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Pantograph I - Analysis on Pantographs & Traction Control


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My 'info'-presentation on basics on electric traction systems for railways and electrical trains (rolling stock).

The presentation cover the following basic concepts:
- types of electrification systems.
- types of collection method
- traction control

Published in: Engineering
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Pantograph I - Analysis on Pantographs & Traction Control

  1. 1. Kelvin Lam Analysis on Pantographs – Pantograph and Traction Control Introduction to electric tractions Summer 2015 19/10/2015 1
  2. 2. Content 1 - Pantograph & Traction Control 1. Introduction 2. Design of Pantograph 3. Working Principle 4. System Interface 2 – Pantograph Safety 5. Safety & Fatigue Evaluation 6. Qualitative Standards of Pantograph 19/10/2015 2
  3. 3. Analysis on Pantographs – Pantograph and Traction Control 19/10/2015 3
  4. 4. Pantographs & Traction Control 19/10/2015 4
  5. 5. Introduction 19/10/2015 5
  6. 6. Introduction i. Introduction to Traction Power System ii. Introduction to Pantograph iii. Categorisation of pantographs 19/10/2015 6
  7. 7. Introduction to Traction Power System • The advancement in electric systems provided trains with enhanced performance. • Pivotal for commuter services: short dwell time, rapid acceleration, ideal for quasi-high speed use. • Lack of fuel storage: a method to transport and deliver energy to electric-powered trains are needed. 19/10/2015 7
  8. 8. Rigid rail conductor • Third Rail System • Fourth Rail System • Ground Level Supply • Guide bar (for rubber tyres system) Overhead Lines (OHL) • Catenary • Rigid Conductor 19/10/2015 8 Introduction to Traction Power System
  9. 9. Rigid rail conductor • Third Rail System • Fourth Rail System • Ground Level Supply • Guide bar (for rubber tyres system) 3rd / 4th Rail • Used in lower speed railways, frequently in metro services. • Direct Current (DC) Supplies only due to skin effect. • Proximity to passengers: usually electrified to 750 volts. • Current obtained from lower/upper contact shoe. • Stray voltage with ground: frequent corrosion due to chemical electrolysis. • Skeptical to flooding/snow (arcing). 19/10/2015 9 Introduction to Traction Power System
  10. 10. Schematic diagram of Third Rail and Contact Shoe. A Contact Shoe. 19/10/2015 10
  11. 11. A London Underground driver placing a safety Short Circuit Device on 4th Rail after shutting traction current. Source: “Live or Dead” video. Third Rail system. 19/10/2015 11
  12. 12. Rigid rail conductor • Third Rail System • Fourth Rail System • Ground Level Supply • Guide bar (for rubber tyres system) Ground Level Supply • “L'alimentation par le sol” • Used in urban trams to avoid scenary implications caused by OHLs. • Remove fatal hazards of 3rd/4th Rail. • Conventionally 10 metres of live track with 3 metres of “neutral zones”. • Trams have 2 shoes, and electrically energise tracks when traction current is required. 19/10/2015 12 Introduction to Traction Power System
  13. 13. Rigid rail conductor • Third Rail System • Fourth Rail System • Ground Level Supply • Guide bar (for rubber tyres system) Guide Bar • Typically used in smaller scales, i.e. People Movers or Rubber Metros. • Same as 4th Rail principle – rubber tyres do not offer an earthing route. 19/10/2015 13 Introduction to Traction Power System
  14. 14. Overhead Lines (OHL) • Catenary • Rigid Conductor Catenary • Power line placed above rolling stocks; current collected by pantographs. • Power transmission can be transmitted either DC/AC (discussed later). • Suitable for high speed application. 19/10/2015 14 Introduction to Traction Power System
  15. 15. Overhead Lines (OHL) • Catenary • Rigid Conductor Catenary • OHL lines held by mechanical tension – both pantograph pushing force and by lineside mechanical tensioner. • Clearance problem: higher infrastructure construction cost; lack of clearance for double-decker carriages. 19/10/2015 15 Introduction to Traction Power System
  16. 16. Schematic diagram of a typical overhead catenary system. Zig-zag OHL is designed to reduce pantograph carbon strip wear. 19/10/2015 16
  17. 17. Mast. The red circles show ceramic insulators. Mechanical tensioner hanged next to a mast. 19/10/2015 17
