Research On Control Method Of Dual-Frequency Single-Phase Rectifier In Weak Power Grid

Single-phase pulse width modulation(PWM)rectifiers with high power factor are widely used in various fields.A representative application is the high-speed railway traction drive system.However,the single-phase PWM rectifier still inevitably injects certain harmonics into the grid,and the grid-side current will be distorted in a weak grid.In order to meet the grid-connected requirements,control measures must be applied to the grid-connected devices.Usually a low-pass passive filter is added to the grid side of the rectifier to ensure the quality of the grid-side current,which will result in larger passive components and higher costs.Although increasing the switching frequency of the rectifier is beneficial to reduce the size of the filter device,a higher switching frequency will cause a greater loss of the electrical energy flowing through the converter,thereby reducing the efficiency of the entire system.Therefore,the contradiction among the efficiency of the rectifier,the size of the passive filter components of the rectifier,and the quality of the grid-side current of the rectifier is a key issue in the field of current converter research.In view of the above problems,this article proposes a dual-frequency single-phase rectifier.The dual-frequency single-phase rectifier consists of two parts,the main rectifier unit with low switching frequency and the auxiliary converter unit with high switching frequency.The one that controls the main rectifier rectifies the AC power from the grid to DC power.The auxiliary converter uses a feedforward control method to generate a current with the same amplitude and opposite direction as the current harmonics in the grid-side current of the main rectifier to offset the current harmonics.Although the introduction of the auxiliary converter increases the cost of the rectifier system,it not only can effectively suppress the harmonics in the grid-side current,but also its operating loss is very small compared to the main rectifier.Compared with the loss that the main rectifier uses low switching frequency to reduce,the added loss of the auxiliary converter is very small.The auxiliary converter uses the feedforward control method to suppress the switching harmonics in the grid current without detecting and extracting the current harmonics,so there is no need for the control loop to have a high bandwidth.In a weak grid,the auxiliary converter can still compensate for the distorted main rectifier current and improve the quality of the grid-side synthesized current.The dual-frequency single-phase rectifier can work normally in a weak grid.Therefore,the dual-frequency single-phase rectifier has relatively good performance in three issues: the efficiency of the rectifier,the size of the passive filter components of the rectifier,and the quality of the grid-side current of the rectifier.In the second chapter of this paper,the topology and working principle of dualfrequency single-phase rectifiers under weak grids are studied,and how to achieve gridside harmonic suppression without introducing circulating current problems.In the third chapter,the core control method of the feedforward control of the rectifier under weak grid conditions is introduced,and how to control the auxiliary converter to achieve grid-side harmonic suppression is studied.In Chapter 4,the control structure of the phase-locked loop under the weak grid is introduced,and the influence of the phase-locked loop in the system under the weak grid on the harmonic suppression of the grid-side current is analyzed,indicating that the phase-locked loop in the dualfrequency single-phase rectifier changed will not have a major impact on the dualfrequency single-phase rectifier,and it can still work normally in a weak grid.The simulation and experiment in Chapter 5 verify the correctness of the topology and control system of the dual-frequency single-phase rectifier.