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1 REACTIVE POWER COMPENSATION AND HARMONIC DISTORTION CONTROL IN ELECTRIC TRACTION SYSTEMS JUAN DAVID MARTINEZ QUINTERO UNIVERSIDAD DE LOS ANDES FACULTAD DE INGENIERIA DEPARTAMENTO DE INGENIERÍA ELÉCTRICA Y ELECTRÓNICA BOGOTA, D.C 2010
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REACTIVE POWER COMPENSATION AND HARMONIC DISTORTION CONTROL IN

ELECTRIC TRACTION SYSTEMS

JUAN DAVID MARTINEZ QUINTERO

UNIVERSIDAD DE LOS ANDES

FACULTAD DE INGENIERIA

DEPARTAMENTO DE INGENIERÍA ELÉCTRICA Y ELECTRÓNICA

BOGOTA, D.C

2010

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REACTIVE POWER COMPENSATION AND HARMONIC DISTORTION CONTROL IN

ELECTRIC TRACTION SYSTEMS

JUAN DAVID MARTINEZ QUINTERO

Trabajo presentado ante la Universidad de los Andes como requisito parcial para optar

por el título de Ingeniero Eléctrico

DIRECTOR

Ing. Gustavo Andrés Ramos López Ph.D

UNIVERSIDAD DE LOS ANDES

FACULTAD DE INGENIERIA

DEPARTAMENTO DE INGENIERIA ELÉCTRICA Y ELECTRÓNICA

BOGOTA, D.C

2010

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To my mother, who always stood by my side and believed in me.

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ACKNOWLEDGMENTS

I would like to express my gratitude to my project director Gustavo A. Ramos and to

Esperanza Susana Torres who aided me in the execution of this project.

And to everyone that taught me valuable lessons that helped me in achieving this goal.

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CONTENTS

SUMMARY ...................................................................................................................... 11

1. INTRODUCTION ....................................................................................................... 12

1.1 Objective .......................................................................................................... 12

2. DC TRACTION SYSTEMS ........................................................................................... 13

3. POWER QUALITY PROBLEM PRESENT IN AN ELECTRIC TRANSPORT SYSTEM ........ 15

4. MODELING AND SIMULATION OF THE SYSTEM IN PSCAD ...................................... 16

4.1 Study Cases ...................................................................................................... 19

4.1.1 Low Load Variability ................................................................................. 19

4.1.2 High Load Variability ................................................................................. 28

5. MODEL AND ANALYSIS OF THE COMPENSATOR ..................................................... 37

5.1 Features of FACTS devices ............................................................................... 37

5.2 Static Var Compensator (SVC).......................................................................... 39

5.2.1 Static Var Generator ................................................................................. 41

5.2.2 Static Var Compensator Control ............................................................... 47

6. ELECTRIC TRACTION SYSTEM COMPENSATED ........................................................ 49

6.1 Compensated Study Cases ............................................................................... 54

6.1.1 Low Load Variability ................................................................................. 55

6.1.2 High Load Variability ................................................................................. 62

7. IMPLICATIONS OF USING MSC-TCR SVG TO COMPENSATE THE ELECTRIC TRACTION

SYSTEM ........................................................................................................................... 70

8. CONCLUSION ........................................................................................................... 72

REFERENCES .................................................................................................................... 73

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List of Figures

Figure 1. DC Electric Traction System Feeding Scheme [1] ............................................ 13

Figure 2. DC Locomotive Diagram (Based on [3]) ........................................................... 14

Figure 3. Electric Traction System without compensation Block Diagram .................... 17

Figure 4. Feeding System Implemented on PSCAD ........................................................ 17

Figure 5. Rectifier Substation Model .............................................................................. 18

Figure 6. Direct Current Electric Traction System PSCAD Model ................................... 19

Figure 7. Measuring Points in Electric Traction System ................................................. 21

Figure 8. Voltage Waveform PCC1 ................................................................................. 21

Figure 9. Current Waveform PCC1 ................................................................................. 22

Figure 10. PCC1 Voltage (p.u) ......................................................................................... 22

Figure 11. Source I Current FFT and THD ....................................................................... 22

Figure 12. Voltage Waveform PCC2 ............................................................................... 23

Figure 13. Current Waveform PCC2 ............................................................................... 23

Figure 14. PCC2 Voltage (p.u) ......................................................................................... 23

Figure 15. Source II Current FFT and THD ...................................................................... 24

Figure 16. 1Prim Voltage Waveform .............................................................................. 24

Figure 17. 1Prim Current Waveform .............................................................................. 25

Figure 18. 1Prim Voltage (p.u) ........................................................................................ 25

Figure 19. 2Prim Voltage Waveform .............................................................................. 25

Figure 20. 2Prim Current Waveform .............................................................................. 26

Figure 21. 2Prim Voltage (p.u) ........................................................................................ 26

Figure 22. 3Prim Voltage Waveform .............................................................................. 26

Figure 23. 3Prim Current Waveform .............................................................................. 27

Figure 24. 3Prim Voltage (p.u) ........................................................................................ 27

Figure 25. Rectifier Substation Current FFT and THD ..................................................... 27

Figure 26. PCC 1 Voltage Waveform .............................................................................. 29

Figure 27. PCC 1 Current Waveform .............................................................................. 30

Figure 28. PCC 1 Voltage (p.u) ........................................................................................ 30

Figure 29. Source I Current FFT and THD ....................................................................... 30

Figure 30. PCC2 Voltage Waveform ............................................................................... 31

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Figure 31. PCC2 Current Waveform ............................................................................... 31

Figure 32. PCC2 Voltage (p.u) ......................................................................................... 31

Figure 33. Source 2 Current FFT and THD ...................................................................... 32

Figure 34. 1Prim Voltage Waveform .............................................................................. 32

Figure 35. 1Prim Current Waveform .............................................................................. 33

Figure 36. 1Prim Voltage (p.u) ........................................................................................ 33

Figure 37. 2Prim Voltage Waveform .............................................................................. 33

Figure 38. 2Prim Current Waveform .............................................................................. 34

Figure 39. 2Prim Voltage (p.u) ........................................................................................ 34

Figure 40. 3Prim Voltage Waveform .............................................................................. 34

Figure 41. 3Prim Current Waveform .............................................................................. 35

Figure 42. 3Prim Voltage (p.u) ........................................................................................ 35

Figure 43. Rectifier Substation Current FFT and THD ..................................................... 35

Figure 44. TCR-TSC and TSC diagram (Based on [5]) ...................................................... 40

Figure 45. TSR-TCR Diagram (Based on [5]) .................................................................... 41

Figure 46. Operating V-I Areas of TCR and TSR (Based on [5])....................................... 43

Figure 47. TSC Diagram (Based on [5]) ........................................................................... 44

Figure 48. Operating V-I Area of a TSC (Based on [5]) ................................................... 45

Figure 49. SVG TSC-TCR Diagram (Based on [5]) ............................................................ 46

Figure 50. Operating V-I area of SVG TSC-TCR with two TSC branches (Based on [5]) .. 46

Figure 51. Control Scheme TSC-TCR SVG (Based on [5]) ................................................ 47

