Equal Voltage And Power Control Strategy For Cascaded H Bridge Solid State Transformer

In the traditional distribution network,the power frequency transformer is the transformation of power distribution and voltage level.Its outstanding advantages are high reliability and efficiency,and low price.However,with the development of smart grid,active power distribution network,energy Internet and other emerging technologies,the intelligent level of distribution equipment is increasing,and the shortcomings of traditional transformers have been highlighted.For this purpose,electrical isolation,voltage change,reactive power compensation,power flow control and other solid state transformers(Solid-State Transfomer,SST)stand out and become an important equipment for the integration and optimization of the traditional transformers and various electric and electronic devices in the power grid and the improvement of the intelligence level of the power grid equipment.First,the principle of the cascaded H bridge rectifier(CHBR)and the carrier phase shifted PWM modulation technology are studied.At the same time,the parameters of the cascaded H bridge rectifier are designed.Then the switch model and the small signal model of the cascade H bridge rectifier in the dq coordinate system are established,and the variables are derived.The transfer function is used to construct the block diagram of open loop structure.The double closed loop decoupling control strategy of voltage and current in dq coordinate system is used to control the cascaded H bridge rectifier.The system can run in various working conditions,and the simulation model of the 3 cascaded module is built to verify.Secondly,based on the unbalanced DC voltage of each module of cascaded H bridge rectifier,a new voltage balance control strategy is proposed to eliminate coupling and achieve DC side voltage and reactive power balance.First,the coupling effect and reactive power are analyzed,and the conditions for realizing the elimination of coupling and the realization of the reactive power need to be fulfilled,that is,the balance between the DC side voltage and the reactive power is realized by correcting the active power and the reactive power ratio according to the certain relationship,and the DC voltage can be realized quickly even under the condition of unequal load.Balance,eliminate coupling effect between controllers and realize reactive power balance under any conditions.At the same time,a system block diagram of voltage balancer is set up,and the stability of the system is analyzed.The effective range of the voltage balance controller is also discussed.The effectiveness and feasibility of the method is verified by simulation and experiment.Finally,according to the isolation level DAB parallel system,the traditional phase shift control of DAB is analyzed first,the mathematical model of DAB is established,the control block diagram of DAB is derived,and the DAB parallel system is analyzed,and the control block diagram of the DAB parallel system is derived.At the same time,in view of the problem of DAB parallel current and power imbalance,a power balance control strategy without current sensing is adopted,and the control strategy is analyzed in detail.The effectiveness and feasibility of the method are verified by simulation and experiment.