The Key Issues Of Wave Energy Farms Based On DC Nano Grid In Islands

Because of the low wave energy density of China Sea, the wave energy power generation cost is high, and the first application of wave energy converter(WEC) must be islands and marine observation instruments and so on, where the electricity cost is high too. Because of the lack of land resources on offshore, the first choice of the energy solution must be ocean energy, then the other choices are solar energy, wind energy and biomass energy et al. Therefore, the power supply of the offshore isolated users and wave energy converters are the two-way selections.Considering the poor of wave energy resource, the dificult of ocean engineering and the difficult of operating maintenance et al, the output power of one WEC is very limited. In order to meet the requirements of the increasingly electricity demand of isolated users offshore, to establish wave energy farm by more WECs is one of the valid solutions. The wave moves back and forth in long period, therefore the output power of WEC is fluctuant. The micro grid of offshore isolated user cannot support big power fluctuant. Therefore, the interconnection of WECs, the stable power supply of wave energy farm, the power generation system characteristic of WEC, the power converter and its control strategy, and the monitoring technology of WECs et al, are the key issues to be solved immediately before wave energy farm establishing.The characteristics of the power generation system of WECs are the basis theory to research the interconnecting of WECs and the power converting of WECs. Under the modeling, the reasoning, simulation and experiment et al, the hydraulic power generation system characteristics formulas of resistance load model and voltage load model are obtained, and the hydraulic power generation system(HPGS) with the “current source” characteristic is analyzed, and a DC nano grid technology based on “current source” characteristic is proposed. The DC nano grid is a smaller grid than micro grid, which is connected by DC bus and constituted by some parts of power generation devices in micro grid, energy storage devices and DC users. In the DC nano grid for wave energy farm, WECs supply DC electricity, the connection among WECs will be finished on offshore, the power transmission will be finished by one submarine cable, the batteries absorb the fluctuant power of WECs, and the nano inverter supply controllable AC power to micro grid. Furthermore, the DC nano grid technology cuts some of electrical converting links off, and improves the route efficiency of the delay consumed wave power above 10%.Before the establishing of the DC nano grid for WECs, there are mayn key issues should be researched. First of all, the rotate speed of HPGS is uncontrollable, and the devices selections of HPGS are not flexible, when the HPGS of WEC is directly connected to DC nano grid through an uncontrollable rectifier. An interleaved boost converter is provided to solve the problem, and the speed closed-loop control structure of HPGS is established, and the rotate speed of HPGS has been controlled. There is no maximum output power of HPGS because of the proportion characteristic between output power and rotate speed. Therefore, the control strategy of the power converter for WEC can’t be maximum power point tracking(MPPT) as the same of other renewable energy. The control strategy of maximum efficiency converting for the power converter for WEC is proposed. The optimum pressure——speed curve of HPGS is obtained from experiment training method, and the interleaved boost converter control HPGS work on the optimum pressure——speed curve in every whole power generating cycle. Furthermore, aiming at the defect of phase control rectifier, the PWM high frequency rectifier is proposed. The mathematical model of the PWM high frequency rectifier of HPGS, the control structure of speed closed-outer-loop and stator current closed-inner-loop of HPGS are established. The power converter realized flexible rectifying and maximum efficiency converting simultaneously. Finally, aiming at the specific assignments and special requirements of WEC monitoring system, several WEC monitoring systems have been designed and realized by redundant design technology.The simulation results and the preliminary operating results of DWS DC nano grid have proved that the power fluctuation in DC nano grid side would not affect the operating stability in AC micro grid side, and the fault in AC micro grid side also would not affect the power generation of renewable energy devices in DC nano grid side, which is very important for WECs, because that WECs can’t work without load. The DC nano grid has solved the difficult of the interconnection and the power supply stability of WECs in micro grid, synchronously, has enhanced the reliability of the power electronic converter of WEC. The simulation results proves that the interleaved boost converter and PWM high frequency rectifier all have realized maximum efficiency converting control strategy, and the PWM high frequency rectifier have realized flexible rectifying for HPGS. The operating results of the monitoring systems of several WECs prove that the WECs monitoring systems designed by redundant technology have many advantages as high pertinence, well designed, comprehensive functions and reliable performance.Therefore, the key issues research of WEC and the results are very importance to establish a DC nano grid for WECs. And it is very significant to improve the total efficiency of wave energy converting, to improve the technology maturation of WEC and to enlarge the application of wave energy.