Figure 52. V-I Characteristic of SVC (Based on [5]) ........................................................ 48

Figure 53. Control scheme of a SVC ............................................................................... 49

Figure 54. Electric Traction System with SVC and Filter ................................................. 50

Figure 55. SVC PSCAD Model .......................................................................................... 50

Figure 56. SVC Control Scheme Block Diagram .............................................................. 51

Figure 57. Susceptance Order PSCAD Control Scheme .................................................. 52

Figure 58. Delay Angle and TSC PSCAD Control Scheme ................................................ 52

Figure 59. Electric Traction System with compensation PSCAD Model ......................... 53

Figure 60. Fifth Harmonic Filter ...................................................................................... 54

Figure 61. Number of Capacitor Stages ON .................................................................... 55

Figure 62. Alpha Order ................................................................................................... 55

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Figure 63. PCC1 Compensated Voltage Waveform ........................................................ 56

Figure 64. PCC1 Compensated Current Waveform ........................................................ 56

Figure 65. PCC1 Compensated Voltage (p.u) ................................................................. 56

Figure 66. Source 1 Compensated Current Wave FFT and THD ..................................... 57

Figure 67. PCC2 Compensated Voltage Waveform ........................................................ 57

Figure 68. PCC2 Compensated Current Waveform ........................................................ 57

Figure 69. PCC2 Compensated Voltage (p.u) ................................................................. 58

Figure 70. Source 2 Compensated Current Wave FFT and THD ..................................... 58

Figure 71. 1Prim Compensated Voltage Waveform ....................................................... 58

Figure 72. 1Prim Compensated Current Waveform ....................................................... 59

Figure 73. 1Prim Compensated Voltage (p.u) ................................................................ 59

Figure 74. 2Prim Compensated Voltage Waveform ....................................................... 59

Figure 75. 2Prim Compensated Current Waveform ....................................................... 60

Figure 76. 2Prim Compensated Voltage (p.u) ................................................................ 60

Figure 77. 3Prim Compensated Voltage Waveform ....................................................... 60

Figure 78. 3Prim Compensated Current Waveform ....................................................... 61

Figure 79. 3Prim Compensated Voltage (p.u) ................................................................ 61

Figure 80. Rectifier Substation Compensated Current FFT and THD ............................. 61

Figure 81. Alpha Order ................................................................................................... 62

Figure 82. Capacitor Stages ON ...................................................................................... 62

Figure 83. PCC1 Compensated Voltage Waveform ........................................................ 63

Figure 84. PCC1 Compensated Current Waveform ........................................................ 63

Figure 85. PCC1 Compensated Voltage (p.u) ................................................................. 63

Figure 86. Source 1 Compensated Current Wave FFT and THD ..................................... 64

Figure 87. PCC2 Compensated Voltage Waveform ........................................................ 64

Figure 88. PCC2 Compensated Current Waveform ........................................................ 64

Figure 89. PCC2 Compensated Voltage (p.u) ................................................................. 65

Figure 90. Source 2 Compensated Current Wave FFT and THD ..................................... 65

Figure 91. 1Prim Compensated Voltage Waveform ....................................................... 65

Figure 92. 1Prim Compensated Current Waveform ....................................................... 66

Figure 93. 1Prim Compensated Voltage (p.u) ................................................................ 66

Figure 94. 2Prim Compensated Voltage Waveform ....................................................... 66

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Figure 95. 2Prim Compensated Current Waveform ....................................................... 67

Figure 96. 2Prim Compensated Voltage (p.u) ................................................................ 67

Figure 97. 3Prim Compensated Voltage Waveform ....................................................... 67

Figure 98. 3Prim Compensated Current Waveform ....................................................... 68

Figure 99. 3Prim Compensated Voltage (p.u) ................................................................ 68

Figure 100. Rectifier Substation Compensated Current Wave FFT and THD ................. 68

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List of Tables

Table 1. Operating States of Rectifier Substations ......................................................... 19

Table 2. Breaker Operation ............................................................................................ 20

Table 3. Operating States Rectifier Substation I ............................................................. 28

Table 4. Operating States Rectifier Substation II ............................................................ 28

Table 5. Operating States Rectifier Substation III........................................................... 28

Table 6. Breaker Operation ............................................................................................ 28

Table 7. Study Cases Relevant Parameters .................................................................... 36

Table 8. Signal Description PSCAD Model ...................................................................... 50

Table 9. Low Variability Comparative Results ................................................................ 69

Table 10. High Variability Comparative Results ............................................................. 70

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SUMMARY

The main goal of this project was to study and analyze the behavior of a direct current

electric traction system and the consequences on the power quality of the distribution

network. Once the problems were identified a solution was proposed through a

Flexible AC Transmission Systems (FACTS) device. For the purposes of this study a

Static Var Compensator (SVC) was selected. This compensator has a control loop that

allows setting a reference value for the desired voltage of the system and corrects it. A

filter was installed also to lower wave distortion in the system in the same node as the

proposed compensator.

To verify that the proposed solution was in fact adequate, Colombian regulation was

revised. The NTC 1340 states that for a voltage level of 34.5kV there is an allowed 5%

overvoltage and a 10% drop. Compared to other countries this is a flexible regulation,

reason why for this project the specifications adopted were the ones currently

employed by the United States. This regulation has a 5.0% voltage drop tolerance; this

value was obtained from Table 3-1 of the IEEE 141-1993 [11], which is based on

ANSIC84.1-1989. For harmonic generation IEEE 519-1992 states the ones expected for

a 6 pulse rectifier such as the one used in the systems rectifier substations, and also

the current Total Harmonic Distortion limits in the network, which were taken as 5%

from table 10.3 from IEEE 519-1992 [10].

Finally the possible events that could occur from changing the original configuration of

the Static Var Generator from a bidirectional thyristor valve to a mechanical breaker

were listed. And these phenomena were evaluated if they had relevance in the

particular characteristics of the proposed model.

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1. INTRODUCTION

Electric traction systems have been acquiring momentum in the nowadays

transportation sector. Since the beginnings of last century they have been a preferred

option due to their high performance, low maintenance cost, and lack of greenhouse-

gas emission to the atmosphere.

In general electric traction systems show very specific characteristics in relation to

their operation. These aspects have a very clear impact on the conception of the

electric infrastructure. Their principal features are: [1]

The AC and DC subsystems should count with backup equipment that can be

switched on or off depending on the actual contingency situation the system is

operating on. These specific requirements are taken into account in order to

be able to achieve reliability and the continuance of the service.

Permanent load variations as a consequence of the operation cycles of the

vehicles. The power demand of the system is non-linear and this has an

important impact on the different aspects of the conception of the system.

Regenerative conditions as a consequence of the operations of regenerative

breaking of the vehicle. In these cases it is necessary that the system is

incorporated with a specific device that stores or uses this energy.

The AC and DC traction systems can generate disturbances to the power quality and as

a consequence, voltage unbalances and wave distortion appear. In AC [6] based

systems the voltage unbalance is the main problem, while in the DC systems the

harmonic generation must be taken into account due to the operation of the AC/DC

converters present in the rectifier substations [12].

