| DC power distribution has been attracting more and more attention due to its advantages such as low power loss,high reliability,simple and flexible control,and easy access to distributed power.In DC distribution network,DC microgrid is an important operation mode.However,the DC microgrid can't interconnect with DC distribution bus of different voltage levels through the electromagnetic induction principle,so the DC/DC converter based on power electronics technology must be used to realize the functions of voltage conversion,bidirectional power transmission and electrical isolation.In the field of low voltage and small capacity,the dual-active-bridge(DAB)converter has been widely applied for its functions of voltage conversion and electrical isolation,and also for its advantages of high safety and reliability,bidirectional power transfer capability,etc.However,due to the voltage and current limits of switching devices,single DAB converter can't achieve high voltage and high power ratings.Therefore,the DC solid state transformer(DCSST)which is composed of multiple DAB units in the structure of input-series and output-parallel(ISOP)emerges as the times require.The ISOP structure enables the DCSST to achieve a higher voltage level on the input side,and a larger transmitted power on the output side.The premises for the normal operation of the ISOP converter are the input voltage and output current sharing among sub-modules,therefore,it is necessary to add the intput voltage balancing control loops to DCSST while realizing stability control of the output voltage,and the coupling effect between the input voltage and output voltage controllers should be eliminated.In this paper,the phase shift control principle and mathematical model of single DAB converter are analysized,and the unified modeling of DCSST is carried out.Based on mathematical model of DCSST,the decoupling condition between the input voltage balancing and output voltage control loops is analyzed,and a decoupling control strategy is proposed.Conventional balancing and decoupling control strategy that based on small-signal analysis would cause decoupling error when great imbalance of input voltages occurs,which affects the output voltage control of DCSST.Aiming at this problem,this paper expounds the causes of decoupling error by analyzing the decoupling principle of the previous method,and proposes a novel balancing and decoupling control strategy.In order to improve the disturbance-rejection ability of the output voltage of DCSST,a novel control strategy based on active disturbance rejection control(ADRC)is proposed to stabilize the output DC voltage.Based on circuit model of DCSST,the ADRC controller is designed,and the tuning method for ADRC that achieve high performance is presented which is based on linear active disturbance rejection control(LADRC)scenario.In this tuning method,all the parameters are expressed as a function of the loop-gain bandwidth by pole-placement.Finally,a DCSST experimental platform is built,and the selection of main circuit components and the realization of control system are introduced.The effectiveness of the proposed control strategy is verified by experiments. |
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