Research On The Control Strategy For Smooth Switching Of P Power C Conversion Onversion System Ystem

The microgrid has two operating modes: grid-connected operation and off-grid operation.When photovoltaic power generation is sufficient and the power conversion system(PCS)is in operating condition,electric energy can be transmitted to the grid through the PCS,it is the grid-connected mode.When the power grid fails or the power line is overhauled,the PCS maintains the bus voltage and frequency,and it is in island operation mode at this time.When switching between on-grid and off-grid,the microgrid voltage and current are required to have a smooth transition without current impact,otherwise the protection will be triggered,and the microgrid will fail to switch and work normally.In severe cases,important equipment may be destroyed or even fire.Therefore,studying the smooth switching control strategy of the PCS for on and off grid is of great significance to the microgrid and its stable operation.First of all,in view of the current impact and voltage fluctuation caused by the PCS in the microgrid during the on-grid and off-grid switching,a smooth grid-connected/off-grid switching control strategy based on wave-by-wave current limiting is proposed.The current closed-loop control process of the PCS is analyzed,The proportional derivative(PD)control strategy used in the switching process can more effectively suppress the current overshoot.By comparing whether the peak value of the output current of the PCS exceeds the limit value,the current inner loop control method is changed to reduce the current overshoot during switching.Secondly,when the PCS is switched from off-grid to grid-connected,there are problems such as voltage oscillation and long switching time.Considering that the ordinary phase locked loop(PLL)only adjusts the error of a certain parameter of the system output,such as frequency or phase,While affecting the voltage stability during switching,this paper proposes a double closed-loop synchronization control strategy of phase and frequency.Before connecting to the grid,the output voltage is synchronously locked through the phase outer loop regulator and the frequency inner loop regulator to achieve.In this way,the PCS is quickly connected to the grid,and the voltage oscillation is reduced.Finally,the simulation study of the proposed wave-by-wave current limiting control strategy and the double closed-loop synchronous control strategy of phase and frequency is carried out,the main parts of the circuit are introduced in detail,and the PCS prototype and experimental platform are designed.The experimental results prove that this control strategy reduces the current impact and voltage oscillation in the process of switching between the on and off grid,and the PCS has feasibility and certain engineering value.