1.1 Objective

The objective of this project is to study and analyze the different events that are

generated by a direct current electric traction system and their correction through

power electronic devices. Using the simulation tool PSCAD (Power System CAD)

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propose a model that can generate the similar power quality problems generated by

an electric traction system in operation. Review Flexible AC Transmission Systems

(FACTS) technology and select the most appropriate for the compensation of these

phenomena, also include a filter in the system to reduce harmonic flow to the sources.

With the filter and the FACTS device selected, simulate the system and evaluate if the

solution proposed places values under regulation. Finally compare the solution

proposed with an alternative choice and analyze what are the implications of making

that change.

2. DC TRACTION SYSTEMS

The system that it is going to be analyzed and compensated is a direct current electric

traction system. Figure 1 shows the typical feeding scheme. Starting from the high

voltage grid, two double circuit substations are connected to each other at each end of

the transmission line; each one of them is connected through a double circuit which

enables each one of the rectifier substations to provide power to the catenary at the

desired DC level.

Figure 1. DC Electric Traction System Feeding Scheme [1]

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Power is fed to the vehicle through a conductor cable connected to the catenary

system that each car has. Nowadays most transportation systems function with

standardized voltage levels of 600V to 750V [1], in this particular case the catenary

system will feed the vehicle at 750V.

As it was mentioned the rectifier substation is a fundamental part of the feeding

system, its basic function is to transform and rectify the AC voltage into a DC desired

voltage. The typical parts of the substation are: [1]

MV electric cells

A mid to low voltage transformer

Rectifier blocks of 6 pulses with firing angle control

High speed breakers

DC network outputs

Electric Locomotive Parts

Figure 2 shows the representative loads that each vehicle has. The three phase AC

motors are the main loads. There are also motor blowers, cooling systems, lights and

controls. The power electronic devices present in the locomotive can also be

appreciated due to the fact that they are a source of harmonic distortion onto the

feeding network.

Figure 2. DC Locomotive Diagram (Based on [3])

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Nowadays the vehicles use for their traction asynchronous squirrel cage motors, due

to their economy and reliability. Each one of the motors installed in the vehicles

consumes between 44kW and 280kW and are fed through inverters which are

synchronized around 400V. The other previously mentioned loads are not comparable

to the motors.

Another important aspect is the regenerative breaking system present in the

locomotives, because the energy generated from this process has to be either

dissipated through a resistor or reabsorbed by the system to avoid over voltages on

the DC feeders.

3. POWER QUALITY PROBLEM PRESENT IN AN ELECTRIC TRANSPORT SYSTEM

Electric transport systems have power electronic devices which have a direct impact on

the normal system conditions and the behavior of certain components in the presence

of contingency situations. The power quality phenomena that can appear are: voltage

fluctuations, voltage and current wave distortion, voltage sags, voltage transient, and

voltage and current unbalances [3]. It is important to point out the fact that all of the

phenomena mentioned above appear in cases in which there is a presence of non

linear loads which is the case in the traction system in study because the power

demand of the rectifier substations depends on the vehicle traffic at the time.

Here are the definitions of the probable phenomenon present.

Voltage Fluctuations [1]: Sudden load changes due to de presence of reactive power

cause this problem and it can be harmful to the control circuits.

Current and Voltage Wave Form Distortion [1]: The current wave distortion is

generated by the operation of rectifiers and increases the losses in the conductors and

transformers demining their load capacity. Distortion in the voltage wave is generated

due to the combination of the current demanded by non linear loads and the system

impedance; it also affects the control and regulation circuits and may also be harmful

to communication systems.

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Voltage Sags [1]: They can be generated by fast load variations. In the specific case of

the electric transport system the simultaneous acceleration of several trains fed by the

network or faults in the system. This phenomenon is damaging to solid state devices.

Transient voltages effect [1]: There are two types, oscillatory and impulse. They are

generated by atmospheric discharges, capacitor charging, transmission line failures,

power electronic device operation, and inappropriate protection action. These events

damage the solid state devices.

Voltage and Current Unbalances [1]: They occur when there is an asymmetry in the

impedance system or the feeding is not balanced. In the electric traction system the

motors are mainly affected because these current unbalances enable a counter torque

that increases loses and if it is excessive it can result in a bigger deterioration of the

device. Voltage unbalances affect the multi pulse system performance.

Rectifier Substation

The rectifier substation is an additional component in the DC electric transportation

system that generates a key problem to the network. It produces waveform distortions

and consequent harmonic generation. The IEEE 519-1992 states that these types of

rectifiers generate odd harmonics except for multiples of three, either it is a 6 or 12

pulse rectifier used.

Direct current electric traction systems do not have just one rectifier substation; in this

particular example three rectifier substations are supposed. In larger systems there

can be more rectifiers. In a two source system harmonics will flow equally to them and

propagate the problem all over the network.

4. MODELING AND SIMULATION OF THE SYSTEM IN PSCAD

The basic idea of the traction system is shown in Figure 3, and this basic model was

implemented on PSCAD with some changes.

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Figure 3. Electric Traction System without compensation Block Diagram

The main feeding system was implemented as Figure 4 shows.

Figure 4. Feeding System Implemented on PSCAD

Both sources have a line to line voltage of 34.5kV, and after that the short circuit

equivalent of the system was assumed to have a 100MVA capacity. The RL equivalent

was calculated through the following expression with a rated frequency of 60Hz:

2

2

(34.5 )11.9025

100

(34.5 )0.031572

100

kVR

MVA

kVL

MVA

Between the two sources there is a 7.5km line divided into 3 stages in which a rectifier

substation is going to be connected. It was assumed that the conducting wire across

the path was an AWG 4/0.

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Each one of the rectifier substations is made up by a 6 pulse bridge, a 10MVA delta-

delta transformer and the load.

Figure 5. Rectifier Substation Model

The transformer has a relation of 34.5/0.6kV due to the fact that the rectifier gives an

output in DC that is around 1.35 times the line to line voltage that is fed, through this

factor the feeding voltage was calculated in order to achieve the 750V desired. Also

the firing control of the delay angle was set to cero in the PSCAD model in order to be

able to rectify the full wave. The substation has a fixed load as soon as it is connected

but it has two additional branches with breakers that as they operate create the effect

of a varying load. The effect of the variation in the load is amplified due to the fact that

each substation has a breaker that takes it in and out of operation. Figure 6 shows the

electric traction system modeled in PSCAD.

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Figure 6. Direct Current Electric Traction System PSCAD Model

4.1 Study Cases

To be able to analyze the behavior of the system two scenarios were created; one with

high variability of the load and the other with more of a stepwise variation. The

purpose of these examples is to recreate a demand on peak hours, and on what is

known as valley hours.

The time window of the simulation was of 5 seconds. It is important to point out that

PSCAD breakers limit their number of operations to just two, and an initial operation

state of open or closed. Each rectifier substation has a total of 3 breakers; the first one

switches on or off the operation of the substation, and the other two breakers increase

or decrease the load in the dc side.

