface) in the right-hand side openings of the Workstation. Place the Three-Phase Transmission Line, Resistive Load, and the Power Supply in the middle openings of the Workstation.

a

In this exercise, the thyristor bridge (bridge 1) powered by a wye-wye (Y-Y) transformer is referred to as 6-pulse converter 1, while the thyristor bridge (bridge 2) powered by a wye-delta (Y-ǻ) transformer is referred to as 6-pulse converter 2.

2. On the Power Supply, make sure that the two ac power switches are set to the O (off) position, and that the voltage control knob is set to 0%. Connect the Power Supply to a three-phase ac power outlet.

3. Connect the Low Power Input of each Power Thyristors module to the 24 V ac power source of the Power Supply. Connect the Power Input of each Data Acquisition and Control Interface to the 24 V ac power source of the Power Supply. Turn the 24 V ac power source of the Power Supply on.

173

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Procedure 4. Connect the USB port of each Data Acquisition and Control Interface to USB ports of the host computer.

5. Turn the host computer on, then start the LVDAC-EMS software. In the LVDAC-EMS Start-Up window, make sure that both Data Acquisition and Control Interfaces (DACIs) are detected (the serial number of each DACI appears in the LVDAC-EMS Start-Up window). 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 LVDAC-EMS Start-Up window.

6. On each Power Thyristors module, set switches S1 and S2 to the I (on) position. This interconnects thyristors ͳ through ͸ of each Power Thyristors module in a thyristor three-phase bridge. Connect the equipment as shown in Figure 82. Before you begin connecting the equipment, record in the space below the serial numbers of the DACI modules used to control the two 6-pulse converters forming the 12-pulse converter. Serial number of the DACI controlling 6-pulse converter 1: Serial number of the DACI controlling 6-pulse converter 2: Use the fixed, three-phase ac voltage output of the Power Supply as the ac power source for the 12-pulse converter. Use the Three-Phase Transmission Line module to implement smoothing inductor . Use the Resistive Load module to implement resistors ͳ, ʹ, and ͵. Meter E2 must first be connected to measure the dc voltage across thyristor bridge 2, as Figure 82 shows.

a

174

Inputs E1, E2, E3, I1, I2, and I3 in Figure 82 are all inputs of the DACI controlling 6-pulse converter 1. Input E4 of the DACI controlling 6-pulse converter 1 provides synchronization of the firing signals for thyristor bridge 1. Input E4 of the DACI controlling 6-pulse converter 2 (represented by the red symbol E4 in Figure 82) provides synchronization of the firing signals for thyristor bridge 2.

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Procedure

6-pulse converter 1 (wye-wye transformer) Three-Phase Transformer Bank module (wye-wye) L1

1

2

5

4

Power Thyristors module



L1 Bridge 1

6

7

10

9

L2

L3 11

12

15

14

L3

L2

AC power source (8821), fixed voltage output

Firing control signals from the digital outputs of the DACI of converter 1

6-pulse converter 2 (wye-delta transformer) N Three-Phase Transformer Bank module (wye-delta) L1

1

2

Power Thyristors module L1

3

Bridge 2

15 L2

6

5

7

L2

8 13

L3

11

10

12

L3

Firing control signals from the digital outputs of the DACI of converter 2

Local ac power network Voltage (V)

Frequency (Hz)

ͳǡʹǡ͵ (ȍ)

 (ȍ)

120

60

171

60

220

50

629

66.7

240

50

686

66.7

220

60

629

66.7

First connect meter E2 to measure the voltage across 6-pulse converter 2. (Meter E2 will then be reconnected to measure the voltage across the 12-pulse converter, as shown by the dotted line.)

Figure 82. Thyristor 12-pulse converter.

175

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Procedure 7. Connect the Digital Outputs of each Data Acquisition and Control Interface to the Firing Control Inputs of the associated Power Thyristors module, using the provided cable with DB9 connectors.

