Research On Short Circuit Fault Protection Of Ship Medium Voltage DC Power Grid

The ship’s Medium Voltage DC(MVDC)system has the advantages of improving prime mover efficiency,reducing fuel costs,saving weight and space,enabling generators to operate at a uniform power,easier to operate in parallel,and easy to access energy storage.With the development of power semiconductor technology and the increase of ship power density,medium voltage DC grid has become the development direction of ship power systems in the future.When a low-impedance short-circuit fault occurs in the ship’s medium-voltage DC power grid,the current increases rapidly and there is no zero-crossing point,and the fault area needs to be quickly removed.In order to ensure the safe operation of the ship’s MVDC system,a mature and effective fault protection scheme needs to be considered.First,in order to accurately reflect the short-circuit fault characteristics of the ship’s medium-voltage DC power grid,a multi-zone ship’s medium-voltage DC power grid was modeled.Among them,the power generation module adopts a modular multilevel converter(Modular Multilevel Converter,MMC)and considers the power distribution strategy among multiple MMCs.Among them,the MMC has the characteristics of being able to completely isolate the fault current and quickly restart.Finally,a multi-zone ship medium-voltage DC power grid model was built in PSCAD/EMTDC,and compared with the existing literature to verify the accuracy of the model.Secondly,aiming at the problem that it is difficult to isolate the short-circuit fault current of the ship’s medium-voltage DC grid,a coordinated isolation strategy of full-bridge MMC and hybrid DC circuit breakers is designed.Firstly,the theoretical analysis and simulation verification of the short-circuit fault between the poles are carried out;secondly,the fault isolation capability of the full-bridge MMC and the hybrid circuit breaker is analyzed and verified,and finally the coordinated isolation strategy of the full-bridge MMC and hybrid circuit breaker is verified.Finally,in view of the problem that it is difficult to accurately detect and locate the short-circuit fault of the DC power grid of regional power distribution ships,a detection and location scheme that can be applied to the short-circuit fault of the medium-voltage DC power grid of ships is designed.First,the transient analysis of the short-circuit fault of the DC power grid is carried out;secondly,the characteristics of the line fault current under different fault transition resistance,operation modes and fault locations are considered;finally,the short-time Fourier algorithm and the partition location algorithm are used to analyze The line fault current processing can quickly realize the detection and location of the ship’s medium-voltage DC grid short-circuit fault,and only needs to use low-bandwidth communication to complete it,with few parameters transmitted,and no need to consider the problem of difficult selection of the setting threshold.