| Compared with the traditional Medium Voltage Alternative Current collection scheme for renewable energy sources,the Medium Voltage Direct Current(MVDC)collection scheme can avoid the bulky line frequency transformers and has no stability or multi-resonant problems of multiple parallel-connected inverters,which has attracted a lot of attentions.The high-power DC/DC converter suitable for MVDC collection system for renewable energy sources is studied in this dissertation,including the topology,the control strategy,the soft-switching technology,the magnetic flux density(MFD)analysis,and the device development.The research work of this dissertation consists of three main parts.The first part includes Chapters 2 to 4,where the topology innovation,the control strategy,and the zero-current-switching(ZCS)implementation of high-power DC/DC converter suitable for MVDC collection system for renewable energy sources are studied.Based on the component-sharing thought,an auxiliary-and-main-current-diversion(AMCD)ZCS full-bridge converter is proposed in Chapter 2.By sharing a half bridge,only six switches(IGBTs)and two transformers are needed to form the main and auxiliary full-bridge cells,where the former operates at 50% fixed duty cycle while the latter adopts pulse width modulation.As a result,the proposed converter can operate in the discontinuous current mode(DCM),achieving ZCS turn-on and-off for the main switches and all rectifier diodes among the full load range along with switching loss effectively reduced.In addition,the AMCD thought is raised,and its key content is to realize ZCS for the main cell which transfers most of the power by the small current turn-off of the auxiliary cell which only transfers a small part of the power.However,the current waveforms of the proposed converter in Chapter 2 are triangular with high peak value.In Chapter 3,hence,the LC series resonance technology is introduced into the AMCD thought and an AMCD resonant ZCS full-bridge converter is proposed with both the peak current and the small turn-off current effectively reduced,leading to lower current stress and switching loss.The influence of transformer turns ratio and resonance parameters on peak and turn-off currents is analyzed in detail and the parameter optimization is given.With the same converter parameters,the peak current can be reduced by at least 19% and the turn-off current can be reduced by more than 50% in Chapter 3.In Chapter 4,furthermore,by improving the auxiliary cell or/and employing traditional neutral-point-clamped three-level circuit as the main cell,the voltage stress of the auxiliary or/and main switches is successfully reduced to half,and three kinds of low voltage stress AMCD resonant ZCS DC/DC converters are obtained.The second part,namely Chapter 5,analyzes the MFD of high-power DCM-SRC.Since the series resonant converter(SRC)operating in DCM(DCM-SRC)can realize ZCS for all switches and has inherent short-circuit protection on the output side,it is suitable for MVDC collection as well.To avoid the saturation problem of high-power high-frequency transformer in DCM-SRC,It is found that there are two different operation cases with different MFDs when the traditional pulse frequency modulation(PFM)with constant on-time is adopted for the high-power DCM-SRC.The respective MFD expressions of the two cases are derived,as well as the boundary condition and the fundamental cause of them.To avoid the case with higher MFD,a turns ratio design rule for the high-power high-frequency transformer is proposed from the perspective of optimization parameter design,and its feasibility is verified.And from the perspective of improving the control strategy,an asymmetrical PFM with constant on-time is proposed with ZCS realized.Except for getting rid of the higher MFD case,the peak current of all switches is reduced by at least 50%.The third part,namely Chapter 6,has developed a ±35k V/250 k W DCM-SRC device to fully demonstrate the feasibility of applying high-power DCM-SRC to MVDC collection system for renewable energy sources.The traditional PFM with constant on-time is adopted for the DCM-SRC with the turns ratio design rule,and the design of key parameters and device selection is conducted,as well as the water-cooling scheme for high-power IGBTs.During the device development process,the high-power high-frequency transformer fails due to the high-voltage creepage phenomenon.Hence,the bobbin structure of the high-voltage side is optimized with the creepage distance significantly increased,and a 250 k W high-frequency transformer is successfully developed.Finally,a ±35k V/250 k W DCM-SRC device is built,and the test experiments is completed,including the temperature rise test of high-power IGBTs and the performance test of 250 k W high-frequency transformer. |
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