Protection Strategy Method Of Dc Distribution Network Based On MMC

With the development of new energy sources and the emergence of a large number of DC loads,the DC distribution network based on modular multilevel converters has received extensive attention due to its advantages such as good output waveforms and strong flexibility and controllability.Due to the DC distribution network has the characteristics of fast fault current rise speed and weak overcurrent tolerance of the converter equipment,relay protection has become the key to the safe and reliable operation of the DC distribution network,and it is also one of the main problems to be solved urgently.In view of the above problems,this paper selects the MMC based double-terminal DC distribution network to study its inter-pole short circuit and single-pole ground fault characteristics and protection methods.This paper analyzes the different characteristics of different network topologies of DC distribution networks in terms of power supply reliability,fault identification and construction costs.By establishing an equivalent mathematical model,the operation principle and modulation control of MMC are studied.The simulation model of DC distribution network was built on MALAB simulation software,and the fault characteristics of DC distribution network with inter-pole short circuit and single pole ground short circuit are analyzed,and the change characteristics of relevant components suitable for protection are obtained.Aiming at the problem that the simple half-bridge submodule cannot block the short-circuit current when the DC side inter-pole short circuit fault occurs,a new sub-module topology with fault isolation capability through self-locking is studied.The working principle of the MMC based on the improved submodule and the current blocking ability in the event of a fault are analyzed,and the improved submodule proposed is compared with other self-clearing submodules in economics.The protection and control strategy for inter-pole short of MMC double-terminal DC distribution network based on improved sub-modules is designed.By constructing the MMC simulation model based on the improved sub-module,its short circuit current blocking capability was verified and analyzed.In view of the fact that the fault current characteristics of MMC DC distribution network are not obvious when single-pole grounding,it is often difficult to accurately identify faulty lines.Utilizing the difference between the transient characteristics of the positive and negative current of the faulty line and non-faulty line,a single-pole grounding fault line identification method based on the correlation analysis of positive and negative currents is proposed.The working principle,implementation method,workflow and influencing factors of this method are analyzed and demonstrated in detail.Through the establishment of related simulation models,it is verified that the effectiveness of this method under transition resistances of different fault locations and different fault points.At last,a single-pole grounding protection device for the DC distribution network is designed,which is used for the installation,debugging and experiment of the DC distribution network system on site.