  18. 18. Overhead Lines (OHL) • Catenary • Rigid Conductor Rigid Conductor • Resistivity of metal, which is a physical attribute, cannot be changed. • To reduce resistance we increase diameter of wire. • Efficient DC transmission, inefficient for AC due to skin effect. • Higher structural integrity required due to increased weight. • Higher wear for pantograph. 19/10/2015 18 Introduction to Traction Power System
  19. 19. Direct Current (DC) • 600V • 750V • 1,500V • 3,000V Alternating Current (AC) • 15 kV Single Phase (50Hz) • 25 kV Single Phase (50Hz) • 50 kV Single Phase • 3000V Three Phase (16-50Hz) (mountain railways only) 19/10/2015 19 (Conventional) Choices of traction current: Introduction to Traction Power System
  20. 20.  Factors in consideration:  Efficiency & Energy Loss (per kilometre)  Infrastructure cost: (Extra clearance; number of transformer substation per electrified section; neutral zones)  Electromagnetic Compatibility (EMC)  Synonymous motor (for terrain railways) uses a 3-phase supply, but speed control compromised.  Stray voltage problem present in DC Power Supplies. 19/10/2015 20 Introduction to Traction Power System
  21. 21. Electric potential is induced around a grounded conductor. As distance increases moderately potential reduces, thus a potential difference is caused, a.k.a. stray voltage. A e.m.f. is said to have formed when a stray voltage is observed. This catalyse the electrolysis of the permanent way, accelerating its corrosion. “Cathodic corrosion of railways” 19/10/2015 21
  22. 22. A ‘neutral zone’ lineside sign. A neutral zone is to separate power supplies of different phases; to avoid harmonic and unsynchronized crashes. Only applicable in AC. A Section Insulator in a DC Supply railway. Analogous to neutral zones of AC railways to prevent harmonic crashes. 19/10/2015 22
  23. 23. • Atmospheric clearances should be maintained when OHLE are used. • From BS EN 50119 (discussed later in Part II): • Table outlining clearances is attached below: • * Maximum value from static and dynamic clearances are taken for justification reason 19/10/2015 23 DC 600V 100mm DC 750V 100mm DC 1500V 100mm DC 3000V 150mm AC 15kV 150mm AC 25kV 270mm Introduction to Traction Power System
  24. 24. • In AC traction system, differing potentials caused by phase should be separated accordingly. • * Maximum value from static and dynamic clearances are taken for justification reason 19/10/2015 24 Nominal Voltage Phase difference (deg) Relative Voltage Clearance 15 kV 120 26 kV 260 15 kV 180 30 kV 300 25 kV 120 43.3 kV 400 25 kV 180 50 kV 540 Introduction to Traction Power System
  25. 25. Introduction to Pantograph • A conducting arm extending above the rolling stock, collecting traction current from OHL system. • Three broad classifications: I. Bow collector (now obselete) II. Trolley Pole III. Pantograph 19/10/2015 25
  26. 26. 19/10/2015 26
  27. 27. Introduction to Pantograph • Trolley Pole: • An extended arm with revolving conductive wheel, rolls below the OHL wire. • Low-speed application. • Chances of “following the wrong wire” at junction. • Unidirectional only – requires rotating. 19/10/2015 27
  28. 28. Introduction to Pantograph • Pantograph: • Folding, mechanical arm pushes against the OHL wire to maintain un-intermittent traction current supply. • Expensive and difficult to construct. • Bi-directional. • Suitable for higher speed application. 19/10/2015 28
  29. 29. Categorisation of Pantographs • Single-arm Lighter and simpler design. • Double-arm Heavier but more forgiving to faults; less aerodynamic for high-speed usage. • Wing (T-) shaped Together to shrouds, designed using F1 technologies to reduce vortex and associated noise. Very high cost to manufacture and maintain and are retired. 19/10/2015 29
  30. 30. A single-arm pantograph from Odakyu Railway Series 3000. A double-arm pantograph from Series 0 Shinkansen trainset. 19/10/2015 30
  31. 31. Wing-shaped pantograph from Series 500 Shinkansen trainset. 19/10/2015 31
  32. 32. Categorisation of Pantographs • Shrouding • Given that fluid always have viscous properties, as skin drag develops. • Trains travelling faster will increase Reynold’s number; an appreciable boundary layer is developed. High-speed trains have higher aerodynamic impacts. • Boundary layer develops, producing noise and unnecessary tractive longitudinal resistance. • With shrouding we create a non-slip boundary condition around pantograph, keeping unwanted noise to minimum. 19/10/2015 32