4.1.1 Low Load Variability

Tables 1-2 contain the operating states of each one of the rectifier substations and if

their operation is at full load or if the load is changing.

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Table 1. Operating States of Rectifier Substations

Rectifier

Substation 0≤t≤1 1≤t≤2 2≤t≤3 3≤t≤4 4≤t≤5

SEE I BRK 1 OFF FULL LOAD FULL LOAD FULL LOAD

SEE II OFF BRK 2 OFF OFF OFF

SEE III OFF OFF BRK 3 OFF OFF

As it was mentioned each one of the rectifier substations has 2 breakers that will

switch in and out the load connected.

Table 2. Breaker Operation

Initial

State

First

Operation (s)

Second

Operation (s)

BRK 1 BRK 1.1 Closed 0.5 0.95

BRK 1.2 Closed 0.3 0.8

BRK 2 BRK 2.1 Closed 1.0 1.5

BRK 2.2 Closed 1.3 1.8

BRK 3 BRK 3.1 Closed 2.5 2.95

BRK 3.2 Closed 2.2 2.6

In this case the load of the system is not very high and the operation of the rectifier

substations is in a stepwise form. There is only one high increase in the load during the

time window of 2 and 3 seconds in which there are two rectifier substations in

operation but only one of them at full load.

In the simulations the most important parameters that were measured were the p.u

voltage, instantaneous current and voltage, the Fast Fourier Transform (FFT) of the

current waves at the sources, and the current Total Harmonic Distortion (THD) of the

sources. Besides the Fourier transform and the current THD, the parameters were

measured at several points: the Point of Common Coupling (PCC) 1 and 2, primary

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sides of each transformer at the rectifier substations and at the sources. Each one of

these points is shown in Figure 7.

Figure 7. Measuring Points in Electric Traction System

Figures 8-15 show the previously mentioned parameters simulated in the low load

variability case.

Figure 8. Voltage Waveform PCC1

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Figure 9. Current Waveform PCC1

Figure 10. PCC1 Voltage (p.u)

Figure 11. Source I Current FFT and THD

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Figure 12. Voltage Waveform PCC2

Figure 13. Current Waveform PCC2

Figure 14. PCC2 Voltage (p.u)

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Figure 15. Source II Current FFT and THD

From the sources and Points of Common Coupling (PCC) graphics there are several

aspects to take into account. First of all, voltage and current waveforms present a

considerable distortion. Harmonic flow to the sources is reflected on the current FFT

and THD of both sources. The p.u voltage at both PCC does not comply with the

assumed regulation of 5% of voltage drop proposed.

Figure 16. 1Prim Voltage Waveform

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Figure 17. 1Prim Current Waveform

Figure 18. 1Prim Voltage (p.u)

Figure 19. 2Prim Voltage Waveform

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Figure 20. 2Prim Current Waveform

Figure 21. 2Prim Voltage (p.u)

Figure 22. 3Prim Voltage Waveform

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Figure 23. 3Prim Current Waveform

Figure 24. 3Prim Voltage (p.u)

Figure 25. Rectifier Substation Current FFT and THD

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The behavior in the three rectifier substations is quite similar; there is a clear voltage

waveform distortion. It is also clear that none of them achieve the voltage regulation

specifications, showing a voltage drop greater that 5% in some cases. Distortion is

present in the rectifier substations although it is expected given the characteristics of

this device in the system.

4.1.2 High Load Variability

The second case designed has two main characteristics: a higher load in the system

and the breaker movement at the rectifier substation is more irregular. Tables 3-6,

show the operation of the substations in this case.

Table 3. Operating States Rectifier Substation I

0≤t≤1 1≤t≤2 2≤t≤3 3≤t≤4 4≤t≤5

SEE I BKR 1 OFF FULL LOAD FULL LOAD FULL LOAD

Table 4. Operating States Rectifier Substation II

0≤t≤2 2≤t≤2.5 2.5≤t≤3 3≤t≤4 4≤t≤5

SEE II FULL LOAD BRK 2 OFF BRK 2 FULL LOAD

Table 5. Operating States Rectifier Substation III

0≤t≤2 2≤t≤2.4 2.4≤t≤3.1 3.1≤t≤4 4≤t≤5

SEE III FULL LOAD FULL LOAD OFF BRK 3 FULL LOAD

Table 6. Breaker Operation

Initial

State

First

Operation (s)

Second

Operation (s)

BRK 1 BRK 1.1 Closed 0.5 0.95

BRK 1.2 Closed 0.3 0.8

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BRK 2 BRK 2.1 Closed 2.0 2.5

BRK 2.2 Closed 3.3 3.8

BRK 3 BRK 3.1 Closed 3.5 3.95

BRK 3.2 Closed 3.3 3.8

The system is much more loaded and has a very fast change in the conditions of the

load. For example, in the time frame of 3 to 3.2 seconds the system goes from 1

rectifier substation to 3. Out of the 3 substations 2 of them have additional load

variation, which has a clear impact on the voltage along the line.

As with the low variability case the same parameters were studied to analyze the

variation in the voltage and the wave distortion present with a higher disturbing load.

Figure 26. PCC 1 Voltage Waveform

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Figure 27. PCC 1 Current Waveform

Figure 28. PCC 1 Voltage (p.u)

Figure 29. Source I Current FFT and THD

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Figure 30. PCC2 Voltage Waveform

Figure 31. PCC2 Current Waveform

Figure 32. PCC2 Voltage (p.u)

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Figure 33. Source 2 Current FFT and THD

This case is clearly worse than the first one in terms of voltage regulation. Both PCC

show a voltage under 0.93p.u which clearly does not comply with the desired

regulation. In terms of harmonic distortion it is practically the same, THD went from

24% to 22.5%.

Figure 34. 1Prim Voltage Waveform

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Figure 35. 1Prim Current Waveform

Figure 36. 1Prim Voltage (p.u)

Figure 37. 2Prim Voltage Waveform

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Figure 38. 2Prim Current Waveform

Figure 39. 2Prim Voltage (p.u)

Figure 40. 3Prim Voltage Waveform

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Figure 41. 3Prim Current Waveform

Figure 42. 3Prim Voltage (p.u)

Figure 43. Rectifier Substation Current FFT and THD

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Looking at the voltage on the 1PRIM, 2PRIM and 3PRIM points it is clear that they do

not satisfy the voltage regulation.

After running these two cases it is clear what are the measures to take and the

parameters to take into account. The compensator has to correct the voltage in both

cases to achieve the desired regulation and the filter has to correct the wave

distortion.

Table 7 summarizes the most important parameters to be analyzed throughout the

compensation. The table shows the minimum value that the voltage reaches and also

the current THD at the sources, which is a parameter that is important to analyze the

wave distortion through the network.

Table 7. Study Cases Relevant Parameters

Node Low Variability

Parameters

High Variability

Parameters

PCC1 0.946p.u 0.922p.u

PCC2 0.945p.u 0.922p.u

1PRIM 0.945p.u 0.923p.u

2PRIM 0.945p.u 0.923p.u

3PRIM 0.945p.u 0.923p.u

THDF1 24.07% 22.68%

THDF2 24.20% 22.50%

The biggest difference between the two cases is the voltage drop that the system

suffers. A voltage fall of 0.022p.u represents an additional decrease of 2.2% in the

voltage in the high variability case compared to the low variability case. The THD

shows that in both cases the wave distortion has an impact on each source in similar

ways, this distortion flow in both directions seeking the sources of the system. The

distortion in the high variability case is around 2% lower than the low variability case,

but are still comparable and beyond the desired values.

