Three Phase Technology for Traction Application PDF

1. THREE PHASE TECHNOLOGY FOR TRACTION APPLICATION 1.0 INTRODUCTION Three phase AC drive technology has become very common and significant for modern rail vehicles. These vehicles are equipped with GTO thyristors and microprocessor control systems....

1. THREE PHASE TECHNOLOGY FOR TRACTION APPLICATION 1.0 INTRODUCTION Three phase AC drive technology has become very common and significant for modern rail vehicles. These vehicles are equipped with GTO thyristors and microprocessor control systems. Microprocessor is used for vehicle control, supervision of health and operations of all major components and diagnostics. It permits electric breaking down to standstill and selection of best PWM technique for improved performance of motor as well as unity pf. The advantages associated with this technology are evident in technical as well as economic aspects. 2.0 WHAT‟S NEED FOR A CHANGE ? Earlier, all the locomotives were using DC traction motors. The speed/torque regulation is achieved by using either tap changer on transformer or through resistance control on majority of these locomotives. Conventional relay based protection schemes are used. In most of the cases, the driver uses his discretion to diagnose and get past the problem. 2.01 FRPCPY for Tap changer and its associated equipments is about 10%. 2.02 DC motor has inherent problems of brush gear, commutator and low power to weight ratio. DC motor is essentially a high current low voltage design which calls for expensive large diameter cables and large electro-pneumatic reverser, contactors, switches etc. 2.03 Thyristorised DC traction motor drives, though made the DC motor drive more efficient, suffer because of high harmonic injection into Power supply. Loss associated large filters had to be carried on Locomotives to overcome this. 2.04 Emphasis on regeneration is increasing day by day to reduce energy bill as well as to save energy for greater national cause. 2.05 With ever increasing need for hauling higher loads, there is need to make maximum use of available adhesion. 2.06 There is need for track friendly locomotives to reduce track maintenance efforts. 3.0. WHY THREE PHASE TECHNOLOGY? Advantage of 3-phase induction motor over DC series motor 3.01 Three phase traction motors are robust and require little maintenance. Apart from bearing, it has no parts subjected to wear. It is insensitive to dust, vibration and heat. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 5 3.02 No restriction on speed of motor in absence of commutators, AC traction motors can easily operate at 4000 rpm in contrast to 2500 rpm in case of DC machines. 3.03 The limit imposed due to bar-to-bar voltage for DC commutator motor is no more relevant with squirrel cage induction motors. Whole power flow from transformer to converter to DC link and down to inverter / motor may be chosen at higher operating voltage. Against nominal 750 V, 1000A system with DC machines equivalent three phase propulsion is configured around 2800 V, 300A. Due to heavy reduction in operating current, power cables are much lighter and losses are reduced. 3.04 Power to weight ratio of induction motor is much higher than the DC motor. As a typical example 1500 KW per axle can be packed per Axle with induction motors compared to 800 KW maximum with DC motors. 3.05 Since the torque speed characteristic of the induction motor is markedly steeper than that attainable by conventional Dc machines, the induction machine can take better advantage of maximum possible tractive effort. A high mean adhesion coefficient can be expected. 3.06 As the adhesion coefficient is high, it is possible to transfer a part of the braking forces for the trailing load to electric brakes of locomotive. That is, in the case where regenerative braking is used, the regenerated electric energy can be increased. 3.07 High power/weight ratio of induction motor, reduction in cable thickness, reduction in number of contactors, switches etc. result in reduction in physical dimension and weight of the entire system. Advantages of microprocessor based control. 3.08 Almost all moving contactors, switches, relays, reversers etc. are eliminated and operation is sequenced by means of solid state logic. 3.09 The microprocessor is used for drive control. The microprocessor allows the redundancy to be built in controls rather than the power equipments. 3.10 Microprocessor based fault diagnostic system guides driving crew about the fault location and suggests remedial action. It also keeps records of faults, which can be analysed by shed staff later. 3.11 Microprocessor control software has flexibility to provide software-based solution to local operational needs. Other advantage of three phase drive 3.12 The induction motor drives are about 20% energy efficient compared to DC drives. 3.13 Three phase drives allow regeneration and unity power factor operation. The energy saving due to regeneration and improved power factor are sizable. 3.14 Electric braking down to standstill is possible. It improves operational efficiency besides reduction in maintenance efforts. