Harmonic Modeling And Research On Collective Effect Of Residential Nonlinear Loads

In recent years,under the drive of subject consciousness on the energy conservation and development,power electronic technology has a rapid development.At present,harmonic sources belonging to the type of domestic appliance containing power electronic rectifying circuits have been widely in the low-voltage distribution system.For this kind of harmonic sources,in general,the single power is small,but the total distortion rates of harmonic current produced by such harmonic sources are around 100%and even higher at times.Then,the whole power system could be polluted as a result of the widespread use of the above harmonic sources.Traditional method for harmonic analysis regards the various harmonic sources as constant current sources and ignores the collective effect of harmonic sources,which often overestimates harmonic level of power grid.The focus of this paper is on the study of harmonic emission level for the residential nonlinear loads.On the one hand,through the analysis of operating mechanism of the equivalent circuit for the typical household appliances,an accurate harmonically coupled admittance matrix model is established in this thesis.Then,based on the above model,this thesis also proposes a kind of calculation theory for the total current of multi-harmonic sources;On the other hand,by use of the established accurate matrix model and the proposed theory,the collective effect of residential nonlinear loads is studied from the point of view of quantitative analysis.The main contents of this thesis and outstanding achievements are as follows:(1)Through the detailed analysis of the operating mechanism of the equivalent circuit for the typical household appliances,and by means of Fourier transform in frequency domain,a harmonically coupled admittance matrix model which can accurately reflect harmonic characteristics of the equivalent circuit is established.During the entire modeling process,any approximate treatment is not involved,which overcomes the original model's problem,dc-side voltages of the equivalent circuits are treated approximately,which leads to error in some degree.In addition,the corresponding turn-on angle and turn-off angle of equivalent circuit are precisely calculated using the methods of Gauss-Seidel and Newton-Raphson together,further improving the accuracy of the established matrix model.The comparison results of model calculation and time domain simulation prove the accuracy of the established model.(2)In this paper,the method for calculating total current of the residential multi-harmonic sources system is established.Based on the above method,the comparison results of programming calculations and the corresponding time domain simulations show that the method has a better applicability.(3)Based on the established matrix model and the proposed theory,harmonic attenuation effect,diversity effect and collective effect of residential multi-harmonic sources system are studied from the point of view of quantitative analysis,respectively.The results show that the system harmonic order is higher,and harmonic attenuation phenomenon is more obvious;the more the number of parallel loads in load-side is,the greater each harmonic amplitude attenuation will be,and the attenuation factor can perfectly measure the degree of system's harmonic attenuation;The corresponding diversity factors of different types of loads can well represent the contribution degree of the different types of loads to the system's total harmonic diversity effect;Collection factor can intuitively reflect the system's harmonic collection phenomenon.(4)The impact of the linear load on harmonic collective effect of the residential multi-harmonic sources system is analyzed in this thesis.The study results show that the addition of linear load can make total harmonic distortion rate of the system's total current reduce,and smaller linear loads are more beneficial to the reduction of total harmonic distortion rate of the system's total current.