| Nowadays,the annual home electricity consumption in the world has reached more than 8.2 trillion kilowatt hours,which accounts for approximately 30%of society's total electricity consumption.According to statistics,the home electricity consumption of urban residents in China has reached 1.025 trillion kilowatt hours in 2019,and the total electricity consumption of residents currently accounts for 14.2%of the total electricity consumption.With the development of social economy and the improvement of residents'living standard,the total home electricity consumption will certainly increase year by year.Meanwhile,with the rapid development of power electronics technology,the proportion of electricity consumption by the nonlinear home load is increasing year by year,which makes nonlinear home appliances become a new type of harmonic source load.However,all kinds of nonlinear home appliances are injected directly into the power grid without any harmonic filter devices when they are connected to the power grid.For example,the current distortion caused by the home loads containing power electronic devices is very serious,and the total current harmonic distortion(ITHD)of most such home loads will exceed 100%.Although the power of a single home appliance is small and the harmonic current injected into the power grid is insignificant,the nonlinear home appliance of a low-voltage residential distribution network has the following characteristics:high quantity,wide distribution,high ITHD and strong randomness.As a result,the harmonic current content in the residential distribution network is high and the harmonic current will also randomly fluctuate,so the harmonic currents generated by nonlinear home loads can not be ignored in the low-voltage distribution network.If the harmonic current can not be controlled reasonably and effectively,it will flow into a higher power grid and cause a series power quality problems,which can result in many adverse effects on the safe and stable operation of the power system.Based on this,this paper will focus on the harmonic characteristics of the individual residential harmonic source load and the aggregate residential load,and establish its general harmonic models,then develop a probabilistic harmonic power flow algorithm that can analyze unbalanced residential distribution network,which can provide a theoretical basis for preventing and suppressing harmonic pollution in power system.The specific contents of this paper are as follows(1)A detailed frequency domain harmonic coupled model is presented for the single-phase diode bridge converter based home appliance.A parameter estimation method based on Z transformation is proposed which can simultaneously estimate the AC side and DC side circuit parameters of the single-phase diode bridge converter based home appliance.This method can estimate circuit parameters quickly without iterative calculation by estimating equation directly.Considering that the circuit parameters of some single-phase diode bridge converter based home appliances are dynamic in operation,a dynamic acquisition strategy is proposed,which can quickly and accurately estimate the circuit parameters during its dynamic change.Finally,the effectiveness of the proposed harmonic model,parameter estimation method and dynamic circuit parameter updating strategy is verified based on the measured data of various typical single-phase diode bridge converter based home appliances.(2)A dynamic harmonic coupled matrix model based on measured voltage and current data is established for nonlinear home appliances.To deal with the inaccuracy of model parameter estimation,the widely linear complex partial least squares method is proposed to estimate the harmonic coupled matrix model parameters.In addition,to increase the voltage adaptation range of the harmonic model,the model parameter adjustment factor is proposed,which makes the model voltage adaptive.A hierarchical K-means clustering method is proposed to determine the operating state of multi-state home appliances,and then harmonic models are established for different states of multi-state load based on the classified voltage and current data.Finally,the effectiveness of the proposed harmonic model is verified based on experiments and simulations.(3)A piecewise probabilistic harmonic model is proposed to analyze the aggregate harmonics of residential loads.Based on the field measurement data,a unified dominant harmonic coupled matrix model(UD-HCMM)is established.This model directly exhibits the coupled relationship between the harmonic voltages and currents of residential loads.In view of the random and fluctuating harmonic characteristics of home appliances,the multi-Gaussian curve fitting method is developed to fit the UD-HCMM parameters.Thereafter,the probabilistic UD-HCMMs are reconstructed using the Newton-Raphson method.Finally,a piecewise UD-HCMM is derived to characterize the time-varying harmonics of aggregate residential loads.The accuracy and validity of the proposed method are verified through simulations and field measurements.In addition,the proposed method is applied to evaluate the harmonic condition of a medium voltage distribution system with massive residential loads.(4)A method based on field theory is proposed to calculate the probabilistic harmonic power flow of unbalanced residential distribution system.In the unbalanced distribution system,the graph theory combined with current injection method is proposed to solve the deterministic fundamental power flow.In the harmonic power flow calculation,the proposed UD-HCMM is used to model the harmonic source load,which can reflect the coupling between the harmonics of nonlinear loads,and the decoupling harmonic power flow calculation formula is established.In view of the probabilistic harmonic power flow,the probability density function of a small number of Monte Carlo simulations is estimated by the probabilistic density estimation based on field theory.The effectiveness of the proposed probabilistic harmonic power flow algorithm is verified in IEEE-13 and 123 node systems. |
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