| Isolated users such as remote maritime islands,large offshore instruments,and intelligent far-reaching Marine aquaculture platform,are difficult to fully utilize solar energy or other renewable energy sources due to their special geographical location.Therefore,wave energy can be regarded as the preferred solution to the problem of offshore energy.The research on efficient wave energy conversion and DC grid-connected transmission technology is very beneficial to solve the problem of inconvenient power supply for isolated users at sea.Compared with developed countries,the research progress and application in China lag behind,and the wave energy current density in China is low,which will also increase the cost of wave energy power generation.In this paper,the aim is to realize the stable voltage output of the hydraulically driven wave energy generating device,improve the conversion efficiency and realize the confluency and transmission connection of wave energy device group.Combined with the characteristics of wave energy,such as rapidity and fluctuation,the off-grid operation and grid-connected operation of the device are centered.The research focuses on system modeling,control strategy formulation and simulation analysis,etc.A more perfect system modeling,efficient conversion control method and grid-connected transmission system are proposed.The main contents are as follows:The research background of wave energy,the current situation of industrial application at home and abroad and the research status of the subject are introduced.The research content is put forward.The wave mathematical model was established by using the Bretschneider wave spectrum as the input of the system.The mathematical model of each component of the hydraulic PTO is established,including the hydraulic cylinder mathematical model,the hydraulic valve mathematical model,the accumulator mathematical model and the hydraulic motor mathematical model considering the friction and inertia effect.The mathematical model of permanent magnet synchronous motor(PM SG)and the PWM converter model are established in D-Q coordinate system,and the modeling process of the power conversion and voltage stabilized output system is completed.A PMSG compound control strategy is proposed in combination with the power conversion voltage stabilizing system of the device,and the low magnetic speed regulation of PMSG is realized.Based on the research of the hydraulic PTO,the W2W Simulink simulation model of the wave energy device is established.In the wavelet state,the simulation results of the corresponding wave state are obtained for the two cases with and without the weak magnetic field control.The control effect is verified by the analysis and calculation of PMSG current,rotational speed and each power of the system.A hybrid AC/DC power transmission system for wave energy devices is proposed.Based on the modular multi-level converter(MMC),the flexible direct current transmission(MMC-HVDC)system of wave energy device group was developed,the internal topology of the system was determined,and the mathematical model of MMC was established.A compound control strategy of the system under the island operation mode is proposed,and the design of each controller is completed.Combined with the project,the transmission system parameters were calculated,and the Matlab/Simulink simulation model was established to simulate two kinds of power grid collapse faults.The operating results of the system show that the system can effectively compensate the transient voltage drop,and has the power quality regulation capabilities such as low harmonic distortion,reactive power compensation and cyclic current suppression. |
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