  33. 33. Shrouding around pantograph from N700 tilting Shinkansen trainset. Boundary Layer Theory: as fluid transverse upon a surface, the contacting particles retard, and subsequent layers slower. Above the transition point a different physical layer – boundary layer is formed. The flow transited from laminar to turbulent and drag increases egregiously. 19/10/2015 33
  34. 34. Similar ideas in aircrafts – winglets. 19/10/2015 34
  35. 35. CFD (Computational Fluid Dynamics) and FEM (Finite Element Method) analysis on pantograph shrouding travelling at 350 km/h. Image extracted from a Chinese academic publication. (Wang et al.) 19/10/2015 35
  36. 36. Design of Pantographs 19/10/2015 36
  37. 37. Design of Pantograph 19/10/2015 37
  38. 38. Design of Pantograph 1. Pan Head A Carbon strip that acts as a conductive, dry lubricant. The contact surface between rolling stock and OHL wires. 2. Upper Arm 3. Lower Arm Pivot support of the pan head. 4. Damper Assembly Offers improved control for pan rising/dropping than solely using compressed air. 19/10/2015 38
  39. 39. Design of Pantograph 5. Air Cylinder Acting like a piston-cylinder. Compressed air pushes the piston, hence extending the piston rod to raise pantograph; vice versa for pan drop. 6. Raising/Counterbalance Spring Provides pantograph articulation against vibrations and harmonics. 10. Base Frame Base of the pantograph assembly. 19/10/2015 39
  40. 40. 1. Carbon Carrier Conductive platform where carbon strip is housed. 4. 25kV Ceramic Insulator A highly insulated material to isolate car body from high voltage. 5. Raising Spring To provide actuator effort to the lower arm, in addition to piston power. 19/10/2015 40 Design of Pantograph
  41. 41. 8. Air Feed Insulator An insulative, solid state device that allows gas to be bled into the cylinder. 19/10/2015 41 Design of Pantograph
  42. 42. 3. Foot Insulator Insulative support that secures the pantograph assembly on the roof of the rolling stock. 19/10/2015 42 Design of Pantograph
  43. 43. • In high speed trains, where dynamic behaviour could be dominant: • Conventional gas power may not be able to keep pantograph in contact with OHL wires. • Rather, aerodynamic effect ∝ velocity2 . • We exploit aero-behaviour to maximise contact. 19/10/2015 43 A Brecknell-Wills high speed pantograph on board a Thai Siemens Class 360 Desiro. The use of aerofoil generates lift above the chord line therein propelling pantograph upwards against the OHL. Design of Pantograph
  44. 44. Structural Design of Pantographs • ‘Pantograph’ is a word with Greek roots, meaning ‘every write’. • Structure with mechanical linkage so it works by forming a parallelogram. • Modern pantograph principle is derived from a linked support structures called “Scissors mechanism”. 19/10/2015 44
  45. 45. Structural Design of Pantographs • The height adjustment is achieved by the horizontal displacement motion in the x-axis of the actuator. • As force is applied, it is transferred to the truss. • Causing elongation to the crossing pattern. • May be thought as ‘conversation of area’. • The diagram represents a perfect model for trusses of the same length. 19/10/2015 45
  46. 46. Structural Design of Pantographs 19/10/2015 46
  47. 47. Structural Design of Pantographs • However, if we consider insulative gauge of the OHLE system. • It is evident that a scissors structure must be elongated vertically to maintain atmospheric separation. • We ‘may be’ able to achieve it by simply lengthening one of the top truss. 19/10/2015 47
  48. 48. Structural Design of Pantographs • Every 1cm that A moves up, W will move up 3 cm. • Mechanical Advantage increases as = 1:3. • However… • With increased bending moments extending towards the weight, the bending stress will inevitably cause beam deflection. “Euler-Bernoulli beam theory” 19/10/2015 48
  49. 49. Structural Design of Pantographs • Single-arm pantograph are used nowadays as it offer greater leverage. • It is important to understand the speed requirement of pantograph concerned. • Extra speed incur extra forces, so reinforcements required. 19/10/2015 49 “One-truss” configuration can save weight but may not offer appreciation torsional resistance.
