Research On Variable Switching Frequency PWM Based On Current Ripple Prediction For Hydraulic Power Generation System

As a clean and low-carbon renewable energy,the efficient development and utilization of water energy is not only necessary for social development,but also an important energy development strategy in China.The traditional hydro-generator unit adopts constant-speed and constant-frequency power generation mode,which has a fixed demand for power generation water condition and can only operate near the rated speed.The operating capacity under variable working conditions is insufficient,which makes it difficult to realize flexible utilization of water energy and low utilization rate of water energy.Variable-speed constant-frequency hydraulic power generation technology has great potential in improving the utilization rate of water energy and has become a research hotspot.In this paper,the improved DPWM variable switching frequency method and the direct-drive permanent-magnet variable-speed constantfrequency technology are introduced into the hydropower system,and relevant theoretical research and simulation analysis are carried out.Firstly,the current space vector PWM,the three-phase converter variable switching frequency scheme based on current ripple prediction and the new DPWMMin method are introduced.Aiming at the switching stress unbalance of the converter upper and lower bridge switching devices in the new DPWMMin method,and high frequency EMI noise,the paper proposes an improved DPWM variable switching frequency method and derives the current ripple prediction model of the improved DPWM method.This paper innovatively proposes a method of injecting common mode components based on the position of the reference voltage vector.The modulated wave is generated by the switching injection of two common mode components.Then the modulated wave is compared with the variable-cycle triangular carrier generated by the current ripple prediction model to generate a PWM signal that satisfies the requirements.And finally the control of the three-phase PWM Converter is realized by the improved DPWM variable switching frequency method.Combining s function programming,the simulation model of improved DPWM variable switching frequency method is built.The simulation results of the improved DPWM variable switching frequency method are compared with the traditional SVPWM and variable switc hing frequency SVPWM methods.The effect of the improved DPWM variable switching frequency method on current ripple control and high frequency EMI noise suppression is analyzed.Secondly,the direct-drive permanent-magnet variable-speed constant-frequency power generation technology is introduced into the hydropower system.Introduce the topology of hydropower systems and related principles of hydropower generation.The mathematical model of permanent magnet synchronous generator,machine side PWM converter and grid side PWM converter in three-phase ABC coordinate system and two-phase rotating dq coordinate system is deduced.Then the control strategy adopted by the back-to-back dual PWM converter system is studied,The machine side PWM converter current converter adopts a control strategy based on rotor flux linkage orientation,and the grid side PWM converter adopts a grid voltage orientation control strategy.And the improved DPWM variable switching frequency method is used to generate the required PWM signal to control the converter system,variable-speed constant-frequency hydropower generation is realized.Finally,the Matlab/Simulink model of variable switching frequency PWM hydroelectric power generation system is built and simulated.Compare and analyze the THD and EMI noise under the traditional SVPWM and improved DPWM variable switching frequency method.Verify the correctness and feasibility of the improved modulation method and control strategy proposed in this paper.