High Efficiency And High Power Density Energy Router For Submarine Observation Network

The ocean is the cradle and home of mankind.It affects the global climate and supports the energy security of the world.China is a big maritime country with vast sea areas and long coastlines,and since 2012,“building maritime power” has become a major national strategy.Exploring the ocean is a prerequisite for its exploitation and utilization.The cabled submarine observation network is a new generation of major infrastructure for learning about the ocean,which enables long-term,uninterrupted and intensive detection of the ocean.In the submarine observation network,the energy routers that connect the high-voltage cable and the low-voltage detection equipment are the “power hearts” of the submarine observation network.The submarine energy routers are of vital importance to ensure the continuous and stable operation of the submarine observation network.However,high-power high-voltage energy routers work in the extreme and complex environments of the deep ocean and face some essential technological challenges,such as conversion with wide input voltage,voltage balancing of multiple submodules,and thermal stress reducing of devices.In recent years,modular multilevel converters have become the preferred technical solution for high-voltage and high-power energy routers due to their high modularity,high extendibility,and flexible regulation.Therefore,this thesis proposes a modular multilevel resonant dc-dc converter for submarine energy routers and focuses on the crucial control of largepower and high-power-density energy routers.There are three main aspects in the research:voltage regulation strategy based on multi-degree-of-freedom,submodule capacitor voltage balancing method for two working stages,and device-level thermal imbalance suppression.Furthermore,a high-voltage,high-power submarine energy router is designed and developed.The main contributions of this thesis are summarized in the following four aspects:Firstly,a high-efficiency control strategy for wide input range combining both switching frequency and inserted submodular number is proposed.Based on the working principle and modulation scheme in the modular multilevel resonant converter,the analytical model of switching frequency and inserted submodule number is established,and then a control method for wide input range is proposed.In this method,the decoupled architectures of feedforward and feedback control loops are developed,and the optimal design principle for circuit parameters and control parameters are analyzed.Consequently,the switching frequency range,as well as the voltage stress on the transformer and arm inductors,is reduced compared with only frequency regulation.The soft switching of the submodule devices in all working conditions is guaranteed Hence,the efficiency of the submarine energy router with a wide input range can be improved.Secondly,the capacitor self-balancing control in the pre-charge stage and the voltage ripple suppression algorithm during steady operation are presented.The oscillatory of the cascaded submodule capacitors during the pre-charge stage due to the negative-impedance auxiliary power supply is investigated.Then a balancing circuit and control method are proposed to solve this problem.This method employs a low-voltage switch and a power compensation resistor after the auxiliary power supply for adjusting the submodular capacitor current,which rebuilds a positiveimpedance characteristic during the pre-charge stage.Besides,the control delay leads to large capacitor voltage ripples and even unbalanced in the steady operation stage.A supervised submodule voltage balancing algorithm considering control delay,is proposed,which could avoid the submodule capacitor being continuously charged for multiple cycles.Hence,the reliability of the large power submarine energy router can be realized enhanced.Thirdly,the loss model for the switching device is established,and a loss reshaping strategy based on modulation wave optimization is investigated.The power device conduction losses and switching losses are calculated.The unbalanced thermal stress on power devices under heavy load conditions is revealed,and the dc voltage modulation ratio is used to establish the corresponding relationship between the degree of thermal unbalance and the voltage modulation ratio.In order to suppress the thermal stress difference between the upper and lower devices of the half-bridge submodule,the optimal modulation wave is proposed,which can reshape the current distribution between the upper and lower devices.Meanwhile,the proposed modulation method reduces the capacitor voltage,thereby reducing the total switching loss and achieving more balanced thermal stress at the device level.The lifetime of the submarine energy router can be promoted.Finally,the first 15 kV@40kW submarine energy router is designed and developed.Aiming at the key requirements of a new generation energy router with large power,high efficiency and high power density for submarine observation networks,the optimized design of the power circuit,control system,durable structure and heat dissipation has been studied and a series of tests have been carried out.The energy router is capable of 40 kW operation with a wide input from 8kV to15 kV.The capacity is three times larger than the existing equipments.Its efficiency can reach96.4%,an increment of 6%.Its power density is about 385W/L,improved by 3 times.The research work of this thesis provide an essential theoretical foundation and experimental support for the practical implementation of the high-efficiency and high-power-density submarine energy router.