Research On High-Gain Tri-Ttate CSI And Its Control Strategy Under Three-Phase Unbalanced Grid Voltage

Inverter is the key point to the renewable energy power generation technology.Renewable energy has the characteristics of low output voltage and volatility,which requires the grid inverter devices have the capability of wide range of voltage and anti-interference.However,most existing grid-connecting inverters have a shortage of voltage boosting capability.Therefore,research on single stage inverters with low cost,high gain and simple structure is of great significance to renewable energy grid-connecting generation technology.Based on the high-gain tri-state CSI,the main and innovative achievements have been made as follows:Firstly,the topology and working principle of high-gain tri-state CSI are introduced,and the high gain characteristics and mechanism of the inverter are analyzed in detail.Based on the state average method,mathematical modeling and theoretical derivation are carried out,and the root locus of the dc side inductance and filter parameters are plotted according to the obtained transfer function,and the stability of the system is analyzed.Due to the uniqueness of the high-gain tri-state CSI dc switch,this paper proposes a SVPWM modulation technique which is suitable for high-gain tri-state CSI,and improves it to reduce the computation time and suitable for digital signal processor.Secondly,the basic principle and control strategy of the unbalanced grid voltage based on high-gain tri-state CSI is studied,and the phase locking technique in the unbalanced voltage is analyzed by comparison.In this paper,an unbalanced control method is proposed by controlling the positive and negative sequence current of high-gain tri-state CSI.There is dc current control on the dc side,which realizes the tri-state operation of the inverter.In the grid side,the secondary power fluctuation is eliminated by controlling the positive and negative sequence current.Simulation results show that the control method realizes the unification of the normal operation and unbalanced operation of grid voltage,and it can meet the requirements of dynamic response speed when the grid voltage drop and reliability in stable operation.It realizes the unbalance control under low dc voltage and achieves high gain characteristics.In the end,the high-gain tri-state CSI experimental platform is built,and the system is validated by the comparison of simulation and experiment results,including:the experiment of SVPWM modulation strategy,different dc voltage.By the comparison of simulation results and experiment results,it can be concluded that the high-gain tri-state CSI can work normally under the proposed modulation strategy and parameter design scheme,and the actual voltage agreed with the theoretical value,which realizes the decoupling of the dc side and ac side.In different conditions,the inverter can output high quality sine wave and achieve high gain booster function.