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5. MODEL AND ANALYSIS OF THE COMPENSATOR

The basic electric traction system in DC impacts different aspects on the network that

is connected to with the previously mentioned phenomena. One way to correct these

problems is through power electronics. Devices as the Flexible AC Transmission

Systems (FACTS) offer the possibility to acquire an important control over certain

parameters of the network, thereby improving its function.

The main effect that the traction systems have on the network is voltage regulation

problems through the system due to the behavior of the trains which basically are

seen by the network as a variable load. The other problem is harmonic generation

which is mainly produced by the 6 pulse rectifier substations and as they flow onto the

rest of the distribution system devices connected to it are harmed. Wave distortion

correction is solved installing a filter at a determined frequency with the compensator.

5.1 Features of FACTS devices

One of the most interesting aspects for the transmission system planning is that the

FACTS technology opens a new window of opportunity in controlling power and

improving the useful capacity of the existing lines. Opportunities are pretty big through

the use of FACTS due to the ability to control correlated parameters such as shunt and

series impedance, current, voltage, phase angle and the damping of oscillations at

various frequencies below the rated. The use of FACTS controllers allows existing

transmission lines to transport more power closer to their thermal capacity.

Basic Types of FACTS Controllers

In general, FACTS Controllers can be divided into four categories [5]:

Series Controllers

Shunt Controllers

Combined series-series Controllers

Combined series-shunt Controllers

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Series Controllers [5]: Could be variable impedance, like a capacitor, reactor, or a

power electronics based variable source of main frequency. As a principle all series

controllers inject voltage in series with the line. As long as the voltage is in phase

quadrature with the line current, the series controller only supplies or consumes

variable reactive power, any other relation would impact real power as well.

Shunt Controllers [5]: As in the series controllers, shunt controllers can also behave as

variable impedances, a variable source or a combination of these. As a principle every

shunt controller injects current to the system in the connection point. As long as the

current is in phase quadrature with the line voltage, the shunt controller only supplies

or consumes variable reactive power, any other relation will involve real power.

Combined series-series Controller [5]: This type of controllers can be a combination of

series controllers controlled in a coordinated manner or with a unified controller. They

can balance reactive as well as active power maximizing the utility of the transmission

system.

Combined series-shunt Controller [5]: These controllers can be a combination of

separate shunt and series controllers controlled coordinately, or a unified controller of

power flow with shunt and series elements. When the controllers are coordinated

each one of them injects current and voltage, but when there unified control there can

be a real power exchange between the series and shunt controllers via the power link.

If the goal of the system is to control current flow, power flow, and damping

oscillations the series controller for a rated power is the accurate choice to make.

Series controllers have an impact on voltage hence current and power flow are

modified directly.

As shunt controllers control current they become a good way to control voltage in the

connection point through the injection of active and reactive currents. The operation

of this controller is also effective in damping the voltage oscillations in the system.

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5.2 Static Var Compensator (SVC)

The Static Var Compensator (SVC) is a shunt FACTS controller designed to control

voltage that was first shown in Nebraska and commercialized by GE in 1974.

Westinghouse started its commercialization in Minnesota in 1975 [5].

This device is defined as a shunt static generator or consumer of reactive power. Its

output can be adjusted to the exchange of capacitive or inductive current. The SVC

definition according to the IEEE CIGRE is that the SVC is a Static Var Generator (SVG)

whose output can be modified in order to maintain or control specific parameters of

the power system such as voltage or frequency [5].

This term is applied generally to the thyristor controlled reactor (TCR), or a thyristor

switched reactor (TSR), and/or thyristor switched capacitor (TSC) or combination. The

SVC includes separate equipment for the reactive power leading or lagging, the TCR or

TSR absorb reactive power while the TSC supplies it to the system.

TCR [5]: A shunt connected thyristor-controlled inductor whose effective reactance is

varied in a continuous manner by partial-conduction control of the thyristor valve.

TSR [5]: A shunt connected thyristor switched inductor whose effective reactance is

varied in a stepwise manner by full or zero conduction operation of the thyristor valve.

The main difference between the TCR and TSR is the fact that the TCR has firing angle

control while the TSR has none and this bounds the amount of applications and

options that one can have in achieving the desired voltage control.

TSC [5]: A shunt connected thyristor switched whose effective reactance is varied in a

stepwise manner by full or zero conduction operation of the thyristor valve. The

reason why the TSC has a stepwise control scheme is due to the fact that they cannot

be switched continuously.

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Figure 44. TCR-TSC and TSC diagram (Based on [5])

The SVC are included in the Static Var Generators (SVG) category, these devises are

defined as a reactive power source that with the appropriate controls can become a

shunt compensator with a specific o multiple purpose.

The purpose of reactive compensation is to change the natural electric characteristics

of the transmission line to make it more compatible with the main load. The general

main goal of applying shunt compensation is to increase the transmittable power; this

change has an impact on improving transmission on steady state operation and the

stability of the system.

To increase power transmission, transient and voltage stability, damping of the system,

there are three specific requirements for shunt reactive compensators [5]:

The compensator should stay in synchronous operation with the AC system at

the compensated bus under all operating conditions including major

disturbances. Under post fault conditions the compensator must be able to

resume synchronism.

The compensator must be able to regulate bus voltage for voltage support,

improve transient stability, or control it for power oscillation damping.

For a transmission line connecting two systems, the best location for var

compensation is in the middle, whereas for a radial feed to a load the best

location is at the load end.

As it was previously mentioned SVG includes SVC and what transforms a SVG into a

SVC are the external or internal system controls. The input of these controllers can be

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a determined reference value according to the operational requirements of the system

and the variables that control it in order to be able to achieve the desired

compensation in the transmission line. So in conclusion the basic operational

characteristic of the SVC is going to be determined by the SVG type and structure.

5.2.1 Static Var Generator

Thyristor Controlled Reactor-Thyristor Switched Reactor (TCR-TSR)

Figure 45 shows a diagram of a TCR or TSR, which consists of a fixed reactor if

inductance L, and a bidirectional thyristor valve. This valve conducts with a current

impulse on the thyristor gate and just like a diode it conducts until polarity changes.

The valve is going to block conduction in the same instant in which the AC current

crosses over zero, unless another gate impulse is applied.

Figure 45. TSR-TCR Diagram (Based on [5])

The current in the reactor can be controlled from maximum to zero by the method of

firing delay angle (α) control. This control consists in delaying the closure of the

thyristor valve with respect to the voltage peak every half cycle; thereby the duration

of the current conduction intervals can be modified. When the valve is delayed in α,

which varies between 0 and π/2 respect to the voltage crest the current in the reactor

can be expressed by the following equation, assuming ( ) ( )v t VCos t :

1( ) ( ) (sin sin )

t

L

Vi t v t dt t

L L

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Since the valve opens when current crosses over zero, the equation is valid only for the

t interval. If the expression is going to be used for the negative half-

cycles the sign of the terms y the equation change.

