Research On Characteristics Of Photovoltaic Output And Its Impact On The Protection Considering Voltage Disturbance

Large-scale photovoltaic power generation systems are mostly integrated into the AC grid through power electronic equipment.which profoundly changes the physical structure of the original power system and various physical interaction laws,and puts forward new requirements for the safe and stable operation of the power system.Under the dual influence of structure and nonlinear controlled source,its transient characteristics after failure are very different from traditional synchronous machines.Traditional fault analysis theory and AC relay protection are both faced with the new environment of large-scale and high-proportion photovoltaic grid-connected It is a great test.External factors such as grid-side voltage disturbance,as well as its own factors such as the dynamic performance of the control link,have a profound impact on the harmonic characteristics of photovoltaic faults.Therefore,it is necessary to study and consider the harmonic characteristics of photovoltaic faults in different scenarios and their protection The impact mechanism of this is an issue that must be faced and urgently needed to be solved in the field of control and relay protection after vigorously developing photovoltaic and other new energy power generation.This paper takes the photovoltaic AC centralized grid-connected system as the research object,focusing on the second harmonic component of the photovoltaic output current and its impact on the relay protection and countermeasures in the scenario of a serious grid-side voltage drop and no-load closing of adjacent transformers.The main research results and innovations are as follows:(1)Aiming at the scene of a three-phase short-circuit fault in the low-voltage side area of the main transformer,the performance of the phase-locked loop when its input voltage signal is severely dropped is analyzed,and the influence of the dynamic characteristics of the phase-locked loop on the photovoltaic output characteristics is studied.In the scenario of photovoltaic access to a weak grid,when the grid connection point has a severe voltage drop,the phase-locked loop error increases,resulting in a large number of harmonic components in the photovoltaic output current,which affects the performance of the main transformer differential protection.Aiming at this problem,a phase-locked loop improvement scheme based on memory voltage is proposed,which effectively improves the photovoltaic output characteristics under three-phase short-circuit faults,and solves the problem of the wrong blocking of the second harmonic braking criterion of the main transformer differential protection.The simulation results verify the correctness of the relevant analysis and improvement plan.(2)In view of the no-load closing of adjacent transformers,analyze the internal connection and law of the voltage disturbance on the AC and DC sides of the photovoltaic inverter,and further write the transfer function of the photovoltaic control structure to reveal the photovoltaic output current under the grid-side voltage disturbance.The generation mechanism of the sub-harmonic component is to study the impact of the higher second harmonic content of the photovoltaic current on the transformer differential protection when the main transformer of the photovoltaic station is slightly faulted in a weak grid with a relatively low short-circuit,and to optimize the photovoltaic inverse from the perspective of the converter control strategy,the suppression measures of adding a notch filter in the control link are proposed,and the simulation results verify the correctness of the relevant analysis and the improvement plan.(3)Aiming at the problem that the traditional distance protection may not operate correctly when the photovoltaic transmission line is short-circuited by the transition resistance,this paper first points out through theoretical analysis that the existing adaptive distance protection has the risk of out-of-zone faults and malfunctions.Based on the action characteristics of the curved-edge quadrilateral distance protection,the action areas with different delays are delineated and coordinated with adjacent lines;in view of the possible misoperation,the corresponding fault location and the size of the transition resistance are analyzed,and the zero sequence is analyzed.The relationship between the current phase and the fault current phase is approximated by the line short-circuit impedance,which constitutes an auxiliary criterion for protection;finally,the effectiveness of the improved scheme is verified through actual fault data and simulation.