| Submarine scientific network and national defense equipment obtain high-voltage power supply through shore based power stations,and use the connection box to realize medium and low voltage conversion and high bandwidth data transmission,so as to achieve the purpose of real-time,continuous,long-term and reliable submarine observation.At present,it is one of the most effective and advanced important means for marine scientific research and maintaining national defense security in sea areas.With the expansion of the scale of submarine scientific network,there are urgent needs for long-distance high-power power power transmission on the seabed,such as high reliable operation,wide input working range,high-efficiency transformation and selfstarting power supply.Therefore,it is of great practical significance to study the topology of submarine medium voltage DC converter and other related basic theories and key technical problems,so as to realize the high efficiency,high reliability and high stability of submarine long-distance power transmission.Considering the problems of complex construction,large line impedance voltage drop and difficult maintenance of submarine medium voltage DC converter system,the modular combined DC converter with the characteristics of modular structure,wide input voltage range and easy to realize redundant switching operation is one of the effective ways to realize submarine remote power transmission.Aiming at the key technology and application of seabed modular combined DC converter,this paper deeply studies the topology,control method,power unit bypass redundant switch and self powered auxiliary power supply method of seabed modular combined DC converter based on the innovation team project of Guangdong Province " Research,development and industrialization of key technologies of special power conversion equipment for offshore engineering".The research focus and useful conclusions are mainly reflected in the following aspects:(1)Aiming at the requirements of medium voltage DC-DC converter with high reliability,high efficiency and wide input range for submarine scientific network and national defense equipment,a submarine modular combined DC-DC converter based on Buck,LLC integration is proposed.Through the high isolation resonant power unit,the input is connected in series to provide high voltage withstand capacity,and the output is connected in parallel to meet the demand of high power.The proposed power unit topology adopts the two-stage integrated transformation mode of series double buck and LLC resonance,so as to adjust the duty cycle of series double buck circuit and meet the requirements of wide input voltage working range.By introducing auxiliary circuit,the zero-voltage-switching(ZVS)problem of all switches in wide input range is solved and the energy transmission efficiency is improved.Based on the analysis of the working principle of the power unit,the DC gain characteristics,power characteristics and ZVS characteristics of the power unit are deduced,the boundary conditions for each switch to realize ZVS are studied,and the auxiliary circuit and control parameters are optimized.Secondly,in order to realize the power balance of each power unit in the modular combined DC converter,the double loop decoupling control strategy of output voltage loop and input voltage equalizing loop is studied to reduce the coupling characteristics between the double loop control parameters.Finally,the prototype experimental results verify the correctness and feasibility of the proposed power unit topology and its control strategy.(2)With the continuous development of submarine science network acoustic instruments and national defense equipment,the required power supply voltage and transmission power level have also been improved.For the demand of submarine remote power supply above 100 k W level,a resonant modular multilevel converter based on asymmetrical pulse-width-modulation(APWMRMMC)is proposed in this paper.It meets the demand of wide input voltage working range by adjusting the switching frequency.The use of a single isolation transformer to achieve high and low voltage zoning is conducive to voltage insulation treatment in a narrow space in submarine applications.It has the characteristics of high power density,easy redundant operation and stable operation.The topology consists of modular multilevel circuit,resonant circuit and passive auxiliary circuit.Compared with the traditional modular multilevel circuit,it can realize the zero-voltage turn-on of all switches in a wide input voltage range.Specifically,by introducing the passive auxiliary circuit,the asymmetric triangular compensation current is provided to flow into the bridge arm,which offsets the circulating current bias caused by the modular multilevel structure in the lower switch of the submodule,so as to realize the ZVS of the switch and improve the power transmission efficiency.Secondly,the bridge arm series structure combined with asymmetric pulse width modulation has a natural high step-down ratio and frequency doubling output.At the same time,the feedforward control is introduced to adjust the number of constant input sub modules in the bridge arm,which solves the hardware design problem caused by too wide switching frequency adjustment range in a wide working range.Based on the analysis of the working principle of the converter and the derivation of the steady-state performance such as DC gain characteristics and soft switching characteristics,the feasibility and effectiveness of the proposed converter are verified by simulation and prototype experiments.(3)According to the fault isolation requirements of submarine medium voltage DC conversion system,a normally on staggered series solid state bypass switch(SSBS)is proposed to realize the rapid isolation of fault units and the effective bypass after power failure.The proposed topology interleaves multiple inner cells in series to improve the overall voltage withstand capacity,and each inner cell is also composed of multiple normally on switches in series.The circuit topology has strong scalability.At the same time,the control scheme of driving the series topology with a single external driving signal is adopted,and the fast response and dynamic and static voltage balance of the series switch are effectively realized through the internal and external double-layer voltage sharing network.Secondly,turn-off and turn-on process of series Si C JFET is described in detail.Based on this,the effects of voltage sharing network device deviation and circuit stray parameters on the dynamic and static performance of solid-state switch are analyzed.Finally,a 1.5k V/10 A solid-state bypass switch applied to the seabed medium voltage DC conversion scene is designed.The feasibility of the proposed solid-state bypass switch is verified by the prototype experimental results.(4)In view of the difficulties and requirements faced by the auxiliary power supply of seabed remote power supply,such as cross high voltage energy collection,mutual isolation of multiple load terminals,no impact on the internal voltage sharing control and bypass redundancy control of the system,an auxiliary power supply circuit and its control strategy based on self return supply are proposed.The auxiliary power supply circuit is composed of charging circuit,starting power supply and return power supply.Firstly,the super capacitor in the charging circuit is charged by using the constant current and voltage limiting operation mode of shore based power supply,and the power supply of the control system is completed by starting power supply;After the self inspection of the control system is completed,disconnect the starting circuit and the shore based power supply begins to boost;After the output voltage reaches the threshold,the self power supply is realized.Secondly,compared with the traditional DC side auxiliary power supply method,it provides technical support for the auxiliary power supply design in the application of submarine remote power supply engineering.(5)An engineering prototype of 6～10k V / 400 V,50k W undersea power supply is developed.The design principles and methods of main circuit parameters including high-frequency transformer,power switches,resonator device,auxiliary circuit device,rectifier circuit and driving circuit are described in detail.At the same time,the overall framework of the control system and some software response processes are described in detail.Through the 10 k V experiment of the engineering prototype,the engineering feasibility of the power unit structure and control strategy,bypass redundant switch design and auxiliary power supply method proposed in this paper are verified. |
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