| Under the dual pressure of the traditional energy crisis and the surge need in human survival,renewable energy power generation technology has been a vigorous promotion and development.However,solar energy itself has the shortcoming of the intermittence and the instability and poor output waveform quality will seriously interfere with the normal operation of the grid,especially when the point of common coupling voltage dips,the PV system output power quality is really poor and will introduce a large number of high frequency harmonics to the grid,which is not conducive to the safe operation of the system.Therefore,based on a large number of literature search and combined with the current industrial background of photovoltaic power generation technology,the large-scale two-stage photovoltaic power station with energy storage equipment on the DC side is taken as the research object and the control strategy of low voltage ride through is studied.The main works are as follows:First of all,according to the low voltage ride through of large-scale photovoltaic power station,the operating characteristics of grid-connected PV inverters under grid faults are extracted.The output power of the inverter contains negative voltage and current component,which will transmit a large number of high order distortion components to the grid.How to quickly realize the voltage detection,phase locking,positive and negative sequence voltage separation,voltage and current control and a series of network technology must be solved.Secondly,a voltage detection method based on LES(Least Error Squares)filter is proposed to solve the problem of low speed and low precision in the traditional low voltage detection method by setting the transition matrix to construct the grid voltage vector equation.The method can extract the harmonic components of the grid voltage and can accurately calculate the amplitude and phase angle corresponding to each component.Compared with the traditional low-voltage detection method,this method can realize the separation of the grid voltage in one cycle,and has a fast response speed.Thirdly,when the asymmetrical voltage fault occurs in the grid,the voltage distorted at the grid point and the voltage component of the negative sequence double frequency appears.In order to suppress the negative sequence voltage component,the fundamental frequency component extracted by the LES filter and the corresponding quadrature components are obtained,which we used as the inputs of SOGI-PLL(Second Order Generalized Integrator Phase Locked Loop).In the al coordinate system,the corresponding negative sequence voltage components are decomposed as the continued controller of the control signal.Finally,in order to eliminate the voltage distortion and suppress the negative sequence voltage component in the large-scale PV system,a low voltage ride through control strategy based on negative sequence voltage component feed forward is proposed for the asymmetric fault condition,which is based on the voltage and current double closed-loop structure.In addition,the charge-discharge control method of DC-side storage super capacitor is studied and the charge-discharge control strategy based on compensation factor is proposed to ensure the stability of the reference input in the feed forward low voltage control strategy. |
