Control Strategy For Diode-clamped Cascaded Single-phase Power Electronic Transformer

As a new type of multifunctional power transformer,power electronic transformer is smaller in size,lighter in weight and has the power quality adjustment ability compared with the traditional power frequency transformer.At present,it has been applied in locomotive traction,new energy generation and grid-connected control.Therefore,how to stabilize and control power electronic transformer is of great research value.In this paper,the single phase power electronic transformer with diode clamps is studied.The input part and isolation part control strategies are mainly studied,mainly including the following:Firstly,the mathematical model of single-phase diode-clamped rectifier used in the input part is deduced from the switch state and extended to the cascade system.Aiming at the delay error of the phase-locked loop in the traditional current control under non-ideal power grid voltage,an improved transient current control,phase-locked loop transient current control,is proposed.Secondly,the principle and implementation process of single-phase Space Vector Pluse Width Modulation(SVPWM)adopted at the input part are analyzed.The reason of unbalance of voltage of supporting capacitor and the influence of voltage of supporting capacitor under different switching state are studied,so a SVPWM voltage blancing modulation algorithm based on redundancy vector is adopted,and the correctness and effectiveness of SVPWM voltage blancing algorithm based on redundancy vector is proved by simulation.Then,the reason of the unbalance of dc side voltage of the module is analyzed,and the influence of the port level of the ac side on the dc side voltage of the corresponding module is analyzed.According to the selection of port level,a fast SVPWM modulation voltage blancing algorithm and a switching frequency optimization SVPWM voltage blancing algorithm are proposed.The simulation results show that the fast SVPWM voltage blancing control has strong voltage blancing ability,the speed is fast,the frequency is optimized SVPWM modulation voltage blancing control switch frequency is low,and the voltage blancing ability is good.Then,the single phase shift modulation and PWM plus phase shift modulation strategy of diode clamping dc-dc converter with isolation part are analyzed.and the power output characteristics under single phase shift modulation are studied.The isolation level direct power control model is deduced.In order to solve the power distribution problem of multi-module output parallel diode clamped dc-dc converter,under normal working conditions,the direct power balance control strategy is applied to the topology structure in order to solve the power distribution balance of each module,Under the condition of excessive transmission power,a power cooperative control strategy is proposed to solve the possible overload of individual modules.The simulation results show the feasibility and effectiveness of power distribution control under different working conditions.Finally,a diode clamp type PET system model with a voltage level of 6kV/380 V was built,and the characteristics of the whole PET,the load changes of the output ports,the characteristics of each port of the system and the efficiency characteristics of power transmission were studied.