| Offshore wind farm low frequency AC transmission can increase power transfer capability according to decrease transmission frequency.There is no need for offshore converter station compared with HVDC,which can save investment and operational costs greatly.LFAC technologies will be a trend in offshore wind farm connecting to grid area.MMC is suitable for LFAC transmission for its highly modularity,low harmonic ratio and low switching frequency.However,there are some problems when MMC is used in LFAC,such as bridge arm circulating current growing,submodular power loss increasing and loss calculation inaccuracy,which affects the MMC's operation reliability.To further improve the reliability of MMC used for offshore wind farm LFAC transmission system focusing on the power loss calculation of MMC and circulating current growing problem,the improved power loss calculation method of MMC and circulating current suppression strategy were studied in this paper.The main contents of this thesis are followings:In view of the MMC sub module capacitor voltage fluctuation aggravated affecting power quality of AC side in LFAC system,a balanced control stratgy of sub module was adopted to suppress capacitor voltage fluctuation.Firstly,operation theory is introduced combined with the topology of MMC and improved carrier phase-shifted modulation algorithm is used to modulate MMC.Secondly,a balanced control stratgy of submodular was adopted to suppress capacitor voltage fluctuation.Finally,21 levels MMC LFAC simulation model is established to verify the effectiveness of the balanced control stratgy of submodular.In view of the MMC circulating current growing under LFAC condition resulting in power loss calculation inaccuracy,an improved MMC power loss calculation method is proposed considering circulating current.Firstly,the phenomenon of MMC losses nonuniform distributions is analysed in theory.Secondly,based on the probability of the inserted sub module,average and effective values of current flowing through the four switching devices are deduced.Then MMC junction temperature can be calculated based on equivalent thermal network of IGBT modules.Finally,21 level FAC MMC simulated model is established based on PLECS/Simulink platform to verify the calculated value.As with the application of conventional module multilevel converter(MMC)in low frequency alternating current transmission(LFAC),circulating current growing may be occurred,a new MMC circulating current suppression strategy is proposed based on Virtual impedance.First,the relationship expression of 2nd and 4th circulating current amplitude with impedance and frequency is derived in this paper by analyzing circulating current generation mechanism.Then,a suppression strategy of circulating current based on virtual impedance is proposed to suppress 2nd and 4th circulating current amplitude.Finally,21 level LFAC MMC simulated model is established to verify the effectiveness of the proposed strategy compared with traditional PI circulating current suppressing method based on coordinate transformation.In order to analyse the performance of offshore wind farm LFAC transmission system with MMC,firstly,directly driven wind turbine with permanent magnet synchronous generators control model is built.Secondly,MMC decoupled control strategy of active power and reactive power is proposed to achieve active power and reactive power decoupling control and DC bus voltage stable.Finally,B2B-MMC offshore wind farm LFAC transmission system connected to grid simulation model is established to analyse the performance of offshore wind farm LFAC transmission system.Improved power loss calculation method established in this paper can provide support for the accurate calculation and analysis of MMC loss.The proposed circulating current suppression strategies can provide technical support for the improvement of the control strategy of MMC used for LFAC,as well as the improvement of the reliable operation of MMC.The established B2B-MMC offshore wind farm LFAC transmission system connected to grid simulation model can provide technical reference for LFAC applications. |
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