Research On Fault Analysis And Protection Adaptability Of Large-scale Photovoltaic Power Stations Considering Fault Ride Through Control

With the rapid development of photovoltaic power generation in my country,as the main form of photovoltaic grid-connected power generation,large-scale centralized grid-connected photovoltaic power stations(PVPS)have an increasing impact on the security and stability of the grid.As the first line of defense for safe operation,relay protection bears important responsibilities.The special distributed inverter centralized grid-connected system structure of large-scale PVPS,the strict Fault Ride-Through(FRT)requirements of the power grid,and the complex fault characteristics of photovoltaic power supply,these three factors are independent of each other,and they influence each other.The research on relay protection based on the analysis of fault characteristics poses severe challenges.This article has carried out in-depth research on different types of FRT control,failure analysis of large-scale PVPS,and the existing protection adaptability and improvement schemes under the combined effect of multiple factors.The main research contents and contributions are as follows:(1)Hundreds of large-scale PVPS photovoltaic inverter units are hierarchically collected by multiple collection lines and tie lines,and then centralized and grid-connected,resulting in the traditional grid based on synchronous machine power supply and distributed photovoltaic distribution network one by one.The fault analysis method of equivalent inverter power supply is difficult to meet the needs of large-scale PVPS relay protection research.For this reason,based on a positive sequence component control strategy of maximum reactive current injection,an inverter power supply equivalent model is established.Then,through the analysis of the influencing factors of the failure model,a universal failure analysis model of the large-scale PVPS collection system is established.On this basis,the universal failure analysis method is studied,and the calculation results of the engineering examples are compared with the simulation results.The analysis verifies the correctness of the fault model and analysis method.(2)The system structure in which the neutral point of the large-scale PVPS is grounded by resistance and the photovoltaic inverter is connected to the grid through the Y/Δ transformer has a significant impact on the characteristics of the ground fault and the current characteristics of the photovoltaic power supply with negative sequence component injection.In view of the limitations of positive sequence component control and the influence of PVPS system structure on fault characteristics,a FRT control strategy with negative sequence components that takes into account the influence of large-scale PVPS system structure on fault characteristics is proposed in combination with grid codes,which can balance the inverter While the phase voltage of the AC port is used,the transfer effect of the Y/Δ transformer is used to provide the maximum current injection beyond the safe peak current of the inverter for the fault phase.Based on the proposed control,a fault equivalent model with negative sequence components and a short-circuit calculation method are established.The simulation results verify the effectiveness of the proposed control strategy and the correctness of the fault equivalent model.(3)The protection of large-scale PVPS must meet the FRT requirements of the grid code.How to quantify this coordination requirement and combine it with the selectivity and sensitivity of protection is the core issue of the research on the adaptability of large-scale PVPS protection.Based on the failure analysis method established in the previous article,different failure test environments were designed.Through quantitative analysis of the influence of multiple influencing factors on the protection performance,it revealed the internal connection between the selectivity,quick action and sensitivity of large-scale PVPS protection.Therefore,the evaluation index of protection performance is proposed,and the quantified index is used as the evaluation standard of protection adaptability.Based on this,the influence of the neutral point resistance and the control strategy on the adaptability of the protection is studied,and a resistance selection method that guarantees the primary safety while taking into account the protection of quick action and selectivity is proposed,which can be used as a zero sequence current protection adaptation.A prerequisite for sexual evaluation.Then,based on the proposed quick-movement and selectivity indicators,the influence of different control strategies with positive sequence components and negative sequence components on protection adaptability is quantitatively analyzed.Finally,a specific example is used to verify the effectiveness of the proposed resistance selection method and the correctness of the analysis of the impact of different control strategies on protection adaptability.(4)When the neutral point resistance of a large PVPS is large,the zero sequence current protection cannot take into account the requirements of selectivity and quick action.For this reason,a universal large-scale PVPS collection system failure analysis model that takes into account the capacitance to the ground is first established.When single-phase grounding faults occur on different types of lines,the zero-sequence current distribution characteristics in the collection system are described.Based on the difference in the characteristics of the resistance and capacitance components of the zero sequence current flowing through the two ends of the tie line during internal and external faults,an economical latching zero-sequence pilot protection scheme is formed.The protection scheme only uses the zero-sequence current,and only logic information is transmitted,no voltage is required,and no synchronous sampling is required.Then,based on specific engineering examples,the influence of neutral point resistance and photovoltaic power on the characteristics of the zero-sequence current of different fault components is quantitatively analyzed.In addition,the short-circuit calculation process of the large-scale PVPS collection system considering the capacitance to ground is given,as well as the specific steps of the setting calculation of the proposed protection scheme.Finally,through the quantitative protection setting calculation and verification process,it is verified that the proposed protection scheme is in compliance with Within the full resistance range of the neutral point resistance of a safety regulation,the faults within and outside the area can be accurately judged.