| With the gradual improvement of the new energy permeability in the energy system,the power electronic equipment in the future energy system will gradually take the leading position.Compared with the traditional energy system,the power-electronics-enabled energy system has the advantages of stronger controllability,higher flexibility,faster dynamic response and diverse functions.For solid state transformer(SST)is the core device in the future power-electronics-enabled energy system,the improved control strategy and hardware implementation of three-stage SST are studied in this paper.The topological structure of three-stage SST is analyzed,which mainly includes the AC/DC converter,DC/DC converter and DC/AC converter.And the corresponding mathematical models of the topological structure are established.The controllers for AC/DC converter,DC/DC converter and DC/AC converter of SST are designed and modified in this paper.In addition,DC offset elimination and frequency self-adaptation of single phase PLL are studied.Specific study highlights are as follows:1.The DC bus voltage control based on virtual inertia is adopted for AC/DC converter.On the basis of traditional voltage/current double closed-loop controller,the virtual inertia control loop and the DC current feed-forward loop are introduced.In order to effectively suppress the voltage fluctuation of the DC bus in the case of the fluctuation of the system power,the inertia of the DC bus is enhanced by adding virtual capacitance.In order to reduce the influence on the DC voltage caused by load current changing,the voltage stability of the DC bus is enhanced by adding the DC current feed-forward link.2.Single phase shift control is applied for DAB stage to ensure stability of DC bus voltage,but the voltage fluctuation of DC bus is obvious when load changes.So,single phase shift control of load current feed-forward is adopted,which decreases the fluctuation of the DC bus voltage and increases the dynamic response time of the DC bus voltage.3.On account of the traditional SOGI PLL can not achieve frequency self-adaptation and the DC offset elimination,a frequency adaptive PLL based on the structure of modified SOGI(MSOGI)is adopted in this paper.Under the condition of the power grid voltage distortion and frequency fluctuation,it still can accurately track the phase and estimate the frequency of the grid voltage due to the introduction of the frequency estimation link and DC component elimination link.At last,a 10kW three-stage SST experiment platform is built.In addition,the software and hardware of the system are designed respectively.The experimental results verify the effectiveness of the designed controller in this paper. |