  50. 50. Working Principles 19/10/2015 50
  51. 51. Working Principle i. Operator POV ii. Mechanism POV 19/10/2015 51
  52. 52. Operator POV • The train operator (TO) (a.k.a. train driver) raises the pantograph via the TMS (Train Management System) onboard the driving cab. • This is done after the ‘sweep’ checks along the exterior and interior of the train is done. • With TMS a TO can isolate individual pantograph-at-fault. • Alternatively, some older trains will contain ‘PAN UP’ and ‘PAN DOWN’ buttons. 19/10/2015 52
  53. 53. Operator POV 19/10/2015 53 “Cockpit” of a SP1950 trainset. Red circle denotes TMS screen.
  54. 54. Driving Cab of a BREL Class 315. Red Circle denotes ‘PANTOGRAPH UP/RESET’ button. Operator POV 19/10/2015 54
  55. 55. Operator POV • In some dual-voltage system, the TO has to manually set which traction current the train will run on. • Failing to do so may cause serious consequence to trainborne equipment as the correct current cannot be transformed and achieved. • These trains are called “multi-system locomotive/electric multiple unit”. 19/10/2015 55
  56. 56. Driving Cab of a dual- voltage BREL Class 313 EMU. The red circle highlighted the two switches for 25kV AC Pantograph & 750V DC Shoe Gear. Reseau-Duplex high speed locomotive are tri- current, making them suitable for use in French, German and Swiss high-speed rail network. 19/10/2015 56
  57. 57. Mechanism POV • Starting from low, folded position: 1. The compressed air is fed into the operating cylinder. 2. Control link extends, moving a slotted rod in between the piston link and pivot arm. 3. Spring originally in stretched position moves, offering extra pivot moment. 4. Spring contracts, leverage causes collector head to rise and make contact with OHLE. 19/10/2015 57
  58. 58. System Interface 19/10/2015 58
  59. 59. System Interface i. Traction Control ii. Neutral Zones iii. Auxiliary Power Supplies iv. Electromagnetic interferences 19/10/2015 59
  60. 60. Traction Control 19/10/2015 60
  61. 61. Evolution of electrical systems • World’s first electric locomotive was built by Werner von Siemens in 1866. • Powered by 150V DC third insulated rail. • Similar trains can be found in Volk’s Electric Railway, the oldest electric railway still in operation. 19/10/2015 61
  62. 62. Volk’s Electric Railway, Brighton, United Kingdom. The VER No. 8 train is built in 1901, powered by a Compagnie Electrique Belge 8 hp DC Motor. The railway is electrified in offset 3rd rail 110V. 19/10/2015 62
  63. 63. Resistor Control • Before the introduction of electrical controls, mechanical systems were used to control trains. • Bank of (series of) resistors were switched on/off to alter current flowing into the motor, hence controlling motor speed. • In EMUs a ‘cam shaft’, driven by a (pneumatic) motor operates the shaft, and it then switches respective resistors on/off to control the traction current – known as Camshaft. 19/10/2015 63
  64. 64. Simple illustration of a camshaft controller. Camshaft controller of a SEPTA BSS B-IV train. Source: 19/10/2015 64
  65. 65. Camshaft • The camshaft makes and breaks electrical contacts consecutively in response of TO’s throttle handle. • In DC propulsion, motor velocity = f(voltage) • Camshaft itself is powered by a smaller motor in a relay circuit. • Between electric contacts arc breakers break any ‘incorrect’ under-loaded electric arcs. • An analogue of today’s electronics. 19/10/2015 65
  66. 66. • After experiments and trials after WWII, 25kV AC OHLE became the standard for future higher speed railways. • AC traction system was not previously introduced due to lack of high-power rectifiers (mercury-arc). • Unstable AC motors – it does not offer the appropriate tractive characteristics (speed vs torque relation). • Hence DC traction motors remained in use. 19/10/2015 66 Tractive Effort/Resistance curve of a locomotive.
