| In recent years,power distribution system is closely related to the production and life of people,and its power quality problem is becoming more and more prominent with the continuous increasing of the total power demand.At the same time,a lots of power quality control methods have been extensively studied by researchers,which stimulates the development and progress of power quality conditioning technologies.Dynamic capacitor(Dynamic Capacitor,D-CAP),which is designed as shunt power quality conditioners through renovating power capacitor with thin AC/AC converter(TACC),it can be used to solve many power quality problems such as reactive power,harmonics and resonance.Moreover,due to its low cost,high reliability and good control performance,D-CAP has potential advantages in power quality control field.This paper focuses on comprehensive regulation of power quality for three-phase Buck-type D-CAP,some key issues about the performance optimization technology are deeply studied,which includes reactive compensation,harmonic suppression and resonance damping.And the following tasks have been mainly finished.In order to analyze overall performances of AC/AC converter-based Buck-type D-CAP in the aspects of reactive compensation and harmonic suppression,circuit structure,switch mode,mathematical model and basic working principle of Buck-type D-CAP are analyzed in detail in this paper.Meanwhile,basic reactive compensation and harmonic suppression control architecture is also designed.By means of analyzing circuit principle of D-CAP,state average model is established by using duty cycle modulation signal.Furthermore,in order to simplify circuit analysis,general control object could be regarded as single power capacitor,thus the basic control equations of Buck D-CAP is deduced.Based on the basic control equations,reactive compensation and harmonic suppression principle of Buck-type D-CAP could be analyzed by introducing different modulation components into duty-ratio modulation signal.And then,a selective current control strategy in multi-synchronous rotation frames is given to regulate reactive and harmonic currents of Buck-type D-CAP effectively.In case of resistive-inductive linear load,experimental test results verifies basic control performance of reactive power compensation for Buck-type D-CAP,but its compensation current will also contain low harmonic components such as 5th,7th harmonics,leading to poor quality of waveform.In case of both linear and nonlinear loadsin the grid,Buck type D-CAP is employed to compensate reactive current and suppress harmonic currents.Experimental results validate the effectiveness of reactive power compensation and harmonic suppression for Buck-type D-CAP.However,some uncontrolled low harmonic current components may be also introduced into compensation current.At the same time,an inherent constraint is that duty-ratio modulation signal should range from zero to one,the regulation of reactive current could affect the generation of harmonic current and vice versa,leading to the interaction between reactive compensation and harmonic suppression.In case of reactive compensation,the capacitive compensation current of Buck-type D-CAP is regulated by changing DC term in the duty ratio modulated signal.Considering compensation current waveform distortion of Buck-type D-CAP,some optimized measures should be designed to improve its control performance through analyzing the causes of current waveform distortion.Based on circuit working principle of Buck-type D-CAP,three main factors resulting in the distortion of compensation current are discussed in detail in this paper,which are background harmonic voltage from the grid,voltage drops on switching devices and the dead time respectively.Meanwhile,in order to mitigate the distortion of compensation current,a feedback control method with current waveform improvement is designed.In addition,under the excitation of background harmonic voltage from the grid,Buck type D-CAP may interact with inductive line impedance to trigger harmonic resonance,leading to the distortion of compensation current.In this situation,harmonic resonance could be by suppressed by using an active damping control method with feedback regulation of power capacitor voltage.Finally,in order to suppress harmonic currents and harmonic resonance,this paper proposes an optimized reactive compensation control strategy in synchronous rotation frames,which includes basic reactive compensation control,current waveform correction and active resonance damping.In case of reactive compensation and harmonic suppression,based on the principle of even harmonic modulation(EHM),the capacitive and harmonic current of Buck-type D-CAP could be respectively regulated by adjusting DC and harmonic terms in duty-ratio modulation signal.Considering the interaction between reactive and harmonic currents,basic reactive compensation and harmonic suppression control method should be improved to minimize the interaction between reactive and harmonic currents by analyzing the quantitative relationship between compensation current and duty-ratio modulation signal.Because an inherent constraint is that duty-ratio modulation signal should range from zero to one.Its DC and harmonic modulation terms should be alsorestricted each other.Otherwise,Buck-type D-CAP would operate in over-modulation zone to generate unwanted harmonic currents,leading to poor control performance.Therefore,this paper presents a coordinated control between reactive compensation and harmonic suppression for Buck-type D-CAP.According to the demand of desired reactive and harmonic current,DC and harmonic terms could be pre-estimated and selected reasonably.Meanwhile,desired reactive and harmonic current reference i C*should be revised to avoid over-modulation.And then,Buck-type D-CAP could generate desired reactive and harmonic currents by a selective current control strategy in multiple synchronous reference frames.Under the condition of resistive-inductive linear and nonlinear loads,experimental results are provided to verify the correctness of proposed coordinated control method.It is easily to cause harmonic resonance when nonlinear load and shunt power capacitors are installed in the power distribution system,leading to serious distortion of PCC voltage and grid current.In this situation,Buck-type D-CAP could not restrain harmonic propagation of both grid current and PCC voltage adequately by means of harmonic suppression.In order to improve the control performance of Buck-type D-CAP in this resonant condition,this paper discuss the mechanism of parallel resonance and the principle of resonance damping for Buck-type D-CAP.On the basis of it,an comprehensive control strategy of reactive compensation,harmonic suppression and resonance damping is proposed.Resonance damping current reference could be extracted by detecting PCC harmonic voltage,and resonance damping gain is obtained by mean of adaptive adjustment,thus it could achieve excellent performance of resonance damping.the effectiveness of proposed comprehensive control strategy is validated by experimental tests.In this paper,a three-phase star-connected Buck-type D-CAP prototype is designed.A series of experimental results are provided to demonstrate optimized power quality control performance of reactive compensation,harmonic suppression and resonance damping for Buck type Buck-type D-CAP.It is hopefully that the researches of this paper would provide the reasonable reference template and promote Buck-type D-CAP to actual project application. |
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