| It is very important for the safe and reliable operation of power grid to cut off faults quickly and selectively.In recent years,with the rapid growth of photovoltaic installed capacity in China,distributed photovoltaic access is facing a series of challenges,such as insufficient distribution network capacity,serious harmonic pollution,outstanding security and stability problems.Based on the advantages of flexible and efficient power conversion,large power supply radius and high power quality,DC multi-port access based on medium and low voltage flexible DC power distribution systems will become an important form of high penetration and large-scale distribution photovoltaic development and utilization in China in the future.However,in the process of flexible DC fault,there are mainly transients which cannot be applied to the traditional AC analysis methods.At the same time,the frequency domain response characteristics of the power electronic device under the transient fault signal are unknown,and it is difficult to establish a transient impedance model,and the protection lacks the theoretical basis.In this paper,from the perspective of high frequency transient information,the frequency domain impedance models of converters and DC systems are studied,and the protection research based on fault transients is carried out accordingly.In this paper,the fault current characteristics of typical converters AC/DC and DC/DC in multi-terminal flexible DC distribution system are analyzed,and the stable high frequency transient information is obtained by time-frequency transformation.On this basis,the fault point is used as a high-frequency source,and the internal high-frequency current circuits of the converters are analyzed.After equivalent simplification,the frequency domain impedance models of the typical converters are obtained,which provides the basis for the protection principle.By analyzing the impedance characteristics of different frequency bands,it is proved that the model established in the higher frequency band can not be affected by the converter control strategy,provide stable impedance value,and ensure the correct operation performance of the protection principle.At the same time,for the multi converter system,the characteristics of system level high-frequency impedance can be superposed are given.It is proved that the proposed impedance model is not only applicable to a single converter,but also can be applied to the multi terminal DC distribution system,and is not affected by the coupling of converter fault control.On this basis,a high frequency distance protection method based on fault transient characteristics is proposed.In this method,the fault area is mapped by the high frequency components of fault voltage and current,and the fault area is identified by combining the setting and cooperation of protection settings.Finally,based on the current development of flexible DC power distribution engineering topology,a detailed model of six terminal flexible DC power distribution system with high proportion photovoltaic access is built on the PSCAD/EMTDC electromagnetic transient simulation platform,and the impedance model and protection method proposed are verified respectively.The simulation results show that the proposed modeling method has good applicability and accuracy,is not affected by the control strategy,and can provide a good theoretical basis for the follow-up protection.At the same time,the proposed protection method has the advantages of simple principle,small amount of calculation,good rapidity and selectivity,reliable identification of fault areas,and can tolerate certain transition resistance and noise. |
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