Research On The Key Techniques Of Power Converters For The Small-scale Grid-Connected Wind Generation System

Small-scale wind generation is one of the most significant components of the distributed renewable energy generation.Also as the important complement for the power supply,it presents good nature of flexible installation,being close to loads and low cost.However,considerable numbers of the small-scale wind field belong to the low-quality wind farm,and thus features the low power density and fluctuant wind speed.In order to promote the utilization rate of wind power and to increase the gridtie energy production,it is inevitable for a small-scale wind generation system(SSWGS)to improve its adaptability to the small-scale wind field.In this dissertation,from the aspect of power electronic conversion technology,the primary objective is to develop advanced power converters with high efficiency and high adaptability.Here,a smallscale grid-tie wind generation system has been proposed.Its circuit configuration,control strategy and the topologies of several single-stage converters are analyzed in details.The major works are organized as below:(1)To satisfy the requirements of high efficiency,high voltage gain and wide gain range for a Boost type DC-DC converter,a topology-morphing multi-resonant softswitching DC-DC converter has been proposed for the grid-tie SSWGS.Based on the thought of topology-morphing and combining the multi-resonant technology,the converter is able to adjust the equivalent circuit of its resonant tank by controlling the on/off states of the auxiliary switches.The resonant tank has two working states.The inverter cell also can be operated as the half bridge or the full bridge.Consequently,the proposed converter possesses at most four working states.This characteristic ensures the converter harvest wide gain range and high efficiency within narrow operating frequency scope.The feasibility of the theoretical analysis is verified by the experiment.(2)In the wind generation system,to satisfy the requirements of high efficiency and wide applicability for loads with different voltages,a dual-transformer multiresonant soft-switching DC-DC converter is proposed.The introduction of the high frequency transformer brings in a unique resonant zero point.The converter not only can regulate the voltage gain flexibly within a narrow frequency range,but also has a very wide output voltage range and a good over-current protection.These features make it suitable for wide load voltages.A simple,practical and widely applicable parameter design method for the multi-resonant converter is also presented.As the design example,the resonant parameters of the proposed converter are selected,which guarantees all the switching devices soft-or quasi-soft-switching during all the turn-on and turn-off transitions.In the end,experiments on a prototype verify the reliability of the operating principle and the design method.(3)In order to pursue high voltage gain and relatively high efficiency for the gridtie system,a three-phase single-switch flyback AC/DC converter with switched capacitors is proposed.The converter integrates the switched capacitors and the flyback cell into the conventional three-phase single-switch rectifier.Thus,it not only maintains the inherent power factor correction and the harmonic suppression,but improves the voltage gain as well.The rectifier can be connected to the inverter directly without the middle-stage DC-DC converter,so the relevant power losses decrease further.By proper design,high voltage ratio is obtained at the low wind speed condition,and the voltages between converters can match with each other easily;meanwhile,relatively high efficiency and low total harmonic distortion(THD)are realized at the rated state.At last,according to the design,the experimental tests have been completed on the prototype.(4)In order to improve the utilization rate of the wind energy and to conduct the grid-tie generation as much as possible,the topology and control strategy of a grid-tie SSWGS are proposed.For the topology,the proposed three-phase high step-up rectifier is adopted here,and a bidirectional storage branch is introduced into the DC output of the rectifier.In consequence,this system becomes a storage-based wind generation cell.For the control strategy,the proposed control logic ensures the controller can invoke corresponding control method according to different wind speed and battery state-ofcharge.This helps to fulfill the advantageous effects,including battery life prolongation,wind energy utilization rate promotion,entire system efficiency improvement and exploitable wind speed range expansion.At last,experiments in both of the laboratory and the real wind farm are conducted and recorded,and the results show the proposed system presents high adaptability to the small-scale wind field.