Research On Controllable Inductive Power Filtering Theory And Application

Generation,transmission and distribution sides of modern power system more present the form of AC/DC hybrid structure,e.g.,renewable power generation,large-power industrial DC supply system.Compared with traditional power system,such power electronic based system faces more complicated and varied power quality issues.Power conversion system,such as,rectifier,inverter a nd DC converter,can provide flexible and controllable power delivery and distribution for power consumer and public grid.However,the interaction of power quality issues between such systems and public grid has serious negative impact on safe operati on of power system,which makes the dynamic characteristics of public grid complex.It brings about harmonic pollution and harmonic resonance to power system,and results in low operation efficiency and poor performance on power devices.The research work is supported by National Natural Science Foundation of China under Grant 51822702 and 51377001,and Hunan Provincial Innovation Foundat ion for Postgraduate under Grant CX2017B167.Focusing on the topic of “Controllable Inductive Power Filtering Theory and Application”,this dissertation conducted a series of study on power quality issues on AC/DC hybrid power system.The main research work inc ludes the following aspects.(1)According to the large amount of measured data of electrolytic manganese factory,wind farm and PV station in Hunan,Guangxi and Chongqing which are as the typical application of renewable power generation and large-power industrial DC supply system,the basic characteristics and variation of power quality of its main transformer are studied.Moreover,the defects and shortcomings of power quality management are introduced by the over-current case in an electrolytic manganese factory.(2)A probabilistic model based harmonic contribution assessment method is proposed.Based on the measured data of an electrolytic manganese factory,the impact of the harmonics generated by the factory on the public grid is evaluated.According to the harmonic voltage distribution at the point of common coupling(PCC),its probability distribution during the specific discontinued day is fitted in divided period by means of three typical probability distribution functions.The goodness-of-fit is obtained by Kolmogrov-Smirnov test method,and the optimal fitting function is identified.The simulation model of the backgro und harmonic voltage is thus built.Finally,the harmonic contribution of the electrolytic manganese factory on the production day is assessed bas ed on the simulation model.The method has the merit of simple calculation,and the harmonic contribution of p ower consumer can be determined effectively,which contributes to provide early guidance for power quality management.(3)A controllable inductive power filtering(CIPF)system is proposed for application in the large-power industrial DC supply system wit h the electric environment of large current/low voltage.Due to the zero-impedance design of the transformer winding,the harmonic components can be cancelled between the secondary windings,and the harmonic flowing path is shortened.Active power filter realizes the bi-directional harmonic isolation between the public grid and power load,and further improves the filtering performance.The three-phase equivalent circuit model of CIPF system is established,and the transfer matrix from bi-directional harmonic source to grid-side current is obtained.The special filtering mechanism of CIPF system is revealed.It's worth noting that the three-phase analytical method is also suitable for most of the filter integrated transformer system.The harmonic distribution inside the transformer can hence be accurately obtained.The implementation of CIPF system can improve the electric environment of the transformer,and reduce the loss.(4)A series of virtual impedance based control strategies are proposed according to the operating characteristics of CIPF system.Based on the bi-directional harmonic transfer matrix,the relationship between magnitude and phase of the virtual impedance and the filtering performance is explored.The virtual impedance phase for optimal filtering performance is obtained.The improved four-quadrant virtual impedance control is designed accordingly.Based on the requirement of dual zero impedance design for implementing inductive power filtering,the zero impedance control is proposed.The inverter can simulate the negative impedance,so that the quality factor of the passive filter is improved.A passive control based harmonic current injecting control method is propo sed.This method has the advantage of accurate tracking of compensating current and goo d robustness.(5)A transformer integrated filtering system(TIFS)is designed for the electric environment of severe harmonic contamination and the installation space with restricted planning area.Considering the influence of the residual weak coupling,the electromagnetic decoupling model of TIFS is established.The relationship between coupling factor of integrated reactor,virtual impedance and filtering performance is discussed.The impact of mutual inductance on the filtering performance is analyzed.Theoretical analysis and experimental results show that the integration of the inductive filtering winding and the integrated reactor is feasible and effective.A down-scaled TIFS protype is developed.The basic structure of the experimental platform is introduced.The system parameters,program code and debugging process are given.The experimental results are analyzed.(6)For the application of renewable energy power station,two kinds of step-up station are designed.A transformer integrated filtering method based two-stage hierarchical structure is proposed for power quality improvement.A topology of PV station which uses the box-type transformer with integrated reactor and 110 k V grid-connected transformer based on inductive power fi ltering method is designed.The two key technologies of filtering reactor integrated method and inductive filtering method are applied in the PV station in the form of primary and secondary filtering station.The power quality at PCC is improved by means o f the active/ passive filters.Theoretical analysis shows that the method has th e advantages of strong antijamming ability,low risk of resonance and high integration.For the application of wind farm,a parallel operating transformer with shared inductive filt er is proposed.According to the special topology of the step-up station which consists of two main transformers,only a set of shared inductive filter is configured to serve for two wind farms(double harmonic sources).The proposed transformer with the share d filter has the merits of lower installation space,higher equipment utilization and better filtering performance compared with conventional scheme.The three-phase mathematical model of the step-up station is established.Its simplifying circuit is obtai ned considering symmetry and asymmetry parameters of the transformer impedance.The filtering mechanism,operating characteristics and realization condition of good filtering performance are further revealed.At last,the proposal is verified to be feasibl e and effective by means of simulation results and field measurements.The theory and method of controllable inductive power filtering take full advantage of electromagnetic potential of power transformer.Combined with power electronic devices,the power quality improvement of publi c grid/power consumer is realized,which meets the interests of both sides.The operating capacity of inverter is reduced theoretically,which realizes the harmonic suppression efficiently with low cost.In sum,the research work in this dissertation has important scientific significance and practical value on the power quality management of large-power industrial DC supply system and efficient grid-connection of renewable power generation.