  67. 67. Tap Changer • AC has varying ‘vectors’ – resistors only hinders the transmission of a linear current. Some changes in impedance is required. • Transformers operate using alternating magnetic flux, and AC coincides with this principle. 19/10/2015 67
  68. 68. • AC Supplied via OHLE to the pantograph. • An electric locomotive/carriage has two autotransformers, housed in same casing to offer better coupling. • Inside casing there is insulation oil present, increasing transformer efficiency. 19/10/2015 68
  69. 69. • As TO changes the notch a different transformer ‘tapping’ is selected progressively. • Each tapping represent a pre- defined displacement along the secondary transformer coil. • In transformers… voltage = f(ratio of coils) 19/10/2015 69
  70. 70. • In between notches, the on- load tap changer has links in between connection to ensure continuous, uninterrupted supply. • However resistor added to prevent short-circuiting between taps. • Tap is changed via a servo motor, usually pneumatic. 19/10/2015 70
  71. 71. Chopper / Thyristor • With advent of semiconductors, more precise and accurate controls of currents are permitted. • Removal of mechanical devices – dramatically increases efficiency. • Analogues previously help us understand how semiconductor controls work. 19/10/2015 71 Semiconductor s A Gate Turn-off (GTO) Thyristor
  72. 72. PWM • To understand chopper we need to appreciate duty cycle and pulse width modulation (PWM). • In digital electronics only binary signals are used: ON or OFF. This is defined by the logic gates. 19/10/2015 72 An illustration of duty cycles.
  73. 73. PWM • Duty cycle refers to the occupancy percentage of an active signal. • Using a varying duty cycles we are able to transmit information (a mixture of different data) or also transmit varying currents (discussed later). • Not to be confused with frequency modulation (FM), where signals are arranged using varying wavelengths. In PWM the frequency remains constant (i.e. the datum pulse time) 19/10/2015 73
  74. 74. Chopper • Chopper controls traction current in similar manner to PWM. • Simplified idea: 19/10/2015 74 Schematic working principle of a DC Current. Chopper controlled current.
