| The cabled subsea observatory is the ’pillars of great power’ on marine resources development and marine defense construction.Subsea observatory negative polarity constant voltage power supply system consists of power feeding equipment,Junction box converter,optic-electric composite submarine cable,etc.,and has the characteristics of power electronification.The research on fault mechanism and electrical characteristics for constant voltage subsea observatory power supply system is not detailed enough.Existing protection schemes have problems such as the fault segment selectivity is insufficient,breaking fault slowly,and damaging submarine cable insulation.As the ’first line of defense’,the protection scheme of subsea observatory negative polarity constant voltage power supply system is studied.The main research contents of this paper are as follows:(1)Taking double-end chain networking subsea observatory as an example,an equivalent model of subsea observatory negative polarity constant voltage power supply system is researched.First,the working principle of subsea observatory negative polarity constant voltage power supply system is expounded,and the topology of double-end chain networking subsea observatory negative polarity constant voltage power supply system is proposed.Secondly,the FB-MMC and the synchronous rectifier converter based on module stack,respectively,are analyzed.The topology structure and control characteristic of FB-MMC and synchronous rectifier converter are expounded.Based on structural simplification and logical merging,an equivalent model of branch unit of constant voltage subsea observatory is established.Based on the principle that the same material can be merged and similar properties can be equivalent to each other,the single-layer armored optic-electric composite submarine cable is equivalently simplified to a DC cable.The relative permeability and resistivity of optic-electric composite submarine cable’s equivalent core are deduced,and the frequency-dependent parameter model of optic-electric composite submarine cable is established.Because the junction box converter model has low-voltage exit characteristics,the branch unit model has the power supply circuit structure,and the optic-electric composite submarine cable model has the ability to respond to the frequency-dependent effect of electrical quantities,the equivalent model of this system is highly accurate.It is suitable for fault characteristic analysis and protection threshold setting.(2)A protection scheme for constant voltage subsea observatory power supply system based on coordination of control and protection is proposed.Firstly,based on system characteristics analysis and protection principle applicability analysis,the feasibility and necessity of protection scheme based on power feeding equipment current-limiting control are studied.Secondly,the current-limiting control startup criterion is established based on current rate of change,and the characteristics of voltage and current direction under the system current-limiting mode are further analyzed.Thirdly,based on the system fault characteristics,the protection start criterion,spur cable fault criterion,backbone cable fault section criterion,backbone cable fault criterion,and protection termination criterion are established.Finally,the protection scheme flow of power feeding equipment currentlimiting control and protection coordination is established.This protection scheme does not require voltage-reversal control and optical fiber communication.This protection scheme uses power feeding equipment current-limiting control to suppress the short-circuit current,makes up for the insufficient breaking capacity of the branch unit relay switch,realizes the fault discrimination using the fault voltage amplitude characteristics and current direction characteristics,which has high reliability and selectivity.(3)The transient simulation model of subsea observatory typical negative constant voltage power supply system is built,and the simulation verification and analysis of the proposed control and protection cooperative protection scheme are carried out.Firstly,the transient simulation model of subsea observatory typical negative constant voltage power supply system is built on PSCAD/EMTDC.Secondly,the characteristics of power feeding equipment current-limiting control,short-circuit fault characteristics of backbone cables,and short-circuit fault characteristics of spur cables have been verified.Based on the electromagnetic simulation model,the protection scheme and the adaptability of its influencing factors are verified.Simulation results show that the protection scheme has good performance of enduring transition resistance,noise and fluctuation of load,can effectively suppress fault current,and realize rapid identification and isolation of submarine cable faults in each section.Thirdly,compared with the existing typical protection schemes,the innovation and advantages of protection scheme in this paper were expounded. |
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