Control Strategy For A Cascaded H-Bridge Converter Based 10kV/380V Power Electronic Transformer

Traditional power transformers play an important role in the power system.With the development of the times,people have some new requirements for the power system,such as the new energy integration,higher quality power,the intelligence of energy management and so on.Due to the single function and poor controllability,the traditional power transformers cannot meet these requirements.The power electronic transformer is an electronic device that has a flexible topology and adopts various advanced control technologies;it may replace the traditional power transformer in the future,and has great development prospects.Based on the medium-voltage distribution network and the cascaded H-bridge topology,aiming at the common three-phase asymmetry of output loads and grid voltages,this paper studies the control strategy to realize DC capacitor voltage balance,so that the three-phase output voltage is symmetrical and stable.The power electronic transformer studied in this paper consists of three stages: input rectifier,intermediate isolation and output inverter.The input stage based on cascaded H-bridge is mathematically modeled,and the main circuit parameters and controller parameters are designed.Then the mechanism of cluster voltage imbalance is analyzed.When the grid volatges and output loads are both asymmetrical,the relationship between the active power per phase and the negative-sequence voltage is established in the dq frame.The negative-sequence reference that is used to control the cluster voltage balance is calculated based on the power equation.The calculation of negative-sequence does not involve grid current,there is no need to calculate positive and negative sequence of grid current,only single current inner loop control is needed.However,the injection of negative sequence voltage also injects negative sequence current into the grid,when the asymmetry of active power of output loads in three phases is not so serious,in order to keep the grid current symmetrical,a zero-sequence voltage injection method is proposed.A hypothesized phase is intentionally introduced by shifting the original zero-sequence voltage by 90°,so the zero-sequence can be represented by DC form.Therefore,an method similar to the negative-sequence voltage injection can be used to achieve cluster voltage balancing control.The active power regulation capability of the negative-sequence voltage and the zero-sequence voltage is also compared.The single-phase inverter of the output stage is mathematically modeled in the dq frame,and the individual voltage balancing control is realized by trimming the duty ratio while ensuring the output voltage is symmetrical and stable.However,the method based on dq frame requires multiple coordinate transformations,which is complicated,so the method based on quasi-proportional resonance(QPR)control is adopted for improvement.The coupling effect between the individual voltage balancing controller and the voltage-current dual-loop controller is analyzed and the decoupling control is performed.When the circuit parameters of each branch are inconsistent,the capacitor voltages on high-voltage DC side will be unbalanced.The imbalance is suppressed by using LLC resonant converters.Finally,the simulation model is built in Matlab/Simulink,the simulation results show the effectiveness of the proposed control strategies.