8. In LVDAC-EMS, open the HVDC Transmission System Control window. A dialog box appears. Select the serial number of the DACI that is used to control 6-pulse converter 1 (recorded in step 6) then click the OK button to close the dialog box and open the HVDC Transmission System Control window. In the HVDC Transmission System Control window, make the following settings:  

 



Set the Function parameter to 12-Pulse Converter. This makes the HVDC Transmission System Control operate as a controller for a 12-pulse converter. 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 Ƚ of 6-pulse converters (bridges 1 and 2) to 0°. Make sure that the parameters ͳ through ͸ are all set to Active. This makes the firing signals of 6-pulse converters 1 and 2 (bridges 1 and 2) dependent on the Firing Angle parameter. Leave the other parameters set to their default values. Start the 12-Pulse Converter function by clicking the Start/Stop button or by setting the Status parameter to Started.

9. In LVDAC-EMS, open the Metering window. A dialog box appears. Select the serial number of the DACI that is used to perform parameter measurements in the 12-pulse converter (i.e., the same DACI that is used to control 6-pulse converter 1), then click the OK button to close the dialog box and open the Metering window. In the Metering window, set a meter to measure the rms value of source voltage ͳǦ (E3). Set two meters to measure the average (dc) voltages (E1 and E2) at the dc side of thyristor bridges 1 and 2.

176

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Procedure

Waveforms and harmonic contents related to the two 6-pulse converters in a 12-pulse converter In this section, you will observe and compare the waveforms and harmonic contents of the ac-side line currents and dc-side voltage in the two 6-pulse converters forming the 12-pulse converter. 10. On the Power Supply, set the voltage control knob to 100%. Turn the threephase ac power source on by setting the main power switch to I (on). The voltage at the fixed, three-phase ac voltage output of the Power Supply now feeds both 6-pulse converters in the 12-pulse converter.

6-pulse converter 1 (converter connected to the wye-wye transformer) 11. Start the Oscilloscope and display the following waveforms: the source voltage ͳǦ (E3), the line current (I1) at the ac side of 6-pulse converter 1, and the voltage (E1) at the dc side of 6-pulse converter 1. Briefly describe the waveform of the line current at the ac side of 6-pulse converter 1 with respect to the waveform of the source voltage ͳǦ.

Is the waveform of the line current at the ac side of 6-pulse converter 1 in phase (i.e., aligned) with the waveform of the source voltage ͳǦ? Explain. 12. In LVDAC-EMS, open the Harmonic Analyzer. Make sure that the fundamental frequency is set to the frequency of your local ac power network. Set the number of harmonics to 40. Set the scale type so that the vertical axis of the analyzer is graduated in percentage of the fundamental frequency component, (1f). Make sure that the vertical axis graduations are set at 10%/div. Display the harmonic content of the line current (input I1) at the ac side of 6-pulse converter 1. Record in Table 3 the order and relative level of each harmonic present in the line current of 6-pulse converter 1.

a

A low third harmonic component is present in the line current at the ac side of 6-pulse converter 1. Do not take it into account.

177

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Procedure Table 3. Harmonics present in the line current at the ac side of 6-pulse converter 1.

Harmonic order

Level (% of 1f)

13. Note and record the value of the total harmonic distortion (THD) of the line current at the ac side of 6-pulse converter 1, indicated by the Harmonic Analyzer. THD of the line current at the ac side of 6-pulse converter 1 ൌ

a

%

The total harmonic distortion (THD) of a waveform indicates the amount of harmonic distortion present in this waveform and is a measure of how much the waveform diverges from a pure sine wave. The higher the THD value, the more distorted the waveform is due to the presence of harmonics.

Does the value of the THD recorded above confirm that the waveform of the line current at the ac side of 6-pulse converter 1 is distorted?

 Yes

 No

14. On the Oscilloscope, observe the waveform of the voltage at the dc side of 6-pulse converter 1. Describe this waveform with respect to the waveform of the source voltage ͳǦ.

178

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Procedure What is the frequency of the ripple in the dc voltage produced by 6-pulse converter 1?

