Research On Topology And Control Strategies Of Modular Multilevel Electric Energy Router In AC/DC Distribution Grids

In recent years,based on the national strategy for the generation of renewable energy and environmental protection,the implementation of hybrid AC and DC distribution grids characterized by efficient energy interconnection and power electronics dominance has been developed to enhance the penetration of distributed energy generation and the reliability of distribution grid operation.The main equipment of the flexible distribution system called Electric Energy Router(EER),which integrates power electronic technologies and information technologies,is being proposed in recent years in order to increase the reliability and efficiency of future AC/DC distribution grids and to support the operation of the Energy Internet.Hence,based on the EER application,the new integration of AC/DC grids is being constructed.However,the research regarding EER implementation is in the beginning stage,which is one of the directions of Energy Internet construction.In the context of the present advancements and participation of EER as key node equipment in the future AC/DC distribution grids raises serious challenges,such as the steady-state and transient operation performance that needs to be investigated.In steady-state operation,the flexible interconnection of AC and DC distribution grids can be achieved,where the renewable energy generation can be maximized utilized through the coordinated control and power routing of each port of the EER.In the transient operating condition,EER involves the fault isolation along with the ability of shortcircuit current limitation and fault voltage support and correction,so as to form a system-level way to deal with the transient fault ride-through of AC/DC distribution grids,therefore the security and continuous power supply of distribution network is improved.In this dissertation,we study and investigate the topology,operating mechanism,control strategies,and coordination control of EER,which is a system-level power supply equipment for AC/DC distribution grids/micro-grids.The examined results demonstrate the stable and high-performance operation of the EER under various steady-state and transient-state complex conditions,and further enhance the penetration of distributed renewable energy and improve the reliability of AC/DC distribution grids.It also provides technical support for the application of EER in the future AC/DC distribution grids/micro-grids.In order to overcome the deficiency of the existing EER topology,this thesis proposes a novel Modular Multilevel Converter based Electric Energy Router(MMC-EER),which is suitable for multi-voltage-level AC/DC power grids.The topology structure is the base of the function realization and fault ride-through of EER.Therefore,the topology of the high-voltage stage and isolation-transformer stage with a negative output voltage sub-module was studied to achieve fault ride through capability.Furthermore,we investigate the EER-based AC/DC hybrid distribution grids structure and propose three types of AC/DC distribution grids based on multiple EERs.In order to improve the control performance of the MMC in high-voltage stage of EER under complex operating conditions,an improved comprehensive control strategy of MMC based on hierarchical voltage outer loop and inner loop repetitive control-based arm current control is proposed.Firstly,the influences of dead time effect,arm parameter asymmetry,ac grid fault,and power quality control of MMC with carrier phase shift-sinusoidal pulse width modulation(CPS-SPWM)on the voltage and current of ac-and dc-side are analyzed.Then,the equivalent decoupled models of upper and lower arms including ac-side component,dc circulating current component,and ac circulating current component are established.Subsequently,a hierarchical voltage outer loop control is proposed,which consists of four parts:dc bus voltage/power control,leg-averaging control based on dc circulating current injection and negative sequence current injection,and their seamlessly switching strategy,arm-balancing control,and individual balancing control.A new arm current control based on repetitive control is proposed to accurately track the multi-frequency components’ current reference value in the ABC frame and suppress the periodic disturbances caused by grid voltage distortion and deadtime effect.Presenting the performance analysis and parameter design principle of the control scheme,the effectiveness of the proposed improved comprehensive control strategy of the MMC high-voltage stage is verified by simulation and prototype experiments.The improved comprehensive control strategy does not require the extraction of positive and negative sequence components and the circulating current suppressor.We further investigate the voltage control of the sub-module capacitor of isolated modular multilevel DC converter(IMMDC)under asymmetric parameters,light load conditions,and propose a comprehensive sub-module capacitor voltage balancing control strategy for IMMDC,including two layers,i.e.,arm voltage balancing control and inner-arm voltage balancing control.We analysis the operation modes of IMMDC with quasi-two-level modulation,and drive the mathematical models of current and transmission power of each mode.Then,a general mathematical model of IMMDC is also constructed taking into account the asymmetry of arm parameters.We also present the detailed mechanism that reveals the asymmetry of the arm parameters leading to the imbalance of the capacitor voltage and the generation of the ac circulating currents under bidirectional power transmission.Subsequently,an arm voltage balancing control method based on pulse width modulation(PWM)combined with phase-shift control is proposed to balance the sub-module capacitor voltage and to suppress the ac circulating currents.In addition,showing the fact that the conventional inner-arm balancing control method may be invalid under different operation modes and voltage gain,therefore to overcome the challenge such as balance the inner-arm sub-module capacitors voltage,an operation-mode adaptation sub-module capacitor voltage balancing control method is proposed.Finally,the effectiveness and feasibility of the proposed method are validated by the simulation and experiment.In this dissertation,we investigate the prevention of the negative effect of periodic disturbances from the degradation of the output current quality in the voltage-source converter and propose a novel method namely the fractional-order harmonic disturbance observer(FOHDO)control method for three-phase LCL-type converter.The proposed method of control taking into account the mathematical model of periodic disturbance caused by the dead time effect and grid voltage distortion is established.In the dq-axis,an infinite impulse response filter design method is used to embed the internal model of the periodic disturbance signals into the disturbance observer loop to ensure accurate estimation of the amplitude and phase of the disturbances.Besides,the FOHDO based on Lagrange interpolating polynomial fractionalorder delay approximation is put forward to enhance the adaptability of the grid frequency.The combination of the current inner loop proportional-integral regulator and the grid voltage direct feedforward method is presented to analyze the stability,the steady-state,and dynamic performance of the proposed FOHDO strategy,and the detailed parameters design process is also given.Finally,the experiment verifies that the proposed method effectively eliminates lumped disturbance,such as dead time effect,grid voltage distortion,and parameters deviation,and also has a fast dynamic response and well steady-state tracking performance to enhance output current quality.In this dissertation,we propose the hierarchical coordinated control strategy and the node energy management strategy with energy storage coordination to ensure the stable operation and optimal energy management of EER.According to the access of high-voltage and lowvoltage AC/DC ports and the power flows of EER,the operation modes and the working states of EER are analyzed in detail.The hierarchical coordinated control strategy of the three-stage converters of EER is presented.In addition,to resolve the problem of the energy management of EER nodes under various operation modes,a grid-connected and stand-alone energy management strategy of EER is proposed to maximize the consumption of renewable energy and to optimize economically low-voltage AC/DC micro-grid users.