Research On Control Strategy Of Cascaded H-bridge Multilevel Rectifier

With the development of emerging technologies such as energy Internet and distributed generation,the application of high voltage and high power intelligent power electronic equipment are more and more widely used.In the future,power electronic transformers to replace the traditional power frequency phase-shifted transformer has become the new trend of power grid construction,received extensive attention and research.This paper focuses on single-phase cascaded H-bridge rectifier part of the front-end of power electronic transformers,mainly studies the DC-link capacitor voltages balance control and unbalanced load limiting range.Firstly,the topology and operating principle of single-phase cascaded H-bridge rectifier are analyzed in depth.The feedforward decoupling control is used to control the system active current and reactive current independently.The two-dimensional modulation algorithm is used to calculate the duty cycle of each bridge,the active power absorbed by each bridge is dynamically adjusted to achieve the DC voltages balance and unit Power factor operation.Aiming at the nonlinear characteristics of cascaded H-bridge,the advanced feedforward decoupling control in single-phase rotating coordinate system is improved to reduce the influence of DC-link voltages and duty cycles fluctuation on the system performance,enhance the anti-interference ability of the system.Second,the degree of unbalanced load connected to the DC side of cascaded rectifiers has an important impact on system stability.When the load imbalance exceeds the critical value,the DC-link capacitor voltage will no longer maintain the balance,thus result in the increase of the harmonic component of the input current accordingly.In view of this problem,this paper analyzes the output voltage waveform of AC side of two H-bridges in detail.According to the principle of system power balance,the unbalance limit range of load under steady state is studied.At the same time,the selection range of DC-link voltage reference is given.Finally,a simulation model based on MATLAB / Simulink and a semi-real-time simulation platform based on RT-lab are set up.The correctness of the proposed capacitor voltage balance control strategy and the unbalanced limiting range made in this paper are verified through relevant simulation and experiments,providing a theoretical basis for the parameter design of cascaded H-bridge rectifier.