Study Of High Power AC-DC-AC Traction Converter

With the rapid development of the high power, high switching-frequency, modularized,intelligent power electronic devices, computer technology, and modern control technology,the level of ac drive technology has been being continually improved. The development ofac drive technology for the new high-speed railway traction system in the world, is towardhigh power, small size, light weight, high reliability, and low cost.In order to increase the system power density, reliability and reduce costs, this paperdid the research on two aspects. One is the rotor flux close-loop observer and speedestimation to replace the speed sensor. Anathor is reducing the dc capacitor bank and/or LCbranch which are used to absorb the ripple power.A high control quality of ac motor is the main objective of railway traction convertersystem. The speed is very important and necessary for the control. Speed sensors increasethe size, weight and costs and degrade the reliability. This paper studied on the speedsensor-less technique based on Field Oriented Control (FOC). The rotor flux observer is thekey to the vector control and speed-sensorless technique. A full order adaptive observer isused to calculate the rotor flux in close-loop manner. In order to eliminate the influence ofparameter variations on observation results, rotor resistance identification is also adopted todo a real-time adjustment for the observer model. Thus the stability and accuracy of theobserver is effectively improved. However, looking for the adaptive law of parameteridentification is much complicated. When the system has more sensitive parameters, whichhave to be identified one-by-one, large amount of calculation requires a high computingcapability for the control chip. An alternative solution is using Active DisturbanceRejection Control (ADRC) technique to consider the parameters’ variations and externalunknown disturbances as the total system’s disturbance, which is extended in the originalsystem as a new state. The total disturbance from the Extended State Observer (ESO) isused to strengthen the certainty and linearity of the system. The simulation results show thatthe observer has a good parameter suppression capability and steady-state accuracy.Railway traction drive systems are usually supplied by a single-phase grid. Thus asingle-phase rectifier is used to provide a dc voltage for inverter. The inevitable existence of a second-order ripple power, which leads to the dc bus voltage pulsating at twice the linefrequency, is generally absorbed by a bulky capacitor bank and/or LC circuit. Generallyspeaking, electrolytic capacitors with relatively short life are selected for the largecapacitance requirement to power decoupling. The filter frequency of LC circuit is sensitiveto frequency offset. At the same time, due to the100Hz resonant frequency, the size andweight of the LC circuit cannot be very small. The use of the passive filter led to the systemlarge volume and poor power density. To solve this problem, we studied a soft controlmethod (modulation index compensation) and hardware shcemes (Active Power Filter, APF)to improve the system’s power density.Making use of the Laplace and modified z-transforms (mixed s-z approach), this paperanalyzes the time-domain of stator voltage vectors, current vectors, and the electromagnetictorque with periodic pulse width modulation. It is found that the voltage and current vectorscontain a100Hz negative sequence component due to the dc ripple voltage. Thus theelectromagnetic torque appears a second-order pulsating, which is harmful to the motor. Atthe same time, extra sub-harmonic current is produced owe to the cross-modulationbetween the ripple dc voltage and the modulation signals of inverter. A feed-forwardcompensation for the pulse width modulation process of the inverter based on a repetitivedc voltage prediction is adopted to eliminate the influence of the ripple voltage on themotor. Simulation results verify that the feed-forward compensation can effectively reducethe beat current and ripple torque due to the ripple voltage.To completely eliminate the ripple voltage and its hazards on the motor and the grid,this paper focused on the APF schemes. By comparison of various topologies, this paperpointed out that the inductive energy-storage APF has a poor energy storage efficiency inlow switching frequency applications. So the capacitive energy-storage APF is preferred forthe high power systems such as railway traction drive system. Based on a dc side APFtopology, this paper proposed a new control method incorporating dual-loop control andrepetitive control to realize a good effect on power decoupling and ripple voltagesuppression. The circuit parameters are designed from both the control-band and the powerdensity point of view.In order to further weaken the system bandwidth restrictions on APF, this paperproposed a new ac side capacitive energy-storage topology. According to the single-phase d, q transform, this paper also presented a new close-loop method to modify the capacitorvoltage reference to enhance the steady-state performance. The relationship amongmodulation indices between the rectifier and APF was discussed rigorously and wasconsidered in parameters’ design process. Simulation and experiment results prove that theproposed topology and reference modification can effectively compensate the ripple powerand eliminate the dc bus ripple voltage ripple. Thus a significantly decrease of the filtercapacitor and higher power density are realized. This APF has a wide range of applicationmarkets, not only to the railway traction drive system but also to other power electronicsystems with a single-phase converter.