The offset of the signal can be obtained also from the previous equation due to the

fact that there is a term that does not depend on time, sinVL

, it just depends on

the delay angle. This value will shift down for positive half cycles and up for negative

half cycles.

Delay angle, α, also defines the conduction angle of the system, σ, through the

expression, σ=π-2α. Thus as α increases as so does the offset, the conduction angle

decreases and there is a reduction in the reactors current. As the delay angle and

offset approach their maximum value the current on the reactor approaches zero.

The fundamental current amplitude of the reactor, iLF (t), can be expressed as a

function of α:

2 1( ) 1 sin 2LF

VI

L

In this equation V is the amplitude of the AC applied voltage, L is the inductance of the

TCR and ω is the angular frequency of the applied voltage. From this expression it can

be analyzed the fact that it is possible to control in a continuous way the fundamental

current of the reactor from zero to its maximum current as if it was a variable

reactance. Hence it can be defined admittance depending on the firing angle:

1 2 1( ) 1 sin 2LB

L

Analyzing this equation it can be seen that the admittance varies with α in the same

way as the fundamental current previously expressed.

The main difference between the TSR and TCR can be appreciated in their V-I curves,

the first one defines a fixed admittance when is connected to the AC system, while the

TCR generates an operation region as it can be seen in Figure 46.

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Figure 46. Operating V-I Areas of TCR and TSR (Based on [5])

When the firing angle is varied non-sinusoidal current waveforms will be produced in

the reactor, this means that in the TCR operation besides the fundamental current a

series of harmonic distortion will be generated. These harmonics will be generated by

positive and negative cycles, and only odd harmonics will appear. The amplitude of

these current harmonics will be in function of de firing angle and can be expressed

through the following equation:

2

4 sin cos( ) cos sin( )( ) 2 1, 1

( 1)Ln

V n n nI n k k

L n n

In a three phase system, three single phase delta connected TCR are generally used

because under balanced conditions the triple-n harmonic currents will not flow into

the power system but they will stay in the delta connection of the compensator.

Thyristor Switched Capacitor (TSC)

Figure 47 shows a single phase TSC, it consists of a capacitor, a bidirectional thyristor

valve, and a relatively small inductor. The inductor function is to limit the surge current

in the thyristor valve under abnormal operating conditions, it can also be used to avoid

resonance in the AC in particular frequencies.

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Figure 47. TSC Diagram (Based on [5])

Under steady state conditions, the thyristor valve in conduction and a voltage over de

TSC of v=Vsin(ωt) the current in the branch can be expressed by de following equation:

2

2

2

( ) cos1

1 C

L

ni t V C t

n

Xn

XLC

The voltage amplitude through the capacitor is given by:

2

2 1C

nV V

n

When the capacitor is being switched there is the risk of generating transients, so

there are two basic conditions to guarantee that the switching is transient free. The

first case is if the residual voltage across the capacitor is less than the peak value of the

AC voltage, so the switching should be done when the AC instantaneous voltage is

equal to the capacitors voltage. The second case is if the residual voltage on the

capacitor is equal or bigger than the peak value of the AC voltage, so the switching

should be done when the AC voltage reaches its peak because in that instant is when

the voltage across the thyristor valve is minimum.

From the previously described cases it can be deduced that the maximum delay in the

switching of a capacitor bank is a complete cycle of the AC voltage applied. It can also

be understood why firing angle delay control is not an option, because capacitor

switching has to be done in a determined instant of the voltage cycle in order to have a

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transient free scenario; this is the case when switching is done and the voltage across

the thyristor valve is zero or minimum.

With the previous description it can be established that current in the TSC varies in a

linear way with the applied voltage, and depends on the value of the admittance of the

capacitor. Figure 48 shows the operating area of the TSC.

Figure 48. Operating V-I Area of a TSC (Based on [5])

Thyristor Switched Capacitor-Thyristor Controlled Reactor Static Var Generator (SVG-

TSC-TCR)

As the principal functions of the SVC are determined by the topology and behavior of

the SVG, the TSC-TCR Static Var Generator is going to provide the function of the SVC

that is going to be used to control voltage on the electric traction system. Figure 49

shows a single phase SVG TSC-TCR.

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Figure 49. SVG TSC-TCR Diagram (Based on [5])

Strictly from a block diagram point of view, the SVG can be considered as a controlled

reactive admittance that follows a reference input when connected to an AC system.

Figure 50 shows a particular feature of the operation areas of the SVG, and it

represents a typical feature of this type of compensator, it has two TSC branches,

generally this type of static generators have n TSC branches, where n is determined by

a series of variables. There are several variables to take into account when designing

this type of compensators, the first one is the capacitor range required, the second

one is the operating voltage of the system, the third one is the rated current on the

thyristor valves, and fourth costs.

Figure 50. Operating V-I area of SVG TSC-TCR with two TSC branches (Based on [5])

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The SVG has a control scheme to operate the different areas and provide de accurate

value of admittance. There are three main functions in the SVG control [5]:

1. Determine the number of TSC branches switched on in order to be able to

reach the desired capacitive current (with a positive surplus) and computes the

amplitude of the inductive current needed to cancel the surplus capacitive

current.

2. Control the TSC branches switching in a transient-free manner.

3. Modify the TCR current through delay angle control.

In Figure 51 the control scheme for the SVG is shown.

Figure 51. Control Scheme TSC-TCR SVG (Based on [5])

5.2.2 Static Var Compensator Control

As it was previously stated the SVC is a SVG whose output is varied in order to be able

to maintain a specific parameter of the electric system. The basic operating

characteristics of the compensator are already defined through the TSC-TCR

characteristics. One of the main elements that are possible to control with the

complementary elements that the SVC requires is the top part of the operating area of

the SVG which is called the regulation slope. This slope gives more operating flexibility

to the compensator. Figure 52 shows the regulation slope.

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Figure 52. V-I Characteristic of SVC (Based on [5])

Through many applications the compensator is not used as a perfect voltage regulation

terminal, instead it is allowed to change voltage proportionally to the compensated

current. The main reasons to do this are [5]:

If a droop is allowed in the regulation, the operating area of the compensator

in the maximum capacitive or inductive current can be extended. This is called

the regulation slope.

When no droop is allowed the system is likely to start oscillating.

The regulation slope in the SVC is given by the following expression:

*Vref Vref Isvc

The control scheme for an SVC in a power system appears in Figure 53. In the SVG

block the internal control of the actual firing of the TCS and TCR are included, the

additional blocks are the PI controller and the regulation slope regulation. The function

of the PI controller is to amplify the error between the measured voltage and the

desired reference.

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Figure 53. Control scheme of a SVC

With the SVC operation and control defined, it is going to be included in the

simulations of the electric transport system to compensate the power quality

problems generated by the model.