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 6 4.0 THEN, WHY SO LATE ? To achieve these advantages of induction motor, it is necessary to supply it with a three phase variable voltage variable frequency (VVVF) source. This could not be achieved under technically and economically feasible conditions, until the advent of GTOs and microprocessor based control system in the last few years. 5.0 SO NICE, NOW DETAILS PLEASE ? 5.01 Three phase induction motor. To appreciate the complexity of the drive for using 3 phase squirrel cage induction motor for traction application, let us start with speed torque characteristic of a conventional fixed frequency, fixed voltage squirrel cage induction motor shown in fig. 1.1. It is described by following equation. T = K (V / F) ² * fS Where V & f are terminal voltage and frequency of supply to induction motor, fS is slip frequency and T is torque developed. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 7   Though, the starting current of typical cage rotor induction motor is 5 to 6 times rated current, the starting torque is small because of the low power factor.  Regeneration takes place, only when rotor is driven mechanically at super synchronous speeds. Variable voltage variable frequency drive In adjustable frequency drive, the supply frequency is reduced for starting, this frequency reduction improves the rotor power factor and this increases the torque/ampere at starting. In this manner, rated torque is available at start and the induction motor is accelerated rapidly to its operating speed by increasing the supply frequency. This method also avoids danger of low frequency crawling, which sometimes occurs when induction motors are started on fixed frequency supply. Fig.1.2 shows T.S. characteristic for constant v/f (constant air gap flux) at different supply frequencies. The breakdown torque is maintained constant by maintaining v/f constant. The stator voltage cannot be increased beyond rated voltage. With voltage remaining fixed further, as frequency is increased above base or rated motor speed, the air gap flux and breakdown torque decreases, as shown in fig.1.3. These characteristics are suitable for traction applications, where a large torque is required below base speed and a reduced torque is sufficient for high speed running. The torque-speed characteristic for a practical traction drive system evolved from the above two strategies is shown in fig.1.4. The variation in motor voltage & current, slip freq. and torque as the function of speed for operating regions shown in fig.1.4 is shown in fig.1.5. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 8 TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 9 TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 10 TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 11  Fig. 2.2 TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 12 5.02 Three phase induction motor drive The block diagram for such an induction motor drive is shown in fig.2.1 Motor-end inverter can be a current source inverter or a voltage source inverter. In the past, when conventional thyristors were the only choice, designers opted for current source inverter. About 70% of all underground railways and light rail transport in the world today are partly or fully equipped with this technology. The voltage source inverter, which required very complicated control electronics, when equipped with thyristors did not become a paying proposition until the development of GTOs and microprocessor based control techniques. The circuitry of the input converter which provides a DC supply for the load side converter depends on the following: 1) Type of input power supply i.e. AC or DC. 2) Electricity utility‟s limits on reactive power harmonics. 3) Type of electric brakes; that‟s regenerative, rheostatic or both. Fig.2.2 shows power schematic of ABB three phases AC locomotive. The following stages are involved in power conversion.  AC voltage is stepped down by main transformer.  AC to DC conversion and boost up by 2.0 to 2.5-boost factor by means of front-end converter.  Filter stage to reduce ripple in rectified DC.  Link over voltage protection. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 13 TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 14 5.03 Microprocessor control VVVF inverter and four quadrant converter controls are quite complicated. Hardware for this control, if configured with convention equipment, will be complicated with large physical dimensions. Microprocessors are adopted as hardware optimization tool in order to make an improvement in this area. The principal features of the microprocessors control are as follows: a) The space, weight and power consumption of the control unit can be reduced. b) It is possible to execute high degree processing operations for control easily, accurately and at high speed, using software. c) Failure occurring in the circuit, if any can be easily identified by self-diagnostic function. Microprocessor technology is used for control of whole vehicle including  Driving and braking control with automatic speed regulation.  