Analysis Of Collective Harmonic Characteristics Of Civil Distribution Network

With the rapid development of China’s power electronics technology and the growing awareness of residents on energy conservation and environmental protection,non-linear power electronic devices are widely used in low voltage distribution networks,especially in civilian distribution networks.When these non-linear loads are turned on,the harmonic current produced by them will be injected into the power system.Although the power of individual device is small,the current waveform distortion is serious.Coupled with the large population of residents in our country and the densely populated areas,the harmonics generated by these devices cannot be ignored.According to the data released by the National Energy Administration in 2017,the total electricity consumption in 2016 was 599.8 billion kw-hr,and the electricity consumption of residents was 805.4 billion kw-hr,accounting for 13.6%of the total electricity consumption.This percentage is The United States is about 37%.So there is still a large space for growth in electricity use by our residents.However,as the electricity consumption of residents increases,the harmonics generated by non-linear civil loads will also become more and more serious.Therefore,Therefore,it has long-term significance to study the collective harmonic characteristics of residential distribution networks.In the past,researches on harmonic devices mainly focused on medium and large-scale harmonic sources,such as rectifier devices in DC transmission systems,variable frequency speed-regulating motors,and electric arc furnaces and so on.Research work on harmonic analysis of low-voltage civilian loads was relatively scarce.Due to the random and time-varying characteristics of harmonics generated by civil loads,the influence of system voltage on each harmonic current is difficult to determine theoretically.Therefore,based on the measured data includes low-voltage civil loads and commercial loads,this thesis analyzes the power consumption characteristics and harmonic characteristics of different public connection points in distribution network from different perspectives.And based on the measured data,a dynamic harmonic model was established to evaluate the harmonic effects of the aggregate load in the distribution network.The data collected in this thesis includes four different test points in the distribution network:a single user’s distribution box,the floor distribution trunk of multiple users,the low-voltage side of the power transformer,and the high-voltage side of the transformer.By analyzing a large amount of data at these measurement points,it can be found that different locations of the distribution network have some of the same and different power characteristics and harmonic characteristics.For example,no matter what test point,the difference between the values of the active electrical energy values obtained by considering the harmonics and without considering the harmonic calculation is very small.There is a clear positive correlation between active power and TDD.However,the major harmonics are not the same in different tests.The main harmonic currents at a single user are 3th and 5th,while the primary harmonic current is 3th at the floor distribution trunk and the low-voltage side of the transformer.And it is obviously different from other harmonic currents.At the high voltage side of the transformer,the major harmonics are changed to 5th and 7th due to the existence of harmonic cancellation.In addition,the distributions of the harmonic currents’amplitude and phase angle also have different characteristics.The amplitudes of harmonic currents at a single household exhibit multi peak characteristics,while the amplitudes of many harmonic currents at the floor distribution lines are unimodal,which is closer to a normal distribution.In terms of phase,the range of change of the latter is also smaller than that of the former,and the distribution is closer to a normal distribution,indicating that as the number of users increases,the characteristics of its harmonics become more regular.According to the harmonic characteristics obtained from the analysis,based on the measured data,this thesis proposes a harmonic modeling method based on recursive least squares method.The modeling method has the advantages of fast calculation speed,small storage space,and real-time identify the characteristics of harmonic model parameters.It is verified by MATLAB simulation that the model proposed by this thesis has a good match.And with the increase of the selected sample data,the description ability is stronger and the accuracy is higher.The research work in this thesis lays the foundation for harmonic evaluation and harmonic control in low voltage distribution system.