Research On A Hybrid Control Strategy Of Harmonic Compensation And Resonance Damping Based On SAPF

With the increasing use of nonlinear loads in power system, large amount of harmoniccurrents have been generated, which flow through the transmission line will lead to harmonicvoltage drop and accordingly weaken the power quality in Point of CommonCoupling(PCC).The flowing of harmonic current and voltage can cause resonant problemsin whole or partial of power system. Active power filters(APFs) is a effective way tosuppress and eliminate harmonics. But when system resonance occurs, harmonic suppressioneffect may not reach the power quality standards requirements. The main purpose of thispaper is to use APFs to compensate harmonic and damp resonant when resonant occurring inpower system, so the power quality of PCC can meet the power supply quality requirements.An introduction to the cause and harmfulness of harmonic current as well as thecorresponding standards of limitation for the harmonic current is presented in this paper. Theanalysis and comparison of the advantages and disadvantages for several existing harmoniccompensation methods are introduced. Then the paper presents the basic principle of APFsand the development of APFs at home and abroad. At last, some problems about APFs usingin the existing practical application(Resonance condition) is discussed.Then the mathematical model of the general conditions APF in synchronous rotatingcoordinate are deduced in the view of mathematics and physics. The paper gives a specificanalysis of the mathematical model, the transfer function of the resonant state and thesimplified equivalent diagram. In order to provide a basis to determine the resonantfrequency to the the proposed resonance damping strategy, four methods to detect theresonant frequency are discussed in the paper. They are harmonic impedance measurementbased on capacitor switching, harmonic impedance measurement based on thyristor branchswitching, harmonic impedance measurement using harmonic current source and spectrumanalysis of parallelled capacitor current, respectively. Then a detailed analysis of the lattertwo methods is presented, and both are verified by simulation or experiment.This paper concerns a harmonic current compensation strategy based on multiplesynchronous coordinates transforms to compensate the specified frequency load harmoniccurrent. Then a composite control strategy which is constituted by resonance dampingcontrol strategy and compensation strategy is proposed. The harmonic current compensationstrategy based on the multiple synchronous coordinates transforms the specified frequencycomponent of harmonic current to a direct-current component. Then after low-pass filter and regulated by a PI controller, which can achieve zero steady-state error. The PI regulator iscarried out in the cycle of carrier wave, which can response rapidly when the load currentschange. Meanwhile, the resonance damping control circuit detects the voltage at the PCCpoint to obtain resonance frequency. The voltage affected by resonant is transformed to thedirect-current variable by multiple synchronous coordinates and regulates by the PIcontroller. Then the output of PI controller superimposed to the current loop to achieveharmonic compensation and harmonic damping.Finally, the simulation and experiment results show that this hybrid control strategy ofharmonic compensation and resonance damping can compensate load harmonic current anddamp system resonance. This is a good solution to the problem of resonant conditions whileAPF compensation effect can not meet power quality standards. Not only provides a newway of thinking to damp resonance, and also to broaden the field of application of the APF.