Research On Protection Scheme Of The DC Integration System Of PV Generation

In recent years,the photovoltaic industry of China has developed vigorously.As a new type of grid-connected mode,photovoltaic DC access mod e has the advantages of high photovoltaic utilization efficiency and strong control flexibility,which has gradually attracted widespread attention.However,there is still a lack of in-depth theoretical analysis and practical application.Moreover,the interaction of AC and DC side faults characteristic and the design of DC protection scheme have became a bottleneck that hinders its rapid development.This thesis takes the photovoltaic DC integration system based on hybrid-MMC as the research object.By means of theoretical analysis and simulation,the system architecture and control strategy of PV DC integration system are proposed.The characteristics of DC faults and its impact on AC system are studied,and the new relay protections scheme which can reliably identify internal and external faults are proposed.Firstly,according to the structure of DC integration system of PV generation,the simulation model was built.The key equipments were introduced,and the control strategy of DC/DC converter and MMC converter were given.Secondly,according to different connection groups of the connected transformers,the discharge circuits and characteristics of unipolar grounded fault were analyzed.The fault stages were redivided according to the charging and discharging process of capacitors,and the equivalent models of each stage were built and solved.Based on the characteristics of unipolar grounded fault,the influence of the fault on DC bias of connected transformer was explored,and the countermeasures was proposed.Then,the fault processes of DC/DC converter and MMC converter side were analyzed when the bipolar short-circuit fault occurs,and the fault stages were divided.Thirdly,the DC protection schemes based on the correlation coefficients of the positive and negative currents and modulus identification of 0 mode and 1 mode parameters were proposed.For the low current of the unipolar grounded fault when the connection transformer was YNd11 connected and Dyn11 connected with high grounding resistance,considering the distribution capacitance characteristics of the dc cable,the correlation coefficients of positive and negative pole currents at both ends of the cable were calculated,and based on this,the pole-to-ground fault identification scheme which can reliably distinguish fault areas was obtained.Subsequently,according to the fault characteristics of Dyn11 connected transformer with low resistance,the correlation coefficients of variation of 0-mode and 1-mode currents was used as the criterion to distinguish the unipolar grounded fault and bipolar short-circuit fault,and the difference between 0-mode current and 1-mode current at the two ends of the line was used as the criterion of internal and external fault,and the fault pole was discriminated based on the posit ive or negative polarity of the ratio of 0-mode voltage to 1-mode voltage.The PSCAD/EMTDC simulation results show that the two protection schemes can identify the fault area effectively and operate reliably.Finally,the main research contents and results were summarized.