Research On Direct-coupled DC Transfromer Applied In Hvdc Grid

Multi-terminal high voltage dc(HVDC)transmission technology and dc grid technology are efficient methods to resolve the problem of renewable energy integration,and the dc transformer is the key equipment to interconnect the apart HVDC systems with different voltage levels.As the voltage level of the HVDC system is typically hundreds of kilovolts,series connection of hundreds of power switches is required if the classic dc/dc converters(e.g.,buck converter)developed for low or medium voltage ranges are applied.However,the additional snubber components and quite complex gate drive unit are required,and the series connection of power switches produces extremely high dv/dt stress,which results in the serious electromagnetic interference(EMI).Furthermore,the switching speed of high voltage power switches is not expected to exceed 1 k Hz,as a result,the inductance and capacitance of the classic dc/dc converter is quite large.However,the large inductor and capacitor at ultra-high voltage level lead to bulk volume,and their manufactures are difficult so far.This paper proposes the systhesis methodology of direct coupled dc transformers(DCDTs)by replacing the switches and passive devices in classic dc/dc converters with cascaded SMs(abbreviated as chain-links),so that the dc/dc converters are no longer restricted by the manufacture of high-voltage power switches and passive devices.The main research contents are summarized as follows:1)The synthesis methodology of DCDTs,i.e.,replacing the the switches and passive devices in classic dc/dc converters with chain-links,is proposed,based on which a family of DCDTs can be derived.2)The stepped 2-level operation of Buck-DCDT is proposed.The existing sin-wave modulation needs high-frequency sorting of hundreds of SM capacitor voltages to ensure their balance,which results in large computation burden.In addition,the switching frequency is higher than the fundamental frequency,so that the power loss is increased.Compared with the sin-wave modulation,the stepped 2-level operation offers the merits of smaller computation burden,smaller ac circulating current and higher efficiency.Furthermore,the simulation and experimental results verify the theoretical analysis.3)The buck three-level type DCDT(Buck-TL-DCDT)is proposed.The buck-DCDT requires large filter inductor to suppress the lower voltage side current ripple.The manufacture of high voltage inductor is difficult now,and cost and volume of converter will be increased.Thanks to the three-level operation,the filter inductance of the Buck-TLDCDT is dramatically reduced,so that the system volume is reduced.4)The three-port DCDT(TP-DCDT)is proposed.The exsiting high voltage dc/dc converters,including dc autotransformer(DC AUTO)and LCL dc/dc converter,requires multiple dc/ac transmission or power-frequency ac transformer,which results in high cost and low efficiency.The proposed TP-DCDT gets rid of the ac transformer and multiple dc/ac transformer,so that the converter cost and efficiency can be improved.5)The operation and control the DCDT under dc fault is analyzed.The distribution of fault current in the DCDT is discussed,the submodule with dc fault blocking capability is proposed,and the dc fault fast recovery strategy of the DCDT is also provided.Furthermore,the simulation results verify the effectiveness of the fault recovery strategy.6)The coordination controls of the offshore wind power integration dc system based on dc transformer,including master-slave control,dc voltage margin control and dc voltage droop control,are discussed.