Research On A Novel Soft Start Method And Key Technology Of High-power And High-voltage Motors

High-power and high-voltage motors are widely used in fields such as transport, ports, docks, iron, building materials, petroleum, chemnetallurgy, coal and other industries. They are prime motors to drag fans, pumpspressors, lifting machines, kiln grinders and other large equipments. With the devnent of national economy, the capacity of high voltage motors in various industrianges from tens hundreds of kilowatts to tens thousands of kilowatts. At present number of high voltage motors in all industries is more than tens of millions, a(?) increasing year by year.When a high voltage motor is started directly, the starting cnt is so large that it can cause a sudden decline in the grid voltage. This is a sas impact on the normal operation of other adjacent equipments, such as relay mnction, automatic control malfunction and so on. If soft starter is used, the harm ampact caused by direct start can be avoided. Though soft starters operate only foshort time during motor start-up, they are very important and indispensable.In recent years, researchers pay much attention to soft st technologies and devices, a lot of important results have been reported. These dnctive results not only have important theoretical significance, but also have a ro.n guiding the soft start technology and the practical application of the device. Neveeless, these results still have some limitations. Firstly, starting current can be reded by using those methods, but there is still large current impact on the power gd when those soft starters are used to start high-power and high-voltage motors. Sondly, most of the results are only concerned with power factor changes in startingrocess, but ignore reactive power compensation in the soft starting process. Thirdly,nost of the results only consider current amplitude limitation, but ignore coordinatd control between start current and start time in the soft starting process.Based on the above considerations, it is necessary to further research the soft start methods and key technologies of high-power and high-volage motors. This dissertation presents an in-depth application research to deal with above limitations. The research works mainly concern soft start principles and techniques, the corresponding design method and control strategy for the soft starting of high-power and high-voltage motors. Specifically, the main contents of this dissertation are as follows.Firstly, a new soft start method based on auto-transformer and magnetic control reactor (ATMCR) is invented to limit start current of high-power motor. Topology and equivalent circuit model of soft starter based on ATMCR are established. The design target of further reducing start current absorbed from power grid is realized.The common soft start methods used in domestic and foreign industries are investigated, and then advantages and disadvantages of the various methods are summarized. Based on the research results achieved by my tutor’s group in last many years and combined with soft start technology advantages of auto-transformer and magnetic control reactor (MCR), a new soft start method based on ATMCR is invented. The new method has dual characteristics of step-down of auto-transformer and regulation voltage of MCR and it can further reduce start current absorbed from power grid. Through the research of electromagnetic field theory, electrical theory and motor principle, topologies and equivalent circuit of soft starter based on ATMCR are established. Current-limiting mechanism is studied. Simulation results using MATLAB show that the soft start method based on ATMCR has excellent effects to limit start current.Secondly, the integrative method of reactive power dynamic compensation with soft start based on ATMCR is proposed to deal with low power factor in starting process.The relationship of power factor with speed changing in soft start process is studied. In order to improve the power factor, a method of reactive power dynamic compensation in soft start process is proposed. Schemes and realization technologies of the reactive power dynamic compensation are researched and integrative topologies are established. Calculation method of reactive power compensation and optimal switching strategy are studied. Simulation results from MATLAB shows that reactive power dynamic compensation can effectively improve power factor, reduce the grid voltage drop and avoid influence on the power grid in the starting process.Thirdly, dynamic programming control algorithm for ATMCR soft starter is proposed to optimize soft start process. Induction motor is a nonlinear, strong coupling, multivariable controlled object, and its mathematical model is analyzed to reveal the reason of current and torque oscillation. Conventional control strategies only concern the intensity of start current while ignore the span of start time. Consequently, long starting time often result in the motor coils overheating and then the motor start process fails. State equations and its solutions of induction motor are researched. According to the ideal start curve of motors and its obvious time sequence, an optimal control strategy based on dynamic programming is proposed. Soft start process is divided into different stages, each stage uses different control strategy. The optimal control strategy can realize the shortest soft start time and minimize the starting current within two times of rated current. MATLAB is employed to simulate ramp control, constant current control and dynamic programming control. And simulation results are compared to verify feasibility of the dynamic programming control algorithm.Fourthly, soft starter based on ATMCR of high-power and high-voltage motor is developed and tested.Based on theories and technologies of ATMCR soft start method, starter structure and operation principle based on ATMCR are designed in the dissertation. The21000kW/10kV soft starter is successfully developed. All theoretical and technical research results are evaluated through the test.Finally, we summarize the results of the dissertation and point out some problems that are worth further study.