Development Of Three Phase Load Unbalance Compensation Device For Distribution Network Based On Three Level DSTATCOM

In our country’s low-voltage distribution network,especially in the vast rural areas,a large number of single-phase and three-phase loads coexist.Due to uneven load distribution and the uncertainty of users’ power consumption,the problem of three-phase unbalance has existed for a long time.That distribution network operates in three-phase unbalanced state for a long time will cause distribution voltage loss,output reduction,serious transformer damage,neutral drift,and ultimately lead to the burning of electrical equipment,which brings hidden dangers to users’ safe in production and life.As a power electronic device to control three-phase unbalance and reactive power compensation,Distribution Static Synchronization Compensator(DSTATCOM)has been widely concered and applied in the field of power quality.In this thesis,threelevel capacitor split DSTATCOM is taken as the research object.DC-link voltage control,reference current detection method and neutral point balance control strategy of three-level DSTATCOM are studied by combining mathematical modeling,mathematical reasoning,circuit analysis and simulation experiments.Firstly,the operation principle of DSTATCOM is introduced.According to the decoupling characteristics of ABC three-phase in three-phase four-wire system,the mathematical model of three-level capacitor split DSTATCOM is established by analyzing the single-phase equivalent circuit of three-level capacitor split DSTATCOM.And a preliminary method to suppress the fluctuation of neutral point potential is obtained.Besides,the overall control scheme of three-level capacitor split DSATCOM is designed in ABC static coordinate system.Secondly,aiming at the shortcomings of traditional phase-locked loop based on SSRF-SPLL,a reference current detection algorithm based on DDSRF-SPLL is proposed,which can be applied to unbalanced power grid.Thus,the installation range of three-level capacitor split DSTATCOM will extended to the whole distribution station area.From the point of view of energy conservation,the causes of secondary DC ripple in three-phase four-wire system and its influence on DC current detection are quantitatively analyzed.On this basis,a DC-link voltage loop error correction control strategy with second-order notch filter is proposed,and the parameters of the secondorder notch filter are designed,which eliminates the error of instruction current detection caused by unbalance of negative sequence current.Thirdly,aiming at the particularity of zero-sequence path in three-phase four-wire unbalanced system,a strategy of suppressing midpoint potential fluctuation on DC-link based on O-state decomposition is proposed.Considering the problem of neutral point potential offset caused by burnout of DC-link equalizing resistance and inconsistency of capacitance between upper and lower capacitors,a neutral point balance control strategy with neutral point potential offset suppression on DC link is proposed to restrain the fluctuation of single capacitor voltage and reduce the demand of DC capacitor capacity effectively.By decomposing the neutral-point potential fluctuation,the main causes of the neutral-point potential fluctuation on the DC link are analyzed,and the influencing factors of the neutral-point balance control ability and their relationship are also explored.Finally,based on the simulation software of Matlab/Simulink,a simulation model is built to verify the correctness and validity of the proposed command current detection method,Error Correction Control Strategy of DC Voltage Loop and neutral point balance control strategy.Moreover,the parameter design method of the main circuit of the three-level capacitor split DSTATOCM is given.The hardware circuit and software program of the control system based on the core of TMS320F28335 and CPLD are designed.Besides the whole compensation effect of three-level capacitor split STATCOM is verified by simulation and experiment.