Research On Protection Scheme Of Ring DC Distribution Network

With the development of power electronics technology,compared to the AC power distribution network,the DC power distribution network has tremendous potential for its obvious advantage in many ways.Due to the ring DC distribution network has more advabtages,such as the power supply reliability is higher and the recovery time is shorter.This thesis takes the ring DC power distribution for urban power supply as the research object.By means of theoretical analysis and simulation with PSCAD/EMTDC software,the key equipments of the system and control strategy are analyzed systematicly,the fault characteristics are studied,a new protection scheme is proposed at last.First,the main circuits and control strategies of the key equipments,such as MMC,VSC,dc transformer and photovoltaic cell,were analyzed.The ring DC power distribution model was built in the PSCAD simulation software.Second,due to various faults will be occurred in DC distribution networks,and the fault characteristics are complex and closely related to the grounding mode.Under three grounding modes,fault characteristics of pole-to-ground fault on MVDC lines and phase-to-ground fault on the AC side of the converter were analyzed.The advantage and disadvantage of different grounding modes were discussed.The conclusion provides a reference for the selection of the grounding mode of the DC distribution network and protection scheme.Third,With regard to MVDC lines in a ring multi-terminal ring DC distribution network.a fault identification approach based on the voltage of the additional inductor was proposed.The calculation method of the initial voltage across the additional inductor after pole-to-ground and pole-to-pole faults,was elaborated based on modulus networks of the system.And the difference of the initial voltage across the additional inductor between internal and external faults was used to realize the fault identifi cation.Furthermore,the fault pole is discriminated based on the difference between the initial voltage across the additional inductor of the faulted pole and that of the normal pole.Fourth,a back-up protection was proposed.The DC distribution network has a high incidence of pole-to-ground faults with a large range of transition resistance,which affects the identification of fault characteristics.To improve the reliability of the protection action when the line is grounded through a large resistance,the model similarity comparison based back-up protection is constrcted.Based on the zero-mode network of the system,the regional fault model and the out-of-zone fault model are established respectively.Furthermore,the Hausdorff distance algorithm is used to compare the similarity between the sampled signal at any time and the calculated value obtained by using the regional fault model and out-of-zone fault model to realize the rapid identification and removal of the fault.Finally,the main research contents and research results were summerized.