Note and record the value of the dc voltage produced by 6-pulse converter 1. DC voltage produced by 6-pulse converter 1 ൌ

V

15. On the Harmonic Analyzer, display the harmonic content of the voltage (input E1) at the dc side of 6-pulse converter 1. Set the scale type so that the vertical axis of the analyzer is graduated in percentage of the dc component. Set the vertical axis graduations to 1%/div. Observe that the voltage at the dc side of 6-pulse converter 1 mainly contains harmonics of the following orders: 6, 12, 18, 24, etc. Explain why.

Record in Table 4 the order and relative level of each harmonic present in the dc voltage of 6-pulse converter 1.

a

Low second and fourth harmonics are present in the dc voltage of 6-pulse converter 1. Do not take them into account. Table 4. Harmonics present in the dc voltage of 6-pulse converter 1. Harmonic order

Level (% of DC)

th

6

th

12

th

18

th

24

179

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Procedure

6-pulse converter 2 (converter connected to the wye-delta transformer) 16. On the Oscilloscope, display the waveform of the line current (I2) at the ac side of 6-pulse converter 2 and the waveform of the voltage (E2) at the dc side of 6-pulse converter 2.

a

Continue to display the waveform of the source voltage ͳǦ (E3), the waveform of the line current (I1) at the ac side of 6-pulse converter 1, and the waveform of the voltage at the dc side of 6-pulse converter 1 (E1).

Observe that the waveforms of the line currents at the ac side of 6-pulse converters 1 and 2 are different. Briefly describe the waveform of the line current at the ac side of 6-pulse converter 2 with respect to the waveform of the source voltage ͳǦ.

Is the waveform of the line current at the ac side of 6-pulse converter 2 in phase (i.e., aligned) with the waveform of source voltage ͳǦ? Explain.

17. On the Harmonic Analyzer, display the harmonic content of the line current (input I2) at the ac side of 6-pulse converter 2. Set the scale type so that the vertical axis of the analyzer is graduated in percentage of the fundamental frequency component (1f). Set the vertical axis graduations to 10%/div. Record in Table 5 the order and relative level of each harmonic present in the line current of converter 2.

a

180

The Harmonic Analyzer does not provide phase information on the harmonic components, and thus all harmonic components are displayed as being of th th th th th st positive polarity. Consequently, the 5 , 7 , 17 , 19 , 29 , and 31 harmonics in the line current of 6-pulse converter 2, which are of negative polarity (i.e., they are phase shifted by 180° with respect to the harmonics of the same orders in the line current of 6-pulse converter 1), are displayed with a positive polarity on the Harmonic Analyzer.

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Procedure Table 5. Harmonics present in the line current at the ac side of 6-pulse converter 2. Harmonic order

Level (% of 1f)

Compare the harmonic content of the line current at the ac side of 6-pulse converter 2 (recorded in Table 5) to the harmonic content of the line current at the ac side of 6-pulse converter 1 (recorded in Table 3). Are they the same? Explain.

18. Note and record the value of the total harmonic distortion (THD) of the line current at the ac side of 6-pulse converter 2. THD of the line current at the ac side of 6-pulse converter 2 ൌ

%

Compare the THD of the line current at the ac side of 6-pulse converter 1 (recorded in step 13) to the THD of the line current at the ac side of 6-pulse converter 2 (recorded above). Are they the same? Explain.

19. On the Oscilloscope, observe the waveform of the voltage at the dc side of 6-pulse converter 2. Describe this waveform with respect to the waveform of the source voltage ͳǦ.

181

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Procedure Note and record the value of the dc voltage produced by 6-pulse converter 2. DC voltage produced by 6-pulse converter 2 ൌ

V

Is the dc voltage produced by 6-pulse converter 2 (recorded above) equal to the dc voltage produced by 6-pulse converter 1 (recorded in step 14)?

 Yes

 No

20. What is the frequency of the ripple in the dc voltage produced by 6-pulse converter 2? Is it the same as for 6-pulse converter 1?

