Grid-Connected Operation Control Of Microgrid With Series Micro-Source Inverters

Due to there being the characteristics of both AC microgrid and cascaded H-bridge inverter,microgrid with series micro-source inverters(SMSI-MG)has simple grid connection form and high output power quality.The current research into SMSI-MG has mainly been focused on islanding operation mode.However its potential for grid-connected operation remains to be explored,and the applicability of the existing methods to the system need to be discussed.In this context,the grid-connected operation of SMSI-MG is studied in this dissertation according to the order of grid-side disturbance from nothing to existence and from minor to large based on the analysis of system characteristics,with an aim of achieving its safe and stable grid-connected operation.The equipment level and system level control are considered under each operating condition,and effective solutions are proposed.1)Firstly,the topology of SMSI-MG is introduced in detail and the output voltage,current and power characteristics are analyzed in depth based on the system equivalent circuit.The implementation method of power tracking control for grid-connected operation of the system is clarified,among which the effective control of grid-connected current is a key.Therefore,the effects of the grid-connected current control under αβ frame and dq frame are compared and analyzed,and the αβ frame is selected as a unified platform for SMSI-MG grid-connected operation control.Solutions to cope with various disturbances are proposed.This section of the research will provide an important theoretical basis for the subsequent research into SMSI-MG grid-connected operation control.2)The coordinated control of generator sub-unit(GU)is another key to realize the power tracking for SMSI-MG system.For grid-connected operation system without grid-side disturbance,a GU power coordinated control scheme is formulated based on the micro-source power after being variational mode decomposed,and the constraints of GU power distribution are explained.By analyzing the effect of GU power difference on the grid-connected current,a method is proposed for judging whether total harmonic distortion(THD)of grid-connected current exceeds limit based on decision tree model,and the corresponding power adjustment strategy is given for the possible overmodulation and current THD exceeds limit in GU coordination.3)On the basis of operation mentioned above,to solve the harmonic interaction problem between SMSI-MG and the power grid under minor disturbances,the system output impedance model is established.The effects of the GU disconnection number and grid impedance on the stability of the interactive system are analyzed in depth,and an impedance reshaping strategy is proposed accordingly based on virtual impedance and multi quasi proportional resonance.Given the difficulty of modeling the disturbances of the GU disconnection location and the difference of remaining GU power after disconnection,a resonance stability evaluator and a resonance frequency band predictor are constructed based on gradient boosting decision tree.Combined with the impedance reshaping strategy,the resonance suppression and stable operation of the interactive system under minor disturbance are realized.4)When the disturbance increases further and SMSI-MG output power quality deteriorates originated from the power grid voltage unbalanced drop,the control interval is divided and the method of coordinated suppression of power fluctuation and fault ride-through scheme suitable for the system are formulated.The calculation methods of the three-phase current and the maximum current peak value are given under the power fluctuation coordinated suppression,and the overcurrent mechanism is analyzed.To solve the overcurrent problem,a generator sub-unit cooperative control strategy is proposed,which has certain advantages compared with the traditional current limiting method of changing the power reference value.Subsequently,the light gradient boosting machine algorithm is used to predict the power reduction in proposed current limiting method,eventually improving the fault ride-through ability of the system.5)Finally,in response to the problem of SMSI-MG operation mode switching,the sudden changes,shocks and offset effects of hard switching on output voltage and frequency from grid-connected to islanding mode are analyzed in detail.Smooth switching from PQ to V/f control mode is achieved by tracking the state of inner current loop controller and locking the phase of the power grid.GU power and carrier amplitude adjustment strategies are proposed for different relationship between microgrid and load power.Thus,smooth switching is realized from grid-connected to active islanding and it is also ensured that the output voltage THD does not exceed the limit and three-phase voltage is balanced after switching.When SMSI-MG returns to grid-connected operation mode,transient current shock is avoided by pre-synchronization of voltage amplitude and phase with the power grid.