  75. 75. Chopper • Using GTO (Gate Turn-off) we can control traction current using a smaller, reference signal. • In chopper circuits unidirectional semiconductors are used (depend upon layering “charged” structure). 19/10/2015 75
  76. 76. Chopper • “Chopper” circuit can be sub- divided into 4 (quadrant) types, dependent on their characteristics. • The ‘quadrants’ are determined by their task demanded and flow of voltage & current.} • However principle remain the same. 19/10/2015 76
  77. 77. Chopper • Some explanation on chopper circuit:  CH: Chopper  D: freewheel diode; to absorb surges from excessive inductance  L: inductance; to smoothen traction current when chopper control = OFF to maintain tractive characteristics  R: source of resistance; a load: i.e. motor 19/10/2015 77
  78. 78. Type A – forward motoring Type B – forward braking 19/10/2015 78 Chopper
  79. 79. Thyristor • Begin with a conventional thyristor: • As pre-requisite we need to appreciate how semiconductor and transistor work. • A transistor works by inserting a signal voltage to gate, therein putting electrons into ‘base’ so that the ‘emitter’ would have enough delocalised electrons in the impurified silicon to conduct current. 19/10/2015 79
  80. 80. Thyristor • A transistor is biased; it is unidirectional. • Different layering configuration would mean different operating directions. • If transistors are placed in a structured manner we can exploit their logic characteristics to achieve new performances. 19/10/2015 80
  81. 81. Thyristor • When opposite current is applied, in forward-biased mode electrons are dragged towards the terminal, leaving no delocalised electrons midway. • Or, the thyristor behave like a conventional semi-conductor and no current passes through when forward current is applied in correct direction, without a gate signal. This is said to be reverse-biased. 19/10/2015 81
  82. 82. Thyristor • Only when the correct signal voltage is applied, and current is at correct direction then thyristor works. • Owing to unidirectional properties, regenerative braking feature is not available for a thyristor. 19/10/2015 82
  83. 83. Thyristor • The third state of a thyristor is the forward conducting mode. • If we consider a thyristor using a circuit diagram… • Once the signal voltage ‘fired’ the thyristor it works eternally: this is because a closed loop is formed. • It does not switch off when signal voltage no longer applies. • Only controllable by entirely switching the power source off. • Known as ‘latches on’. 19/10/2015 83
  84. 84. Gate Turn-Off Thyristor • Because of the ‘latching’ limitations of normal thyristors, we need a safer design in order to protect traction characteristics. • General Electric invented a new high-powered thyristor that can be gate-controlled. • In retardation, some of the traction current is ‘stolen’ to form a negative voltage. This reverse current is fed into the GATE. • However some residual current is left within the GTO so the reverse current has to be applied longer to induce forward current to fall and eventually disappear. • Long application of reverse negative signal = long switching time. This means inefficiency (but still much efficient than mechanical controls previously). Limitations to 1 kHz. 19/10/2015 84
  85. 85. Phase-fired Thyristor • Given AC’s repetitive characteristics, we cannot simply ‘chop’ and imagine if a pulse as an average current. • A newer way to ‘chop’ current is needed. • Any sinusoidal wave has ‘phase’. We imagine a single-phase AC supply will complete a 360 degree circle. 19/10/2015 85
  86. 86. Phase-fired Thyristor • If the thyristor is fired at a calculated, pre-defined time that matches the ‘phase angle’ of the AC current, we are able to capture a desired output voltage. • This is known as ‘phase cutting’. • Analogous to current ‘chopping’ using phasor feature. 19/10/2015 86
  87. 87. “Snubbing” • GTO make use of a reversed signal voltage to gradually reduce the forward current, eventually powering off. • However, introducing a gigantic reverse current rapidly would be excess current within the GTO, exceeding the safe operating area. • Hence GTO requires a snubber as safeguard in voltage transient stages. 19/10/2015 87 Snubbers. Source: Wikimedia
  88. 88. IGCT • An advancement from GTO technology • IGCT = Integrated Gate Commutator Transistor • A fully controllable device: switched purely by gate signals instantly, where GTO can only be switched off gradually. • However based upon obsolete technology: it suffers from high power loss. • Not commonly seen in railway vehicles. 19/10/2015 88
  89. 89. • Older EMUs utilises DC traction motors and controls. With introduction of AC drives in late 1980s these controls become obsolete. 19/10/2015 89
  90. 90. Inverter • With advancements in AC motors, it is apparent that DC motors do not offer the best traction characteristics (i.e. does not exactly obey tractive curve/Davis formula). • A new set of controls are needed. • Understanding how current are converted from one to another; and strong appreciation in single- and three-phase AC is a pre-requisite. 19/10/2015 90
  91. 91. Inverter • Numerous attempts to use AC controls were made without semiconductors. • AC-DC-AC motors were used. An AC motor runs directly from AC OHLE supply drives a DC dynamo. • This is very inefficient – a lot of energy is wasted via friction. This also increases maintenance cost. 19/10/2015 91
  92. 92. Inverter • An inverter oscillates, which convert a DC to an AC current at pre- defined frequency. • AC motor works by applying current to stators at different angle at different times. To control speed and torque we need different frequency output. • VFD = Variable Frequency Drive is introduced. Also known as VVVF (Variable Voltage Variable Frequency). 19/10/2015 92
  93. 93. Inverter • AC Motors on EMUs are 3- phase motors, so three switches are needed in the matrix inverter. • GTO is not fast enough for high frequency switching, so newer semiconductor is needed. • Born of IGBT – Insulated Gate Bipolar Transistor. • Advantage: low signal current needed, also allowing higher forward traction current. 19/10/2015 93 Application of inverter in a water pump.
