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Study On Control Strategies Of Parallel Operated Virtual Synchronous Generators In Microgrids

Posted on:2019-12-22Degree:DoctorType:Dissertation
Country:ChinaCandidate:Full Text:PDF
GTID:1362330548970363Subject:Power system and its automation
Abstract/Summary:PDF Full Text Request
Renewable energy sources(RESs)have been widely used in the area of electric power generation because of its advantages of free production cost,and free greenhouse gas emission.The advent of microgrids and advanced of converter-interfaced distributed generations(DGs)has opened the door to increase the penetration level of RESs.Unfortunately,a majority of RESs basically cannot participate in the frequency regulation with other conventional generation units because they typically have the inherent feature of very low or non-existent inertia and damping properties which could make the microgrid more susceptible to variations in generation or load.In this dissertation,the virtual synchronous generator(VSG)control concept is discussed which is a novel control strategy recently developed based on emulating the swing equation of synchronous generators(SGs)for friendly integration of DGs.The central idea of this study is the VSGs parallel operation,due to the fact that most DGs are small in capacity,operating VSGs in parallel will increase the system reliability.The main contributions of this dissertation are summarized as follows:(1)An inverter parallel operation control strategy based on the virtual impedance and the virtual power methods is proposed.By using this method multiple control objectives related to the power distribution and the power quality control are implemented.The method overcomes the deficiencies of the single-used virtual impedance method and the virtual power method,improves the system stability and the power-sharing of the parallel operated droop controlled inverters,and achieves the power decoupling of a grid-connected inverter.Comparisons with the traditional methods are made via simulation to verify the effectiveness of the proposed method.(2)Aiming at achieving stable operation of multi-paralleled VSGs,this dissertation proposed the design criteria for the characteristic parameters of VSGs(Viz.,inertia,damping factor,and droop coefficients).Revealed the principle of inertia matching when VSG operating in parallel.It is assumed that the per-unit inertia constant of multi-VSGs can be exploited and unified to suppress the power oscillation of the parallel system and to reduce the variation in the frequency(decrease the high rate of change of frequency)towards realizing the multi-machine parallel operation.The particular results obtained aimed at illustrating that the intelligent parameters design significantly improves the appearance of the output responses and ensures for the multi-paralleled VSGs keeping work in peace.By the simulation and the experimental setup,the effectiveness and correctness of the criteria are evaluated.In addition,for the proportional power sharing the study introduces a control scheme for a microgrid with multi-paralleled VSGs of different capacities based on the adaptive PI controller.(3)The influence of the damping power on the dynamic adjustment of the system frequency and the microgrid stability is studied.From the aspects of planning,modeling and analyzing of the VSG damping effect,a simplified VSG model is established.The study discovers the correlation between the virtual inertia and the damping factor of the simplified model.Then,a modified damping control approach is proposed to handle the parameters coupling.The improved damping approach simply bases on the PI controller and uses a very simple computational process,nonetheless,achieves an efficient performance.The effectiveness of the method is verified by matlab/simulink and experimental platform.(4)Establishing a third-order VSG model by introducing the stator voltage equation and the number of the pole-pair of the rotor into the VSG model.This is due to the fact that,maintaining the normal operating voltage in the parallel operation with SG requires mimicking the stator voltage equation,and comparing a VSG with its analogue SG requires mimicking the number of the rotor poles.Then,the model is used to study a microgrid consisting of a VSG and a traditional synchronous generator(SG).The simulation model is established to analyze,compare and verify the dynamic matching performance of the VSG and SG.
Keywords/Search Tags:microgrid, parallel operation, virtual synchronous generator, power decoupling, virtual inertia, virtual damper, synchronous generator
PDF Full Text Request
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