Research On Induction Motor Direct Torque Control System Fed By Dual Three-level Converters

With the development of industry and economy, a series of new problem emerges, such as reactive power and harmonic pollutions on electric grid, deteriorated environment, and energy crises. Under this background, together with the development and increasingly mature of multilevel technology, employing dual three-level converters control large capacity induction motor drives system has become a hot topic in recent years. The system has several merits. For example, high power factors, bidirectional power flow ability, remarkable energy-saving effect, excellent control performance, trivial pollution on electric grid, and so on. It has extensive application foreground on medium and/or high voltage high-power AC drive domain.The dissertation mainly focuses on the induction motor direct torque control (DTC) system fed by dual three-level converters. The paper firstly discusses the control system of three-level PWM rectifier and three-level induction motor DTC system separately, and then analyzes the whole system. The studies carried out in the paper and corresponding results are as follow.(1) In this paper, the method of how to attain space voltage vector by space vector pulse width modulation (SVPWM) strategy in three-level converters is discussed in detail. And a SVPWM method that can reduce switching loss and limit dv/dt is introduced, too. And the paper introduces the concept of current modulation. By using coordinate transformation, the neutral-point balancing problem of three-level converters with SVPWM modulation strategy is quantificationally analyzed in dq two-phase rotating reference frame.(2) The direct current control method of three-level rectifiers is detailedly analyzed in this paper. And the neutral-point balancing Bang-Bang control scheme of three-level PWM rectifiers is proposed. According to the direction information of the three-phase leg current and neutral-point potential, the neutral-point balancing control scheme abandons the switching states of short vector in SVPWM sequence, which has disadvantage effect on neutral-point balance. The scheme achieves preferable neutral-point balance results.Simultaneously, the switching loss and dv/dt of output voltage will not increase. Simulation analysis and experimental results proves the feasibility of proposed method. (3) A uniform mathematic model of three-level induction motor drive system inαβtwo-phase stationary reference frame is set up in this paper. And then, the influence of different kinds of neutral-point unbalance on motor performance is analyzed for the first time. The conclusions are able to instruct the neutral-point balancing method design of three-level induction motor drive system. This paper also takes two-level as an example, introduces the principle of DTC system and the corresponding essential technology.(4) This paper proposes a novel fixed synthesizing vectors method. Furthermore, the real-time neutral-point balancing Bang-Bang control method for the novel fixed synthesizing vectors is proposed, too. The shortcomings of directly using three-level's own voltage vector for DTC system are discussed. Also, a DTC method, which bases on three-level fixed direction synthesizing vectors, is introduced. Theoretical and simulation analysis shows that, the method lacks real-time control ability of neutral-point balancing. To solve this problem, this paper modifies the vector synthesizing technique, finds the DTC algorithm of three-level induction motor drive system with real-time neutral-point balancing Bang-Bang control ability. The neutral-point balancing control method also only requires the direction information of leg output current and neutral-point unbalance. Simulation and experimental results illuminates the feasibility and practicality of the novel fixed synthesizing vectors three-level induction motor DTC method.(5) To reduce the stator flux linkage and torque ripple of induction motor, the paper modifies the novel fixed synthesizing vectors. Proposes"24 vectors"method and"24 vectors + alter modulation index"method for the first time. Both of them are suitable for three-level induction motor DTC system. By comparison, the"24 vectors + alter modulation index"method has better performance. Combined with these methods, the paper also proposes the design method of relevant 5-level flux linkage hysteresis comparator, 11-level flux hysteresis comparator, and switching table. The contrastive simulation and experimental results showed that, the above two methods can effectively reduce the flux linkage and torque ripple. Moreover, a series of experiments, such as start-up experiment, load experiment and variable-speed-rate experiment, are carried out to validate the characteristic of"24 vectors + alter modulation index"method. The experimental results are all satisfied. (6) The induction motor direct torque control system fed by dual three-level converters has been studied in this paper for the first time. By ideal assumption, this paper in-deep analyzes the neutral-point balancing problem of dual three-level inverter. By comparison, the paper obtains the conclusion, that dual three-level converters can achieve better neutral-point balancing results than three-level converters. The simulation model of dual three-level converters feed induction motor drive system is established in the paper. And the simulation verifications are also carried out. The simulation results show the feasibility of the control scheme proposed in this paper.