21. Is there a phase shift between the ripples in the dc voltages produced by 6-pulse converters 1 and 2? Explain.

22. On the Harmonic Analyzer, display the harmonic content of the voltage (input E2) at the dc side of 6-pulse converter 2. Set the scale type so that the vertical axis of the analyzer is graduated in percentage of the dc component. Set the vertical axis graduations to 1%/div. Record in Table 6 the relative level of the harmonics present in the dc voltage of 6-pulse converter 2.

a

Low second and fourth harmonics are present in the dc voltage of 6-pulse converter 2. Do not take them into account. Table 6. Harmonics present in the dc voltage of 6-pulse converter 2. Harmonic order

Level (% of DC)

th

6

th

12

th

18

th

24

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Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Procedure Compare the harmonic contents of the dc voltage of 6-pulse converter 2 (recorded above) to the harmonic contents of the dc voltage of 6-pulse converter 1 (recorded in step 15). What can you conclude? Explain.

23. In the HVDC Transmission System Control window, stop the 12-Pulse Converter function by clicking the Start/Stop button or by setting the Status parameter to Stopped.

Waveforms and harmonic contents related to the 12-pulse converter In this section, you will observe the waveforms and harmonic contents of the ac-side line currents and dc-side voltage in the 12-pulse converter. You will compare your results to those previously obtained for the 6-pulse converters.

24. On the Power Supply, turn the three-phase ac power source off by setting the main power switch to the O (off) position. Leave the 24 V ac power source of the Power Supply on. (Refer to Figure 82.) Disconnect input E2 of the DACI used to perform parameter measurements in the 12-pulse converter (i.e., the DACI controlling 6-pulse converter 1) from the dc side of 6-pulse converter 2 (bridge 2). Reconnect input E2 across the dc side of the 12-pulse converter, as shown by the dotted lines in Figure 82. On the Power Supply, turn the three-phase ac power source on.

25. In the HVDC Transmission System Control window, start the 12-pulse converter.

26. On the Oscilloscope, display the following waveforms: the source voltage ͳǦ (E3), the line current (I3) at the ac side of the 12-pulse converter, the line currents (I1 and I2) at the ac side of 6-pulse converters 1 and 2, and the voltages (E1 and E2) at the dc side of 6-pulse converter 1 and the 12-pulse converter. Observe the waveform of the line current at the ac side of the 12-pulse converter. Does this current tend towards a sine wave at the frequency of the source voltage voltage ͳǦ?

 Yes

 No

183

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Procedure Describe the waveform of the line current at the ac side of the 12-pulse converter with respect to the waveforms of the line currents at the ac side of 6-pulse converters 1 and 2.

27. On the Harmonic Analyzer, display the harmonic content of the line current (input I3) at the ac side of the 12-pulse converter. Set the scale type so that the vertical axis of the analyzer is graduated in percentage of the fundamental frequency component (1f). Set the vertical axis graduations to 10%/div. Record in Table 7 the order and relative level of each harmonic present in the line current of the 12-pulse converter.

a

Do not take the low third-harmonic component into account.

Table 7. Harmonics present in the line current at the ac side of the 12-pulse converter. Harmonic order

Level (% of 1f)

Compare the harmonic content of the line current at the ac side of the 12-pulse converter (recorded in Table 7) to the harmonic contents of the line currents at the ac side of 6-pulse converters 1 and 2 (recorded in Table 3 and Table 5, respectively). Record and explain your observations.

184

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Procedure 28. Note and record the value of the total harmonic distortion (THD) of the line current at the ac side of the 12-pulse converter. THD of the line current at the ac side of the 12-pulse converter ൌ

%

Compare the THD of the line current for the 12-pulse converter (recorded above) to the THDs of the line currents for 6-pulse converters 1 and 2 (recorded in step 13 and 18, respectively).

Does this confirm that the waveform of the line currents at the ac side of the 12-pulse converter is closer to a sine wave at the frequency of the fundamental component than the waveforms of the line currents at the ac side of the 6-pulse converters? Explain.