  94. 94. Inverter • IGBT is a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) device. • To understand a MOSFET we need to first understand what exactly is a FET (field effect transistor): 19/10/2015 94 Illustration of a FET. IGBT
  95. 95. Inverter • Consider a normal n-type transistor. Because the ‘source’ and ‘drain’ are all insulated by the material. • However if we apply a small current, then free electrons are ‘induced’, by Field Effect, so charged particles exists to carry the forward current. 19/10/2015 95 Illustration of a FET.
  96. 96. Inverter • Inverter control itself does not produce traction sound. • The oscillator oscillates at varying frequency in which electromagnetic interferences produced are absorbed by the car body. • “Photons” are absorbed by car-body material and are converted to kinetic energy in bond vibrations. • Sudden noises propagate as train acceleration due to material strong resonance at harmonic frequencies. 19/10/2015 96
  97. 97. SiC • In DC power supply, the MOSFET modules oscillates to ‘pretend’ that there is an AC current output. • Impurities and ‘noises’ in AC current can cause disturbance and deteriorate AC traction characteristics. • New methods that utilise low ‘gate signal’ consumption and higher switch frequency is preferable. 19/10/2015 97
  98. 98. SiC • SiC = Silicon Carbide • Developed by Mitsubishi Corporation. • Compared to IGBT and IGCT (an advancement from GTO) it has 40% higher switch frequency. This allow traction characteristics to be achieved using lower energy, which is previously impossible. • Achieved by improved reverse resistance and better breakdown performance. • Smaller mass and dimension: coincide with ‘weight reduction’ trend. 19/10/2015 98
  99. 99. • Modern trains employs inverter control which offer higher efficiency and better traction characteristics. 19/10/2015 99
  100. 100. Associated Systems 19/10/2015 100
  101. 101. Neutral Section 19/10/2015 101
  102. 102. Neutral Section • In AC railways traction current are supplied from different power sources. • Electric sources are unsynchronised and has different phases. • To prevent harmonic crashes they are separated by a section of insulated wire without any current. • Trains through neutral sections have to move through it with residual momentum due to lack of OHLE power supply. 19/10/2015 102 UK Signage System: a trackside board reminding TO of an approaching neutral section.