6. ELECTRIC TRACTION SYSTEM COMPENSATED

The first issue to address is the location of the compensator. In radial systems the

compensation is better located at the end of the line, while in a two source system

such as the one of the electric traction, compensation is better in the mid point. That is

why the SVC is connected in the 2Prim point. Along with the compensator a filter is

connected to correct the 5th harmonic, with these two elements the system can assure

regulation in voltage and wave distortion. The location of the SVC and the filter in the

whole system is shown in Figure 54.

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Figure 54. Electric Traction System with SVC and Filter

6.1 SVC PSCAD Model and Filter Design

The PSCAD model for the SVC is shown in Figure 55.

Figure 55. SVC PSCAD Model

The compensator has a direct connection with the system through a 1Ω resistance and

has several input signals that come from the control loop. Table 8 describes each signal

input.

Table 8. Signal Description PSCAD Model

Signal Name Description

NCT-NCaps Output Number of capacitor stages on in

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TSC

CSW Capacitor Switch Signal: 1 adds a

capacitor and -1 removes a capacitor

Alpha Order

(AO)

Delay Angle Control of the TCR

Kb Block/Deblock, 1 deblocks signals

These input signals along with the internal parameters fixed in the SVC allow the

system to have the desired behavior of the compensator through the control loop. The

characteristics of the SVC implemented are a TCR of 5MVAR, and a TSC of 9MVAR

divided into 2 branches each of 4.5MVAR.

The control logic that has to be applied to generate the adequate signals to the SVC

can be simplified in Figure 56.

Figure 56. SVC Control Scheme Block Diagram

The construction of the control loop in PSCAD can be divided into two phases: one in

which the susceptance order is obtained, and the second one in which the TSC

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branches are switched and the delay angle control of the TCR is done. The first part of

the control loop is in Figure 57.

Figure 57. Susceptance Order PSCAD Control Scheme

In this first part of the control loop the regulation slope was defined with a droop value

of 3%, and a reference voltage of 1.0p.u. There is a filtering stage due to the fact that

the input values are real time measurements. The second part of the control loop

takes the non linear susceptance order, BSVS, and generates the delay angle control

and the TSC branch switching.

Figure 58. Delay Angle and TSC PSCAD Control Scheme

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53

The entire control loop generates the inputs for the SVC previously mentioned. Once

the control loop was defined and the best location was chosen for the compensator,

the Electric Traction Model was updated. Figure 59 shows the PSCAD compensated

model.

Figure 59. Electric Traction System with compensation PSCAD Model

Once the SVC was installed wave distortion did not comply with the desired regulation

reason why in the same node as the SVC a filter was installed. Due to the

characteristics of the 6 pulse bridge the first harmonic with important generation was

the 5th harmonic, so the filter had to be tuned to reduce this distortion.

To avoid sudden current rises the tuning was set in the 4.7th harmonic, so the

resonance frequency to design the filter was 282Hz on a 60Hz based system such as

the one addressed. The capacitor value was set to 15μF and the inductance value was

calculated with the following expression.

12

0.021235

frL C

L H

This filter helps to reduce the wave distortion and satisfy the 5% maximum current

THD limit established by regulation. Figure 60 shows the schematic of the filter.

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Figure 60. Fifth Harmonic Filter

With these two elements introduced to the proposed system the two designed cases

were simulated to verify that voltage and current THD satisfied regulation.

6.2 Compensated Study Cases

Once compensated the two designed cases were tested to verify the proper voltage

compensation and the harmonic distortion correction. In this analysis a new set of

graphics were analyzed in the same points as before to prove that the SVC and the

filter corrected the problems in the network.

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6.2.1 Low Load Variability

Figure 61. Number of Capacitor Stages ON

Figure 62. Alpha Order

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Figure 63. PCC1 Compensated Voltage Waveform

Figure 64. PCC1 Compensated Current Waveform

Figure 65. PCC1 Compensated Voltage (p.u)

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Figure 66. Source 1 Compensated Current Wave FFT and THD

Figure 67. PCC2 Compensated Voltage Waveform

Figure 68. PCC2 Compensated Current Waveform

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Figure 69. PCC2 Compensated Voltage (p.u)

Figure 70. Source 2 Compensated Current Wave FFT and THD

Figure 71. 1Prim Compensated Voltage Waveform

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Figure 72. 1Prim Compensated Current Waveform

Figure 73. 1Prim Compensated Voltage (p.u)

Figure 74. 2Prim Compensated Voltage Waveform

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Figure 75. 2Prim Compensated Current Waveform

Figure 76. 2Prim Compensated Voltage (p.u)

Figure 77. 3Prim Compensated Voltage Waveform

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Figure 78. 3Prim Compensated Current Waveform

Figure 79. 3Prim Compensated Voltage (p.u)

Figure 80. Rectifier Substation Compensated Current FFT and THD

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6.2.2 High Load Variability

Figure 81. Alpha Order

Figure 82. Capacitor Stages ON

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Figure 83. PCC1 Compensated Voltage Waveform

Figure 84. PCC1 Compensated Current Waveform

Figure 85. PCC1 Compensated Voltage (p.u)

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Figure 86. Source 1 Compensated Current Wave FFT and THD

Figure 87. PCC2 Compensated Voltage Waveform

Figure 88. PCC2 Compensated Current Waveform

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Figure 89. PCC2 Compensated Voltage (p.u)

Figure 90. Source 2 Compensated Current Wave FFT and THD

Figure 91. 1Prim Compensated Voltage Waveform

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Figure 92. 1Prim Compensated Current Waveform

Figure 93. 1Prim Compensated Voltage (p.u)

Figure 94. 2Prim Compensated Voltage Waveform

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Figure 95. 2Prim Compensated Current Waveform

Figure 96. 2Prim Compensated Voltage (p.u)

Figure 97. 3Prim Compensated Voltage Waveform

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Figure 98. 3Prim Compensated Current Waveform

Figure 99. 3Prim Compensated Voltage (p.u)

Figure 100. Rectifier Substation Compensated Current Wave FFT and THD

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The two parameters that are being studied with the compensation revealed an

improvement towards achieving regulation and giving support to the different

scenarios that the system could upfront.

The action of the SVC changes with the two cases. The capacitor stages show

difference depending on the load amount and variability. When there is low load

variability 1 TSC branch is enough, but in the high load variability the 2 stages are

longer on due to the demand of the system.

This two parameters show that the SVC with the filter is the proper solution for the

network problems. The parameters that suffer a greater impact with load variability

are corrected in the presence of this FACTS and filter device. Tables 9-10 summarize

the most relevant results comparing each case with its compensated counterpart. In

the case of the voltage, the minimum value on the simulation was taken to be

compared with the compensated case.

