Research On The Control Strategy Of The Dual Stator Induction Machine In The Transformerless Traction Drive System

Multiphase machine traction drive systems have the distinct advantages,such as reduced torque ripple,high efficiency and reliability and also easier to realize high-power levels using existing power switch devices.They are suitable for the electric propulsion of naval ships,railway traction systems and so on,where require low input voltage,high output power and high reliability.This dissertation is based on the research and development of the transformerless traction drive system and aims to solve some key problems of the control and modulation of the dual stator induction machines.The main contents are as follows:Using the powerful vector space decomposition theory,the decoupling mathematical model of the dual stator induction machine is established.Based on the decoupling mathematical model,the current control strategies in the fundamental subspace and the harmonic subspace are introduced.In the fundamental subspace,the complex vector analysis method is adopted to establish the complex-vector mathematical model.Using the principle of pole-zero cancellation,the complex vector current regulator is designed,which can realize the decoupling control of the machine.In the harmonic subspace,the physical meaning of the harmonic currents is derived,the comparative analysis of the five asymmetric current suppression methods is also demonstrated.Simulation and experimental results have achieved good control effects.A modified 24-sector SVPWM technique based on the vector space decomposition is proposed,which can be applied to the two-level six-phase voltage source inverter fed asymmetrical dual stator induction machines.The new SVPWM technique can reduce the torque ripples and suppress the harmonic currents flowing in the stator windings.In order to simplify the calculation process and the implementation process of the SVPWM technique,the equivalent relationship between the proposed SVPWM technique and the carrier-based PWM technique has been demonstrated,which is simple,occupies less computing resource and allows for easy implementation by a digital signal processor(DSP).In order to meet the modulation requirements under low switching frequency of the dual stator induction machines,two kinds of synchronized modulation strategies are demonstrated,which are the synchronized modulation strategy based on the traditional 24-sector SVPWM technique and the synchronized modulation strategy based on the modified 24-sector SVPWM technique.By comparing the harmonic performance of the synchronized modulations and considering the limit of the maximum switching frequency of the inverter,the authors present a set of multi-mode SVPWM strategy,which is suitable for the high-power traction drive systems in the entire speed range.At the same time,the switching strategy for transitions between different modulation modes is proposed as well.According to the characteristics of the multi-mode SVPWM modulation strategy,the vector control strategy under the condition of low switching frequency for the dual stator induction machine has been proposed.In order to solve the inaccurate rotor flux orientation problem caused by a variety of reasons,the correction strategy based on the q-axis estimated rotor flux suitable for the working condition before the square waves and the correction strategy based on the q-axis current error suitable for the working condition under the square waves have been adopted,respectively.Finally,the four-level experimental platform for the transformerless traction drive system is established.As the active front end of the platform,the control strategy of the cascaded H-bridge rectifier is studied,as well as the voltage balance control strategy for the suspending DC bus voltages.Based on the analysis of the stable control field of the cascade H-bridge rectifier,the coordination control strategy between two motors is illustrated.The relevant experiments are carried out on the experimental platform.