Inherent Submodule Voltage Balancing Of Modular Multilevel DC-DC Converter

With the world consensus of low-carbon,environmentally friendly,and sustainable development,the volume of dc sources,dc storages and dc loads grows rapidly with more and more renewable energy generations,battery-based electrochemical energy storage,and electric vehicles connected to the power system.Thus,the medium voltage dc distribution becomes a promising solution to integrate all these dc form components,since it enables fewer conversion stages,a simpler control system and stronger power supply capability.Among the existing medium voltage dc conversion topologies,the modular multilevel dc-dc converter(MMDC)is one of the most competitive candidates because of its superiority of low operating losses,high power density,great expandability and outstanding fault-tolerant capability,which makes it receive widespread attention from both academia and industry.However,the modular structure enables the MMDCs applied in higher voltage applications and brings it flexibility of operating control,but it also introduces the issue of distributed floating capacitor balancing.The traditional submodule closed-loop control schemes highly depend on real-time submodule voltage sampling and sorting,which results in high computational burden and high implementation costs.These schemes also rely on high-speed communication,which limits the further extension of MMDC operating frequency.And the existing self-balancing modulation methods cannot meet the demand for flexible operation in different kinds of MMDCs.This thesis analyzes the constraints on submodule voltages naturally brought by the submodule switching patterns.A series of circulant modulation methods based on the full-rank switching matrix principle are proposed with the common conditions of inherent submodule balance derived and modulation characteristics optimized.Firstly,the features and inherent balancing principle of the circulant modulation are summarized,and the limitations of the existing basic circulant modulation are listed to direct further improvement.Based on the arm stack structure and dc voltage conversion principle of the MMDC,the relationship between the switching matrix and the submodule capacitor steady-state voltage is modeled and analyzed.The main features and key requirements of inherent submodule balancing are concluded for the circulant modulation,which is the switching matrix should be a full-rank circulant matrix.Further,considering the flexibility of modulation,the wide-range operation capability and the demand for high power-density for the MMDC,it is concluded that the existing basic circulant modulation has problems including lack of multilevel voltage generation capability,unbalance in non-coprime cases and large submodule capacitor voltage ripple.Secondly,to realize the flexible modulation in MMDC,the multilevel circulant modulation method based on the duty-cycle matrix is proposed.As the excitation of the internal ac stage of MMDC,the two-level voltage results in high dv/dt stress and could damage the magnetic components and insulating materials,which decreases the lifespan of the MMDC.In this thesis,the circulant modulation is expanded from the square-wave form to a flexible and variable voltage form with inherent submodule balancing capability kept.The multilevel voltage output can be generated by varying the duty cycle of each submodule within a fundamental cycle,and the switching frequency of the submodules is still lower than the fundamental frequency.Then,based on the voltage loop analyses of the arm stack,the duty-cycle matrix is used to represent the circulant switching patterns,and the matrix eigenvalues are solved to determine its full-rank feature.it can be concluded that so long as the numbers of inserted submodules in each voltage level have no common factor other than1,the submodule capacitor voltages can be inherently balanced.furthermore,the inherent balancing criterion is simplified by introducing the common design constraints of MMDC modulation.A generalized coprime criterion is identified for multilevel circulant modulation for the cases with equal magnitude of each voltage level and maximum output voltage,which provides the intuitive direction for practical multilevel circulant modulation design.Thirdly,to promise the inherent balancing capability in arbitrary operation cases,the dual circulant modulation method based on coprime polynomial is proposed.When the number of submodules in one stack is a non-prime number,the basic circulant modulation cannot guarantee the linear independence of the sub-module switching patterns,which leads to the possible problem of unbalanced submodule voltages.In order to complete the constraints and solve the issue of submodule voltage clustering in non-coprime cases,the dual circulant modulation is proposed with two sets of circulant switching patterns preset and combined.The associated polynomial of circulant matrix is introduced and the coprime of polynomials demonstrates the full-rank feature of the extended switching matrix that promises the inherent balancing for any operation cases.then,due to the direct alternation of two switching sets results in the patterns of each submodule varying and the absence of submodule uniformity,the switching patterns are reallocated and rearrangement to keep the submodule uniformity and reduce the capacitor voltage ripple at the same time with the submodule voltage inherently balanced.Finally,to meet the demand for capacitor ripple suppression,the optimal circulant modulation based on the generalized-circulant matrix is proposed.The submodule capacitance increases with the submodule capacitor voltage ripple under the unit power transmission,and the large ripple increases the implementation costs and decreases the power density of the MMDC.Focusing on the large ripple brought by the successive charging and discharging cycle allocation in basic circulant modulation,the intrinsic ripple is extracted to identify the ripple component related to the switching pattern sequence.The equation expression of the intrinsic ripple is derived to choose the optimal pattern cycle-bycycle for minimizing the submodule voltage deviation.Then,it is proven that all the local optimal solutions for a single fundamental cycle can be satisfied at the same time resulting in the global optimal solution for a circulant cycle with the lowest submodule ripple,which obtains the theoretical optimal switching pattern.The switching matrix of the optimal circulant modulation is formulated by introducing the generalized circulant matrix.it verifies the circularity and full-rank feature of the optimal switching matrix,which promises the uniformity of submodule actions and the inherent balancing of submodule voltages.Overall,in this thesis,the circulant modulation methods with a full-rank switching matrix are designed for the MMDC.The submodule capacitor voltage can be inherently balanced with the flexible modulation demands satisfied.All the analyses of the proposed methods are validated by both simulations and experiments,which provides the theoretical and implementational basis for practical MMDC systems with higher power density and higher reliability.