  103. 103. Neutral Section 19/10/2015 103
  104. 104. Neutral Section • Configuration of a typical neutral section • Track Magnet: activates on-board bogie magnetic sensor, hence trips/resets VCB in pantograph cars. • Run Off: section of energised wire before/after the neutral zone to permit control to decrease traction current and (de)activates circuit breaker. • Ceramic Insulator: section of wire made of ceramic that deters electric potential due to differing phases. • Dead Wire: A non-energised wire connected to current return path, acting as earth. • Skid and ceramic insulator makes up an section insulator. 19/10/2015 104 Track Magnet 300mm Run-off zone 5 ft Ceramic Insulato r 5ft Dead Wire 5ft Ceramic Insulato r 40 ft Run-off Zone Track Magnet Total: 155 feets
  105. 105. Skids Ceramic Insulator Dead Wire 19/10/2015 105
  106. 106. Neutral Section • If an electric train/locomotive did not disconnect its power supply as it moves through a neutral section it could cause damage to trainborne electrical system (from harmonics, phase crash, surges etc.) • Trackside signage are placed to remind drivers to ‘coast’ trains before entering neutral section. • Human errors can still happen! Safeguard system(s) introduced to deter preventable errors. 19/10/2015 106
  107. 107. Neutral Section • Trackside magnets can be installed, which activates a bogie-mounted sensor. • This deactivates traction current using a circuit breaker. • Common type of circuit breaker is VCB – vacuum circuit breaker. 19/10/2015 107
  108. 108. Neutral Section • A VCB is a type of circuit breakers that exploit vacuum’s excellent dielectric properties to insulate two terminals and quench any arcing. • Vacuum has 8x higher dielectric ability than air; SF6 (sulphur hexafluoride) has 4x. • This is because there are no gas atoms present; with no (un)-stable electrons in sub- shells there are absence of charge carrier, making it a perfect dielectric material. 19/10/2015 108
  109. 109. Neutral Section • Gas sealage can be apparent; glass insulation are needed to prevent condensation trapped in moving contacts. • Thermal problem: Using perfect gas law, expansion inside & outside of gas are drastically different, and egregious movements is undesirable. The VCB is therein housed within an insulation vessel. 19/10/2015 109
  110. 110. Neutral Section • Within a VCB, there is one fixed terminal and one moveable terminal. • Upon receiving command, the moving terminal retracts and arcing happens. • Arcing is caused by ionisation of metallic atoms. • In vacuum vessel arcing is quickly suppressed by insulating vessel, retracting contacts and in minor extent, metallic vapours re-condensed onto the cooled moving terminals. 19/10/2015 110
  111. 111. Neutral Section • There is no need for a long neutral section for DC traction supply. • A section insulator is installed in between two energised sections fed by separate DC source. This is to avoid power supply interferences. 19/10/2015 111
  112. 112. Auxiliary Power Supplies 19/10/2015 112 Dark room…? Imagine what trains would look like without an APS.
  113. 113. Auxiliary Power Supplies • Trains traversing in between section insulator/neutral sections do not have power supplies. • Loss of traction current would stop power supplies to power- thirst equipment, i.e. traction motor, air conditioning… etc. • APS are installed on board to ensure limited lightings, emergency ventilation and communications are not jeopardised for safety reasons. 19/10/2015 113
  114. 114. Auxiliary Power Supplies An APS battery pack of an electric train. 19/10/2015 114
  115. 115. Auxiliary Power Supplies • Lightings and ventilation need low voltage (LV) supplies. Normal traction current is excessive for its use. • Methods to obtain LV currents are needed. • Previously wiring for step-down devices are bulky and space- consuming. • DC is also not preferable for use: there are excessive power loss. At the time solid state rectifiers were not available and current conversion are very inefficient. • (Motor alternator: motor-alternator-motor) 19/10/2015 115
  116. 116. Auxiliary Power Supplies • A motor generator is placed on each car. • Part of the DC traction current drives a DC motor, which is permanently coupled to an AC generator. • Voltage regulator included to act as a buffer: safeguard from surges/dips in 3rd rail gaps/section insulator. 19/10/2015 116
  117. 117. Auxiliary Power Supplies 19/10/2015 117
  118. 118. Auxiliary Power Supplies • With introduction of solid states, conversion from DC/single- phase AC to three-phase AC 380V (industrial & railway wagon standard) became easier. • If supply is DC, part of the DC supplied into the train will charge up battery and remainder is fed into a static inverter (SIV) and to be converted to a 3-phase current. • If supply is single-phase AC it has to be rectified first. 19/10/2015 118
  119. 119. Auxiliary Power Supplies • Contrary to motor controls, SIV only outputs a constant voltage and frequency. 19/10/2015 119
  120. 120. Auxiliary Power Supplies • Some older locomotives only provide traction to trailing coach; no electricity is fed into hauled coaches. • Power supplies from loco are known as head-end power or electric train supply (ETS). • These trains require a generator van: coach that contains a diesel generator to provide 380AC 3-phase. 19/10/2015 120