Reactive power compensation and harmonic distortion control in electric traction systems

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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 1.

(2) 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. 2.

(3) To my mother, who always stood by my side and believed in me.. 3.

(4) 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.. 4.

(5) 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. 5.. 6.. 4.1.1. Low Load Variability ................................................................................. 19. 4.1.2. High Load Variability ................................................................................. 28. 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. ELECTRIC TRACTION SYSTEM COMPENSATED ........................................................ 49 6.1. 7.. Study Cases ...................................................................................................... 19. Compensated Study Cases ............................................................................... 54. 6.1.1. Low Load Variability ................................................................................. 55. 6.1.2. High Load Variability ................................................................................. 62. IMPLICATIONS OF USING MSC-TCR SVG TO COMPENSATE THE ELECTRIC TRACTION. SYSTEM ........................................................................................................................... 70 8.. CONCLUSION ........................................................................................................... 72. REFERENCES.................................................................................................................... 73. 5.

(6) 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 6.

(7) 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 7.

(8) 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 8.

(9) 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. 9.

(10) 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. 10.

(11) 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.. 11.

(12) 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 greenhousegas 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) 12.

(13) 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]. 13.

(14) 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]). 14.

(15) 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. 15.

(16) 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.. 16.

(17) 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:. (34.5kV ) 2 R  11.9025 100MVA (34.5kV ) 2 L  0.031572  100MVA 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.. 17.

(18) Each one of the rectifier substations is made up by a 6 pulse bridge, a 10MVA deltadelta 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.. 18.

(19) 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.. 19.

(20) Table 1. Operating States of Rectifier Substations Rectifier. 0≤t≤1. 1≤t≤2. SEE I. BRK 1. OFF. SEE II. OFF. BRK 2. OFF. OFF. OFF. SEE III. OFF. OFF. BRK 3. OFF. OFF. Substation. 2≤t≤3. 3≤t≤4. 4≤t≤5. FULL LOAD FULL LOAD FULL LOAD. 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. BRK 1. BRK 2. BRK 3. Initial. First. Second. State. Operation (s). Operation (s). BRK 1.1. Closed. 0.5. 0.95. BRK 1.2. Closed. 0.3. 0.8. BRK 2.1. Closed. 1.0. 1.5. BRK 2.2. Closed. 1.3. 1.8. 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. 20.

(21) 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. 21.

(22) Figure 9. Current Waveform PCC1. Figure 10. PCC1 Voltage (p.u). Figure 11. Source I Current FFT and THD 22.

(23) Figure 12. Voltage Waveform PCC2. Figure 13. Current Waveform PCC2. Figure 14. PCC2 Voltage (p.u) 23.

(24) 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. 24.

(25) Figure 17. 1Prim Current Waveform. Figure 18. 1Prim Voltage (p.u). Figure 19. 2Prim Voltage Waveform. 25.


(27) Figure 23. 3Prim Current Waveform. Figure 24. 3Prim Voltage (p.u). Figure 25. Rectifier Substation Current FFT and THD. 27.

(28) 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. SEE I. 0≤t≤1. 1≤t≤2. 2≤t≤3. 3≤t≤4. 4≤t≤5. BKR 1. OFF. FULL LOAD. FULL LOAD. FULL LOAD. Table 4. Operating States Rectifier Substation II. SEE II. 0≤t≤2. 2≤t≤2.5. 2.5≤t≤3. 3≤t≤4. 4≤t≤5. FULL LOAD. BRK 2. OFF. BRK 2. FULL LOAD. Table 5. Operating States Rectifier Substation III. SEE III. 0≤t≤2. 2≤t≤2.4. 2.4≤t≤3.1. 3.1≤t≤4. 4≤t≤5. FULL LOAD. FULL LOAD. OFF. BRK 3. FULL LOAD. Table 6. Breaker Operation. BRK 1. Initial. First. Second. State. Operation (s). Operation (s). BRK 1.1. Closed. 0.5. 0.95. BRK 1.2. Closed. 0.3. 0.8 28.

(29) BRK 2. BRK 3. BRK 2.1. Closed. 2.0. 2.5. BRK 2.2. Closed. 3.3. 3.8. 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. 29.

(30) Figure 27. PCC 1 Current Waveform. Figure 28. PCC 1 Voltage (p.u). Figure 29. Source I Current FFT and THD 30.

(31) Figure 30. PCC2 Voltage Waveform. Figure 31. PCC2 Current Waveform. Figure 32. PCC2 Voltage (p.u). 31.

(32) 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. 32.


(34) Figure 38. 2Prim Current Waveform. Figure 39. 2Prim Voltage (p.u). Figure 40. 3Prim Voltage Waveform 34.

