Research On Photovoltaic Grid-connected Inverter Control Strategy Under Grid Voltage Unbalanced

Photovoltaic(PV)grid-connected generation system is an popular way to utilize the solar energy,which achieves a great development in recent years in China,the total capacity of the PV generation has been the NO.1,then which can make a steady foundation for us to face the crisis of the resource and energy in the future.The PV grid-connected generation is a system that can transfer the DC electricity generated by PV batteries into AC electricity of the same frequency and phase angle with the grid voltage,and then connect to the “big grid” by the main transformer.Three phase photovoltaic grid-connected inverter is the most critical equipment in photovoltaic power generation system.It is responsible for controlling the power generation efficiency,output power quality and operation safety of photovoltaic power generation system.Under the condition of unbalanced grid voltage,grid connected inverter output power quality is seriously affected,the probability of three phase unbalanced fault occurring in the power grid is large,this dissertation is in this background of research on the control strategy of PGI.Firstly,three-phase PGI mathematical model on the condition of unbalanced grid voltage is established in three-phase static coordinate system abc,two-phase stationary coordinate system alpha beta and two-phase synchronous dq coordinates,respectively.The dissertation analyzes the degree of decoupling of the grid-connected inverter control system,the controller design difficulty and engineering fact under various coordinate systems,choosing to design the control system of the grid connected inverter in three-phase two-phase synchronous rotating dq coordinate system.In addition,when the network voltage is three-phase asymmetrical,the fundamental negative sequence component of the grid voltage will lead to the harmonic distortion of the output current of the grid connected inverter,the output reactive power fluctuation and the DC bus voltage fluctuation of the inverter.Under the condition of unbalanced grid voltage,according to the theory of digital signal processing,this paper proposes a cascaded delayed signal cancellation method based on(Cascade Delay Signal Cancellation,CDSC)of the positive and negative sequence separation algorithm,this algorithm can quickly and accurately extract the positive sequence component and negative sequence component in the signal parameters by simple design,also contains a large number of harmonics in the signal,in the extraction speed and accuracy compared with other algorithms have good effect;on this basis,this paper proposes an improved instantaneous reactive power control algorithm,the algorithm of grid connected inverter power output flow instructions through the flexible design coefficient based on K,realize different control objectives;at the same time,the simulation model is built in MATLAB/Simulink on the platform,the control algorithm presented in this paper are verified,simulation results also confirm the validity and correctness of the proposed algorithm.Finally,under the condition of asymmetric three-phase grid voltage,three-phase unbalanced peak value of output current may exceed the maximum current allowable by the operation system.In view of this,this dissertation proposes a kind of inverter output current peak suppression algorithm,the output current of the inverter control system within the maximum peak current range;at the same time,the dissertation builds simulation model on MATLAB/Simulink,the performance and effectiveness of suppression algorithm is verified.