| In recent years,the penetration of renewable energy resources(RES)in domestic power system has been rapidly escalated.However,the randomly power fluctuation in distributed power generation,as well as enormous of scattered distributed sources with small capacity,always make an effective coordination and control of power system very difficult.Microgrids have been identified as the best option to integrate distributed generation(DG)units in terms of flexibility and reliability.By efficiently utilizing the local resources,micro-grids can significantly reduce the directly influence of DG on the power system.In general,DG resources can be integrated into micro-grids through power electronic converter.Despite having rapidly response,such converter-based generation usually does not provide any mechanical inertial response,and hence compromises frequency stability of power system.The technique of virtual synchronous generator(VSG)has been developed,through an imitation of rotator motion and voltage/frequency modulation in mechanical synchronous generator,to enhance the support of DG to micro-grid.In this design,by providing additional virtually inertia for power electronics converter/converter,the converter-based interface of DG unit exhibits a dynamical behavior and variable voltage/frequency character similar as a synchronous machine does in conventional centralized power generations,which can be expected to insure the frequency stability of power system.Therefore,the effectively implementation of VSG in micro-grid has attracted intensive interests in both academia and industry.This dissertation focuses on the parallel converters control technology based on micro-grid with implementing of VSG.The main contents and the highlights of the dissertation are listed as follow:(1).Based on the features of bi-directional voltage source converter,a suitable control strategy of VSG in d q coordinates is proposed.The dissertation demonstrates that how to implement it practically through various control strategies,such as active powerfrequency control,voltage-reactive power control,voltage-current dual inner-loop control,and load grid connection pre-synchronous control.The dissertation constructs a specific small-signal analysis model for VSG,for which the proper selection of major control parameters to ensure the system stability is manifested and explained.Through simulation,the active power-frequency operational characteristics of VSG is analyzed in three different circumstances.The distinct performances of VSG,droop control and synchronous generator in an active power-frequency response process are compared and discussed.An experiment platform for VSG converter is fully constructed,by which the operation of VSG under various working environment is inspected.Finally,a grid friendly photovoltaic and battery energy storage system(PV/BESS)hybrid distributed generation(DG)is proposed based on virtual synchronization generator(VSG)grid interface.Furthermore,a self-adaptive response control method for grid frequency is proposed to switch the operation modes between photovoltaic power generation mode,frequency regulation mode according the grid frequency,and off-grid mode.(2).For parallel VSGs in microgrid,the mismatching between output impedance and line impedance can result in an adverse influence on power distribution.To compensate this effect,an improved VSG control strategy involving active/reactive-power decoupling control is proposed.In particular,to enhance voltage quality and the dynamical properties of power grid,one can insert voltage-decoupling link into active power/frequency control,while inserting frequency-decoupling link into reactive power/voltage control,respectively.For a proper selection of parameters in newly-added control setting,the dissertation proposes an improved particle swarm optimization(PSO)with parameters-tuning in offline manner.The advantage of this improved PSO in resolving nonlinear problem is investigated.For specific microgrid system,the methods of calculation of optimal target function and fitness value are given,of which the iterated solutions are analyzed.Finally,simulation verifies the feasibility and validation of our proposal,and in particular the effectiveness of the tuning parameters.The experiment verifies that the proposed method can significantly enhance the accuracy of power distribution for parallel VSG.(3).The dissertation addresses on the transient stability of parallel VSGs in microgrid,with particularly focusing on the influence of virtual moment of inertia(VMI)on the stability of multiple VSGs system.Employing the conventional method with transient energy function,the transformation of transient energy during the swing of VSG rotator under certain perturbation is analyzed.Based on above research,the dissertation proposes a novel way to dynamic adjust the VMI of parallel operating VSG in micro-grid.In particular,it is proposed that,according to local information on the motion of VSG rotator,one can dynamically adjust the virtual inertia to significantly degrade the transient energy of oscillating process,which can heavily restrain the power overshoot and made the system arrive to a stable status rapidly.The simulation and experimental data has verified that our novel method can effectively suppress the power oscillation for parallel VSGs system in various circumstances,and significantly enhance the transient stability of system.(4).To study the practical application of VSG in microgrid,the dissertation proposes the topology and coordinate control strategy for the microgrid system with solar/storage/diesel hybrid microsources.Based on the analysis of properties of photovoltaic,lithium battery,lead-acid storage battery and diesel generator,the proposal endows these microsources with distinct roles and corresponding local control strategies.According to the frequency variation in multi-sources micro-grid,the dissertation proposes that a frequency divisional control strategy,which specifics the control with respect to different time-scale and frequency deviation.In particular,taking two parallel VSGs energy storage units as the primary frequency control units,a primary/secondary frequency control strategy is given,according to the power and energy status of energy storage units.To verify our operation control strategy,the dissertation constructs a solar/storage/diesel hybrid experimental platform,and implements the micro-grid central controller.Both the switching control between diesel generator and VSG,as well as a primary/ secondary frequency control strategy have be verified in the platform. |
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