Research On Control Technology Of Three-phase Photovoltaic Grid-connected Inverter With Active Power Filtering Function

With the rapid development of the economy,the energy crisis and environmental pollution problems have become more and more serious.As a new type of pollution-free renewable energy,photovoltaic power generation is an important means to promote energy transformation and replace fossil energy.At the same time,the use of power electronics in the grid and the non-linear load on the user side can bring harmonic pollution to the public grid.In order to alleviate the harmonic problem of the power grid,the photovoltaic grid-connected inverter and the active power filter are combined in the same topology and control method to combine the photovoltaic grid-connected inverter and the active power filter.The photovoltaic grid-connected function and active power filtering function are implemented on the inverter to enable the three-phase photovoltaic grid-connected system with active power filtering to perform grid-connected power generation or grid-connected power generation and active power filtering simultaneously.When the lighting conditions do not meet the power generation requirements,the active power filtering function is utilized to expand the function of the inverter.This paper studies the changes in the function of the inverter and the change of the control strategy.Firstly,the similarities and differences between photovoltaic grid-connected inverters and shunt-type active power filters are compared and analyzed,which provides theoretical premise for realizing photovoltaic grid-connected function and active power filtering function on the same inverter;The selection was made to determine that the front stage is a boost boost circuit,the latter stage is a voltage type inverter,and the main circuit adopts a two-stage grid-connected structure of a three-phase full-bridge topology.The principle of designing the main circuit parameters is explained,which provides the basis for constructing the grid-connected inverter simulation system.In order to make full use of solar energy,this paper selects the disturbance observation method with fast tracking speed and convenient implementation as the maximum power point tracking technology.After that,the two methods of p-q and i_p-i_q harmonic detection based on instantaneous reactive power theory under the condition of distorted power grid are analyzed.The results show that the i_p-i_q detection method is more accurate.However,the i_p-i_q detection method requires accurate grid voltage phase information,and the single synchronous coordinate phase-locked loop cannot meet the demand.Therefore,this paper presents a biquad generalized integral that can accurately lock the grid voltage phase under non-ideal grid conditions.The phase-locked loop is applied to the i_p-i_q detection method to detect harmonic current more accurately.In order to realize different functions of the system,the command current components in the combined working mode of grid-connected mode,active filtering mode,simultaneous grid-connected power generation and active power filtering are analyzed,and the command current synthesis in different working modes is performed.Finally,in the aspect of grid-connected control strategy,this paper studies the grid-connected control strategy of voltage-current double closed-loop control structure in dq coordinate system and the grid-connected control strategy of hysteresis current control in ABC coordinate system.The design method of current inner loop PI controller and voltage outer loop PI controller under dq coordinate system control strategy is given,and the inverter is connected to the grid by SVPWM modulation strategy.The traditional hysteresis current control principle in ABC coordinate system is analyzed.An improved hysteresis current control method is proposed,which can make the switching frequency of power electronic device more stable than traditional hysteresis,and verify the improved method.The grid-connected simulation experiment of the inverter was carried out with improved hysteresis control.The simulation results of the grid-connected control strategy of the inverter in the ABC coordinate system and the dq coordinate system are compared.The results show that the improved hysteresis current control in the ABC coordinate system has better tracking speed and faster dynamic response than the dq coordinate system.The harmonic control effect of the power grid is more obvious,and is more suitable for the grid-connected control of the inverter with active power filtering function.The work of this paper has completed the accurate acquisition of grid harmonics under non-ideal grid conditions.The improved hysteresis current control method can make the switching frequency more stable,the current tracking effect is good,and the ability of grid-connected inverter to control harmonics can be improved.Therefore,the grid-connected power generation and active power filtering functions are realized on the same inverter,which improves the utilization rate and cost performance of the inverter.