(35) Figure 41. 3Prim Current Waveform. Figure 42. 3Prim Voltage (p.u). Figure 43. Rectifier Substation Current FFT and THD 35.

(36) 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 Low Variability. High Variability. Parameters. 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%. Node. 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.. 36.

(37) 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. 37.

(38) 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.. 38.

(39) 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.. 39.

(40) 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. 40.

(41) 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 ) : t. iL (t ) . 1 V v(t )dt  (sin t  sin  )  L L. 41.

(42) 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 halfcycles 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. V fact that there is a term that does not depend on time,  L sin  , 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: I LF ( ) . V  2 1  1    sin 2  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: BL ( ) . 1  2 1  1    sin 2  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. 42.

(43) 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:. I Ln ( ) . V 4  sin  cos(n )  n cos  sin(n )    n  2k  1, k  1 L   n(n2  1) . 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.. 43.

(44) 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:. n2 C cos t n2  1 XC 1 n  2 XL  LC. i (t )  V. The voltage amplitude through the capacitor is given by: VC . n2 V n2  1. 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. 44.

(45) 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 (SVGTSC-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.. 45.

(46) 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]). 46.

(47) 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.. 47.

(48) 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.. 48.

(49) 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.. 49.

(50) 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 50.

(51) TSC CSW. Capacitor Switch Signal: 1 adds a capacitor and -1 removes a capacitor. Alpha Order. Delay Angle Control of the TCR. (AO) 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. 51.

(52) 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 52.

(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 4.7 th 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. 1 L C L  0.021235 H. 2  fr . 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. 53.

(54) 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.. 54.

(55) 6.2.1 Low Load Variability. Figure 61. Number of Capacitor Stages ON. Figure 62. Alpha Order. 55.

(56) Figure 63. PCC1 Compensated Voltage Waveform. Figure 64. PCC1 Compensated Current Waveform. Figure 65. PCC1 Compensated Voltage (p.u). 56.

(57) Figure 66. Source 1 Compensated Current Wave FFT and THD. Figure 67. PCC2 Compensated Voltage Waveform. Figure 68. PCC2 Compensated Current Waveform 57.

(58) Figure 69. PCC2 Compensated Voltage (p.u). Figure 70. Source 2 Compensated Current Wave FFT and THD. Figure 71. 1Prim Compensated Voltage Waveform 58.

(59) Figure 72. 1Prim Compensated Current Waveform. Figure 73. 1Prim Compensated Voltage (p.u). Figure 74. 2Prim Compensated Voltage Waveform. 59.

(60) Figure 75. 2Prim Compensated Current Waveform. Figure 76. 2Prim Compensated Voltage (p.u). Figure 77. 3Prim Compensated Voltage Waveform 60.

(61) Figure 78. 3Prim Compensated Current Waveform. Figure 79. 3Prim Compensated Voltage (p.u). Figure 80. Rectifier Substation Compensated Current FFT and THD. 61.

(62) 6.2.2 High Load Variability. Figure 81. Alpha Order. Figure 82. Capacitor Stages ON. 62.

(63) Figure 83. PCC1 Compensated Voltage Waveform. Figure 84. PCC1 Compensated Current Waveform. Figure 85. PCC1 Compensated Voltage (p.u). 63.

(64) Figure 86. Source 1 Compensated Current Wave FFT and THD. Figure 87. PCC2 Compensated Voltage Waveform. Figure 88. PCC2 Compensated Current Waveform. 64.

(65) Figure 89. PCC2 Compensated Voltage (p.u). Figure 90. Source 2 Compensated Current Wave FFT and THD. Figure 91. 1Prim Compensated Voltage Waveform 65.

(66) Figure 92. 1Prim Compensated Current Waveform. Figure 93. 1Prim Compensated Voltage (p.u). Figure 94. 2Prim Compensated Voltage Waveform. 66.

(67) Figure 95. 2Prim Compensated Current Waveform. Figure 96. 2Prim Compensated Voltage (p.u). Figure 97. 3Prim Compensated Voltage Waveform 67.

(68) Figure 98. 3Prim Compensated Current Waveform. Figure 99. 3Prim Compensated Voltage (p.u). Figure 100. Rectifier Substation Compensated Current Wave FFT and THD. 68.

(69) 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 Parameter. Compensated. Value. 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%. Node. 69.

(70) Table 10. High Variability Comparative Results Parameter. Compensated. Value. 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%. Node. 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 70.

(71) 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.. 71.

(72) 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.. 72.

(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.. 73.

(74) [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.. 74.

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