Research On The Modular Multilevel Converter With SiC Devices And Its Modulation Scheme

Modular multilevel converter(MMC)has the advantages of flexible control,fast dynamic response,weak network connectivity,easy multi-terminal operation,etc.It has become the preferred solution in the field of DC transmission and distribution,and is an effective means of large-scale development and utilization of renewable energy in China.In the future,with the development and utilization of renewable energy,offshore photovoltaic,offshore wind power and other specific occasions have greater demands on the lightness of MMC.However,the traditional MMC generally uses silicon insulated gate bipolar transistor(Si IGBT)as the power device.Limited by the physical limit of Si material,the performance of Si IGBT is close to the theoretical upper limit,making it hard to reduce the number of MMC sub-modules and improve the switching frequency.This makes it difficult to reduce the number of MMC sub-modules and increase the switching frequency,thus constraining the reduction of the size of the device.In recent years,the development of wide band semiconductor devices represented by silicon carbide(SiC)has been rapid,among which silicon carbide metal oxide semiconductor field effect transistor(SiC MOSFETs)have higher blocking voltage,faster switching speed and lower switching losses than Si IGBTs.Replacing Si IGBTs in MMCs with SiC MOSFETs can help the MMC reduce device size,increase control bandwidth,and reduce switching losses.Therefore,this paper focuses on the application challenges of SiC MOSFETs in MMCs.Firstly,based on the basic half-bridge MMC structure,Si IGBTs are directly replaced by SiC MOSFETs,and corresponding optimized control methods are proposed for the device junction temperature unevenness and electromagnetic interference caused by common mode voltage in SiC MMCs,respectively.Then,for the problems of high cost and high conduction loss of all-SiC MMC,this paper proposes Si/SiC device hybrid submodule scheme and Si/SiC submodule hybrid converter scheme to further optimize the efficiency and economy of the device.The main research contents are as follows:1.To solve the problem of uneven junction temperature distribution of SiC MMC submodule devices,a submodule device junction temperature equalization method based on the bi-directional conducting characteristics of SiC MOSFETs is proposed.First,a SiC submodule loss analysis model is established by drawing on the Si sub-module device loss analysis method.On this basis,the operation of SiC MOSFETs are divided into unipolar operation mode and bipolar operation mode according to whether the SiC MOSFET channel conducts bidirectionally or not,and the device loss distribution of the two modes is compared.Then,an adaptively adjustable threshold parameter is introduced to control the autonomous switching of the two modes,so as to achieve the device loss equalization.Finally,it is demonstrated by simulation that the method can reduce the junction temperature imbalance of SiC submodule by up to 72.5%% under the parameters set in the paper.2.To solve the electromagnetic interference problem caused by the common-mode voltage of SiC MMCs,a common-mode voltage suppression method is proposed.First,a common-mode voltage analysis model is established,and the impact of SiC device application on the common-mode voltage problem of MMC is analyzed.On this basis,the implementation principle of the common-mode voltage suppression method based on the nearest zero common-mode voltage vector is revealed,and the defects of its degraded output power quality are analyzed.Then,an improved common-mode voltage suppression method is proposed based on the switching vector analysis method,which can suppress the capacitor voltage fluctuation and improve the output power quality while achieving zero commonmode voltage modulation.Finally,it is demonstrated through simulations and experiments that the proposed control method can achieve common-mode voltage suppression while reducing the AC-side output current THD and capacitor voltage fluctuation by 23.8% and22.4%,respectively,under the parameters set in this paper.3.To solve the problems of high cost and high conduction loss of the all-SiC submodule,the Si/SiC hybrid full-bridge sub-module scheme is proposed.First,this paper compares the loss characteristics of all-Si MMC and all-SiC MMC and compares their advantages and disadvantages under different operating conditions.Based on this,the topology of the Si/SiC hybrid full-bridge submodule is proposed by combining the advantages of Si IGBT and SiC MOSFET;By optimizing the submodule modulation method,the SiC half-bridge operates at high frequency and the Si half-bridge operates at low frequency to realize the advantages of complementary devices.Finally,a prototype of Si/SiC hybrid submodule is built and the feasibility of the proposed scheme is experimentally verified.A further comprehensive comparison demonstrates that the proposed scheme reduces the loss and cost by 16.3% and 41.0%,respectively,compared to the all-SiC full-bridge submodule with the parameters set in the paper.4.To solve the problems of high cost and high conduction loss of MMC composed of all-SiC submodules,the Si/SiC submodule hybrid MMC converter scheme is proposed.First,this paper analyzes the defects of all-SiC MMC based on its topology architecture and operating characteristics.On this basis,the topology of Si/SiC submodule hybrid MMC is proposed by connecting a small number of SiC shaping submodules and multiple Si bearing submodules in series.By optimizing the commutator modulation method,the SiC submodule operates at high frequency and the Si submodule operates at low frequency while keeping the submodule capacitor voltage equalization,which achieves the stable operation of the topology and improves the utilization efficiency of the device.Finally,a prototype of Si/SiC submodule hybrid MMC converter is built and the feasibility of the proposed scheme is verified by experiments.A further comprehensive comparison proves that the proposed scheme reduces the loss and cost by 31.0% and 60.7%,respectively,compared with the all-SiC homogeneous MMC under the parameters set in this paper.