Research On Periodic Frequency Modulation Strategy Of Single Phase Shunt Active Power Filter

Active power filter(APF)copes with the current harmonics and distorted supply voltages in power system,which has been employed in many areas such as residential,commercial,and industrial.The major part of the shunt APF is a converter using pulse width modulation(PWM).This converter produces a compensation current to neutralize the harmonic and reactive components of the nonlinear load current.As a result,the grid current remains sinusoidal and in phase with grid voltage.However,in the fixed switching frequency PWM mode,the high frequency action of switching devices will cause the converter to generate carrier frequency harmonics with large amplitude at the integer multiple carrier frequency of the output waveform.As a result,APF induces carrier frequency harmonics while compensating low harmonic current of nonlinear load.The magnitude of the carrier frequency harmonics will directly affect the severity of electromagnetic interference in APF,which will degrade the performance of APF.In order to suppress the carrier frequency harmonic of APF,this thesis presents a reliable idea to apply the periodic carrier frequency modulation strategy to an APF.The innovative work and research results of this thesis are as follows:(1)A single phase shunt APF system is designed.On the one hand,the APF based on discrete Fourier algorithm simultaneously realizes the phase-locked of grid voltage and the detection of harmonic components in load current;On the other hand,the APF based on double closed loop(the current inner loop and the voltage outer loop)PI control system has high control precision for the low frequency reference signal.(2)The spectrum distribution characteristics of periodic carrier frequency SPWM is analyzed.Taking the uni-polar SPWM as the research object,the dual-Fourier analysis method is used to calculate the spectrum of the fixed switching frequency SPWM.Theoretical analysis shows that the fixed switching frequency SPWM has large harmonic peaks at the switching frequency and its integral multiple.The above spectrum calculation method is further applied to the spectrum quantization calculation of periodic frequency SPWM,and the spread spectrum effect of periodic frequency modulation in different modulation signals,different frequency deviations and different modulation ratios is theoretically analyzed.Finally,theinternal mechanism of different spreading effects under different modulation signals is discussed.(3)The suppression law of APF carrier harmonic current by periodic frequency modulation is analyzed.The harmonic equivalent circuit of APF is established.The double-Fourier analysis method is used to calculate the spectrum of the fixed-frequency PWM APF carrier frequency harmonic voltage and current.Theoretical analysis shows that the fixed frequency PWM APF has carrier frequency harmonic voltage and carrier frequency harmonic current at the switching frequency and its integral multiple.The above spectrum calculation method is further extended to the quantification calculation of carrier frequency harmonic spectrum of periodic frequency PWM APF.The selection method of modulation signal and frequency deviation with optimal harmonic suppression effect is analyzed theoretically.(4)The suppression law of APF Differential Mode(DM)EMI by periodic frequency modulation is analyzed.The APF DM EMI equivalent circuit is established,and the mechanism of APF DM EMI is analyzed.A spectrum calculation method of the source DM voltage of fixed frequency and periodic frequency PWM APF is put forward to predict the effects of the modulation parameters of periodic frequency modulation on the the source of DM voltage of APF.Then the modulation parameters with optimal DM EMI suppression effect and the DM EMI with optimal distribution can be obtained.The research results of this thesis show that DFT detection technology,double closed-loop PI control system,periodic frequency PI controller,periodic frequency PWM APF quantitative design,periodic frequency PWM APF carrier frequency harmonic current suppression,periodic frequency PWM APF common mode EMI suppression,APF compensation performance optimization and other aspects have achieved the desired results.The experimental results show that,compared with the fixed switching frequency PWM APF,the sawtooth periodic frequency PWM APF with a 800 Hz frequency deviation enables the THD of grid current to lower 1%,the peak value of carrier frequency harmonic current on the grid side lower about 50%,and the DM EMI of APF is lower 6.6 dBμV.Verify that the periodic frequency modulation technology can effectively optimize the compensation effect of APF,suppress APF carrier frequency harmonic current,and reduce APF DM EMI.