Supervision of all functions and for the operation of all major components of locomotive with automatic changeover or shutdown in the event of failure.  Diagnostic system for all electrical and electronic devices on the vehicle.  Inversion of DC to variable frequency AC by means of drive end inverter. We shall now discuss briefly the modules used in 3 phase ABB locomotive. a) Voltage source inverter. A voltage source single pulse inverter is used for supplying variable voltage from 0 to 2180 V and variable frequency from 0 to 160 Hz. b) DC link DC link is made of the DC link capacitor, series tuned filter and over voltage protection circuit. DC link is reservoir of energy, which supplies periodic and non- periodic energy requirement of load and decouples it from supply source. The task of the DC link capacitor is to supply the reactive power needed by induction motor. The current supplied to DC link by 4-Q converter consist of second order harmonics, which is absorbed by series tuned filter. c) Input side converter. A 4-quadrant pulse width modulated converter is used for converting the AC to DC. It is capable of obtaining unity power on line current using proper control strategy and thereby eliminating the need for separate power factor correction equipment in the locomotive. Further more, the line current has insignificant harmonic content so that signaling and telecom circuits are disturbed. The principle of working of the converter is explained in the diagram given in fig.3.1. The input voltage to the converter Ec is controlled by pulse width modulation of DC TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 15  link voltage Ed. The fundamental component of the modulated voltage Ec acts against source voltage Es as shown in equivalent circuit of fundamental component. The converter input current Is in quadrature with EL i.e. voltage across reactor. Since Es is equal to Vector sum of EL & EC, it is possible to ensure that IS is in phase with ES by changing amplitude and phase of EC through PWM and thus achieve unity PF. It could be seen that fundamental component of EC is nothing but modulating wave itself. Thus by controlling the modulating wave, it is possible to achieve the unity pf. 5.04 Diagnostics. The structure of the diagnostic system on these locomotives can be distinctly dived into three portions: 1) Hardware of the diagnostic system is based on 80186/16-bit microprocessor and is programmed similar to main processor. All data in main processor is also available to diagnostic Hardware for analysis. 2) Firmware is project-independent software. This takes care of special task to be performed by hardware. The firmware processes and stores the diagnostic messages. It can be viewed as the expert system for diagnostic computer. 3) The application software on the other hand is project dependent. It is written to take care of varying working conditions. It defines the rules for the expert system which evaluates and stores the diagnostic messages. Diagnostic system processes the data available and classifies the diagnosis into three levels. These levels are programmed based on running experience of locomotives. Level I: Audio visual indication and record only. For faults in level-I category, the diagnostics will automatically take necessary corrective action to maintain normal locomotive operation. Wheel slip, oil temperature reaching maximum limit, DC link over voltage etc. fall under this category. Level II: Audio – Visual indication and one bogie isolation. This kind of action is taken when the fault is in major equipment of any one bogie. The faculty bogie is isolated and thereafter power to the locomotive is supplied by only one bogie. Examples are earth fault of DC link, opening of tuned filter, failure of traction motor, converter etc. Level III: Audio visual indication and locomotive shut down. This indicates eventual failure of equipment associated with both the bogies. Such faults are associated with malfunctioning of main circuit breaker, transformer, mechanical breakdown etc. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 16 5.05 Instrumentation A highly sophisticated data acquisition system using state of art instrumentation is used all over the locomotive. 5.06 Braking Regenerative braking down to standstill is possible reducing the break shoe wear. 5.07 Pneumatic system A new modular pneumatic panel supplied by Devis and Matcalfe & SAB-WABLO is used on these locomotives. Triplate structure of pneumatic panel has significantly improved reliability of pneumatic system. It uses brake electronics compatible with MICAS operating system. 5.08 Bogie and suspension system Flexi float bogie with two stage suspension is used for track friendly design. Fully sprung traction motor in passenger locomotive with significantly reduce stress on track. 5.09 To Sum up…. The three-phase technology brings together state of art technologies in the area of devices, control, instrumentation and communication. It puts up great responsibility on all of us to equip ourselves to assimilate this technology, as it will soon pervade other areas like AC/DC traction, EMUs etc. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 17