Research On Hybrid Multilevel Converters And Their Key Technologies

Power electronic converter is the key to achieve efficient power conversion.The quality of power conversion becomes increasingly demanding in modern societies.Although different converters have diverse performance requirements,high efficiency,high power density,and low cost are the common goals of most converters.Aiming at achieving such common goals,it is of importance to reduce the number,volume and weight of components.Being different from two-level converters,multilevel converters have received more attentions owing to their advantages of low harmonic distortions,low dv/dt and low voltage stress of power semiconductor devices.However,the traditional multilevel converters face challenges for the above-mentioned common goals.While topologies and modulation methods are the key technologies to deal with these challenges.Power semiconductor devices are the core components of a topplogy,which fundamentally determine the performance of a converter.Modulation and topology complement each other.The modulation is the basis of a converter to achieve a series of functions,and it is the link to achieve control strategy optimization.Aiming at achieving the common goals of high efficiency,high power density,and low cost of multilevel converters by reducing the number,volume and weight of components,a series of exploration and research are carried out from the perspective of topological innovation and modulation optimization.The specific research contents are as follows:(1)A Modular Multilevel DC-Link Based T-type Thyristor Converter and Its Modulation This dissertation proposes a modular multilevel DC-link based T-type thyristor converter(MMD-TTC)derived from the traditional T-type three-level topology.MMD-TTC is a hybrid topology constructed by low-voltage IGBTs and high-voltage thyristors.The cascaded modular floating capacitors replace the high-voltage capacitors of DC-link to generate multilevel voltages.While the T-type structure is constructed by high-voltage thyristors to synthesize three-phase multilevel line-voltages.It can avoid the series-connection of switches and reduce the switch count owing to the high-voltage rating of thyristors.Moreover,the series-connection technique of thyristor is relatively mature.The modular multilevel DC-link can operate under high frequency,while the thyristors only operate under low frequency by designing the proper modulation strategy,which utilizes the low-voltage and high-frequency features of IGBTs,and the low-frequency and high-voltage features of thyristors.However,it is a technical problem to ensure the thyristors turn ON and OFF accuratly due to their semi-controlled drawback.To address this issue,this dissertation proposes a novel thyristor commutation scheme with the assistance of full-bridge module embedded in upper-and lower-arms,which can allow four-quadrant operation of the converter.Assuming the same output voltage and power,compared with cascaded H-bridges multilevel converter and modular multilevel converter,MMD-TTC has the advantages of fewer devices,smaller voltage fluctuations of submodule capacitors,lower power losses and the stronger resistance to power grid voltage imbalance.Besides,in order to reduce the number of double active bridges(DABs)for MMD-TTC-based power electronic transformer,one construction method of constant medium voltage DC-bus is proposed by utilizating hybrid PWM modulation strategy.(2)A T-type Cell and Half-bridge Cell Combined Nine-Level Converter and Its ModulationThis dissertation proposes a T-type cell and half-bridge cell combined nine-level(9L-TCHC)converter,which consists of only eight power devices and two dc voltage sources with equal voltage amplitude.A hybrid PWM modulation scheme for 9L-TCHC is proposed based on its working feature.And then a power balancing control strategy is proposed to ensure the output power balancing between both dc voltage sources.In addition,the hybrid configuration with SiC and Si devices for 9L-TCHC converter is investigated,in which only 50%of switches are SiC devices and the remaining 50%of switches are Si devices.The high-frequency PWM switching seqeunces are concentrated on the SiC devices,while the Si devices only operate under low frequency,which utilizes the high-frequency feature of the SiC devices fully.Compared with the configuration of all SiC devices,the hybrid configuration can achieve the same switching frequency at the lower cost.(3)A Third Harmonic Voltage Modulation of Boost Seven-Level Active Neutral-Point-Clampled ConverterThe boost seven-level active neutral-point-clamped(B-7L-ANPC)converter is derived from the traditional 5L-ANPC utilizing switched-capacitor technology.It has the advantages of high output voltage gain,capacitor voltage self-balancing and more output levels.However,the switched-capacitor of B-7L-ANPC converter has a long discharging interval due to the absence of redundant switching states,which results in a large capacitor-voltage ripple.The large capacitance is usually equipped to suppress voltage ripple,which increases the size,weight,and cost of the converter.To address this issue,a third harmonic modulation method is proposed to reduce the capacitor voltage ripple by shortening or even eliminating the long discharging interval of the switched-capacitor.In order to minimize the capacitor voltage ripple,the amplitude and phase-angle of third harmonic voltage injected should be dynamically adjusted following the changes of modulation depth and power factor,which are obtained by the particle swarm optimization algorithm.