Table 9. Low Variability Comparative Results

Node Parameter

Value

Compensated

Parameter Value

PCC1 0.946p.u 0.975p.u

PCC2 0.945p.u 0.975p.u

1PRIM 0.945p.u 0.975p.u

2PRIM 0.945p.u 0.975p.u

3PRIM 0.945p.u 0.975p.u

THDF1 24.07% 3.44%

THDF2 24.20% 2.93%

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Table 10. High Variability Comparative Results

Node Parameter

Value

Compensated

Parameter Value

PCC1 0.922p.u 0.957p.u

PCC2 0.922p.u 0.957p.u

1PRIM 0.923p.u 0.957p.u

2PRIM 0.923p.u 0.957p.u

3PRIM 0.923p.u 0.957p.u

THDF1 22.63% 4.26%

THDF2 22.50% 4.30%

Without compensation the current THD in the sources of low load variability was

around 24%, and in high load variability was around 22%. With the inclusion the

designed filter in the low load variability source 1 had a THD of 3.44%, and source 2 a

THD of 2.93%; both THDs were lower than 5% which is the limit for that voltage level.

With the high load variability there is a similar scenario, source 1 has a THD of 4.26%

and source 2 has a THD of 4.30%, both under the previously stated limit.

The voltage along the system also improved showing a drop not higher than the 5%

established by IEEE 141. In the low variability case it raised voltage in around 3.0% and

in the high variability in 3.5% achieving regulation and managing the load increase in

the system

7. IMPLICATIONS OF USING MSC-TCR SVG TO COMPENSATE THE ELECTRIC

TRACTION SYSTEM

One alternative solution that can be suggested for the SVG is the use of a Mechanically

Switched Capacitor-Thyristor Controlled Reactor (MSC-TCR). This option provides the

capacitive support and it is cheaper than the TSC-TCR configuration. However, this

alternative choice is not as efficient as using the SVC proposed for several reasons.

The MSC-TCR configuration does not have the response or the repeatability of

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operation generally needed for the dynamic compensation of systems such as the one

addressed. Another issue is the fact that the response of mechanical breakers

employed to switch the capacitors will determine mostly the elapsed time between

the capacitive var demand and the actual capacitive var output. Also precise and

constant control of the mechanical breaks switch control is not possible because the

capacitive bank must be switched without any appreciable residual charge to avoid

high and possibly transient generation. As a consequence, whenever the capacitor is

switched out it is discharged before the next switching takes place. Considering a

practical discharge time of 3-4 cycles, a typical breaker closing time of about 3-7cycles

the MSC delay may be 6-11 cycles. This means that in a 60Hz system like the one

implemented the operation can fluctuate between 0.1 and 0.1833 seconds, which is

considerable based on the dynamic nature of the system [5], [7].

The typical life of a breaker is of 2000 to 5000 operations, this implies that depending

on the variability of the system periodic repairs are necessary. On the other hand,

FACTS devices provide several benefits in comparison with the previous alternative.

These devices have the ability of rapid and precise switching in and out of large

capacitor banks. This is made possible by solid state switches like the thyristor

bidirectional valve. This device is able to operate orders of magnitude faster, more

precise, and more reliable than the mechanical switching counterpart. Additional to

these operational characteristics they give the possibility to control phase angle,

impedance, voltage and current in ways that would not be possible with mechanical

breakers switching [5],[6].

The main problem that FACTS devices have is that they are not cost competitive,

particularly in terms of the initial investment. However, in the long run it might be a

cheaper option because the SVC allows a larger and more efficient expansion of the

network with better parameter control.

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8. CONCLUSION

The main power quality problems were successfully replicated with the

proposed model. The rectifier and load variability had the expected impact on

the voltage drop, as well as current and voltage waveforms.

The compensation showed that the SVC is an effective device that can adapt to

different conditions present in the system; it can handle situations in which

there is an irregular power demand correcting the voltage along the system to

desired values. In the low variability case, voltage compensation was of 3.0% to

regulate the system to the desired values. With no change in the SVC

configuration or control loop and an increase in the load and its variability

caused a compensation raise to 3.5%.

The wave distortion present in the case of the DC traction can be properly

corrected by adding a filter in the same node in which the SVC is connected.

Without compensation the current THD in the Source 1 was of 24.07% in the

low load variability and 22.64% in the high variability. With the installed filter

the current THD dropped to 3.44% in low variability and to 4.26% in the high

variability. This shows that the filter installed successfully reduced wave

distortion in the system and managed to lower current THD to the desired

regulation limits.

The alternative to the TSC with MSC brings a new set of events on the system

that can easily be avoided by the use of the thyristor valve; such is the case of

transient generation, slower response and limited control action. Based on

these reasons the replacement of the TSC by an MSC for cost reduction is not a

good option due to the behavior of the system. In constantly changing systems,

the fact that the speed of response is harmed directly affects the quality of the

compensation because it will slow down the correction of the voltage along the

feeding line.

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73

REFERENCES

[1] GARCIA Gabriel, NIÑO David, RODRIGUEZ Fredy, CARRO Oscar, RIVERA Carlos.

“Compensación de Reactivos Para Sistemas de Transporte Eléctrico Masivo”, Eighth

Latin American Congress Electricity Generation and Transmission” 2009.

*2+BATTISTELLI L., CARAMIA P., CARPINELLI G., LAURIA D., PROTO D., “A Power Quality

Compensation Device for interacting AC-DC Railway Systems”, Power Tech, 2005 IEEE

Russia.

[3] “Railway Technical Web Pages, Electric Traction” http://www.railway-

technical.com/elec-loco-bloc.shtml#Modern-AC-Electric-Loco [En Línea] [Citado el: 03

de 06 de 2010]

[4]WATSON Neville, ARRILLAGA Jos “Power Systems Electromagnetic Transients

Simulation” The Institution of Engineering and Technology 2007

*5+ HINGORANI G. Narain, GYUGYI Laszlo “Understanding FACTS Concepts and

Technology of Flexible AC Transmission Systems”, Chapter 1 and 5, IEEE Press 1999

*6+GRÜNBAUM Rolf, HALVARSSON Per “The FACTS about Power Quality in Traction

Power Systems”, Electric Machines & Drives Conference, 2007. IEMDC '07. IEEE

International .

*7+RABINOWITZ Mario, “Power Systems of the Future (Part 2), Just the Plain FACTS”

IEEE Press Series on Power & Engineering March 2000.

*8+ “Example: SVC CONNECTED TO AN INFINITE BUS”, PSCAD/EMTDC V.4.2.1,

Manitoba HVDC Research Center, August 11 2006.

*9+ “Example: SIMPLIFIED ACTIVE FILTER IN PARALLEL CONFIGURATION”,

PSCAD/EMTDC V.4.2.1, Manitoba HVDC Research Center, August 11 2006.

[10] IEEE Std 519-1992, IEEE Recommended Practices and Requirements for Harmonic

Control in Electrical Power Systems.

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[11] IEEE Std 141-1993, IEEE Recommended Practice for Electric Power Distribution for

Industrial plants.

[12] HOSSEIN HOSSEINI, Seyed, SHAHNIA, Farhad, SARHANGZADEH Mitra, BABAEI

Ebrahim, “Power Quality Improvement of DC Electrified Railway Distribution Systems

Using Hybrid Filters”, Electrical Machines and Systems, 2005. ICEMS 2005. Volume 2.

[13] Norma Técnica Colombiana NTC 1340 “Electrotecnia. Tensiones y Frecuencia

Nominales en Sistemas de Energía Eléctrica en Redes de Servicio Público” 25 Agosto de

2004.


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