From your observations, explain the major advantage of using thyristor 12-pulse converters instead of thyristor 6-pulse converters.

29. On the Oscilloscope, observe the waveform of the voltage at the dc side of the 12-pulse converter. Describe this waveform with respect to the waveform of the source voltage ͳǦ.

Note and record the value of the dc voltage produced by the 12-pulse converter. DC voltage produced by the 12-pulse converter ൌ

V

Is the dc voltage produced by the 12-pulse converter (recorded above) twice the dc voltage produced by each separate 6-pulse converter (recorded in steps 14 and 19)? Explain.

185

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Procedure Explain why the power transfer efficiency obtained with the 12-pulse converter is higher than that obtained when using either of the 6-pulse converters individually.

30. Compare the frequency and amplitude of the ripple in the dc voltage of the 12-pulse converter to those of the ripple in the dc voltage of either of the two 6-pulse converters. Are they the same? Why?

31. On the Harmonic Analyzer, display the harmonic content of the voltage (input E2) at the dc side of the 12-pulse converter. Set the scale type so that the vertical axis of the analyzer is graduated in percentage of the dc component. Set the vertical axis graduations to 1%/div. Record in Table 8 the order and relative level of each harmonic present in the dc voltage of the 12-pulse converter.

a

Low second and fourth harmonics are present in the dc voltage of the 12-pulse converter. Do not take them into account. Table 8. Harmonics present in the dc voltage of the 12-pulse converter. Harmonic order

Level (% of DC)

Compare the harmonic content of the dc voltage produced by the 12-pulse converter (recorded in Table 8) to the harmonic contents of the dc voltages produced by 6-pulse converters 1 and 2 (recorded in Table 4 and Table 6, respectively). Record your observations and explain.

186

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Conclusion 32. From your observations, explain why thyristor 12-pulse converters provide a smoother voltage on dc transmission lines than thyristor 6-pulse converters.

33. On the Power Supply, turn the three-phase ac power source off by setting the main power switch to the O (off) position. Turn the 24 V ac power source of the Power Supply off. Close the LVDAC-EMS software. Disconnect all leads and return them to their storage location.

CONCLUSION

In this exercise, you learned that using a thyristor 12-pulse converter instead of a thyristor 6-pulse converter greatly helps in mitigating the undesirable effects of ac line harmonics. This is because the waveform of the line currents in a thyristor 12-pulse converter is closer to a sine wave at the frequency of the ac power network, and thus contains much less harmonics than the line currents in a thyristor 6-pulse converter. Also, you learned that the power transfer efficiency obtained with a thyristor 12-pulse converter is higher than that obtained when using either of the two 6-pulse converters individually; this is because the amount of power transferred with the 12-pulse converter for any value of the dc line current is twice that transferred with either 6-pulse converter, while the  ʹ losses remain virtually the same. Furthermore, the lower ripple amplitude and higher ripple frequency of the 12-pulse converter makes ripple easier to filter and results in a smoother voltage on the dc transmission line.

REVIEW QUESTIONS

1. Describe the waveform and harmonic content of one of the line currents at the ac side of a thyristor 6-pulse converter connected to a wye-wye (Y-Y) transformer (firing angle Į set to 0°).

187

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Review Questions 2. Describe the waveform and harmonic content of one of the line currents at the ac side of a thyristor 6-pulse converter connected to a wye-delta (Y-ǻ) transformer (firing angle Į set to 0).

3. Describe a basic thyristor 12-pulse converter. Explain how the thyristor converters and transformers must be connected and set to permit proper operation.

188

Exercise 5 – Harmonic Reduction using Thyristor 12-Pulse Converters  Review Questions 4. What is the major advantage of using the thyristor 12-pulse converters? Explain by comparing the harmonics contained in the line currents of the 6-pulse converters and 12-pulse converter.

5. Explain why the thyristor 12-pulse converter provides a higher power transfer efficiency than that obtained when using either of its 6-pulse converters individually.