Research On Key Technology Of Inductive Filtering Transformer In Low-voltage Distribution Network

In the modern urban low-voltage distribution system(LVDS),the operational conditions of transformers are challenged by the integrated nonlinear loads,which poses a threat to the power quality and secure operation.Under the Ampere-turn equilibrium principle,an inductive filtering transformer(IFT)can keep the harmonics at the secondary side using a special structure design.This isolation effect can improve the operational condition of the transformer and power quality.IFTs thus attract intensive attention in the high-voltage direct current transmission.Particularly,three-phase-three-wire systems witness the wide-spread application of IFTs.In this thesis,some key techniques of IFTs are investigated to solve the threephase imbalance problem in the three-phase-four-wire urban LVDSs:(1)By analyzing the operational characteristics of the LVDS,a Ddy IFT and its filtering system are designed.Specifically,the grid-side and load-side windings use the D and y connections,respectively.The filter winding uses d connection and is connected with the fully-tuned filter branch.The mathematical model of the newly developed Ddy IFT is deduced,and the design of filter winding and passive filter branch is presented.Performance of the proposed Ddy IFT is compared with the conventional10 k V distribution system transformer and its filtering system,proving the heterogeneous characteristics of harmonics when transmitting in the transformer.Simulation studies show that the proposed Ddy IFT has better harmonics attenuation capability.Moreover,the Ddy IFT can improve the electromagnetic environment of the transformer and prevent harmonics from penetrating into the grid-side winding.(2)Considering the limited filtering capability of passive filtering,an active inductive filtering system(AIFS)for the urban LVDS is developed.The AIFS is composed of a Ddy transformer,a fully-tuned filter branch and a voltage-source converter(VSC).First,the circuit and mathematical models of the AIFS is deduced,and the filtering principle of AIFS in the urban LVDS is studied.Further,the impedance calculation method for the filter winding and fully-tuned filter branch is presented,based on which the harmonic damping control of VSC and zero-impedance control scheme is designed.The simulation shows the superiority of the proposed AIFS in harmonic attenuation considering both balanced and unbalanced loads.(3)A novel DC magnetic biasing attenuation approach based on AIFS is developed.By studying the effect of single-phase DC injections upon excitation currents,the positive correlation between the AC harmonics and DC magnetic biasing is revealed.Further,the harmonic attenuation of the proposed AIFS under DC magnetic biasing is studied.By comparing the passive and active inductive filtering performance,the DC magnetic biasing attenuation strategy using the Ddy AIFS is presented.The simulation shows the explicit attenuation performance under DC magnetic biasing,and the active filtering performance is better than the passive one.(4)A 10 k V 30 k VA inductive filtering transformer and the distribution system platform are developed.The capacity of the grid-side,load-side and filter-side windings is calculated based on the Ampere-turn equilibrium principle.Then,the iron core and winding structure is given based on the zero-impedance requirement.The experimental results show the developed transformer platform contains less harmonics and has reactive power compensation capability.There are 123 figures,16 tables and 190 references in this dissertation.