Research On Power Quality Comprehensive Treatment Device For Low Voltage Distribution Network

Since entering the new century,with the development of emerging technology,power electronic equipment are widespread used in industry,medical treatment,transportation and other aspects,so that human production and living standards greatly improved.However,these equipment usually have nonlinear feature,which will consume a large amount of reactive power and generate harmonics during operation.Moreover,due to the uneven distribution of three-phase load,the three-phase current on the grid side will be unbalanced,which seriously affects the power quality of the distribution network.In order to meet the user’s demand for power supply quality,the study of low-voltage distribution network power quality comprehensive treatment device has practical engineering significance and economic value.When using active power filter to treat,it has good real-time and accuracy.But using only active power filter to treat will causes high costs and limited capacity due to stability.In order to resolve these problems,this thesis designs a joint treatment device of static var compensator(SVC)and active power filter(APF),which increases the compensation capacity and reduces the cost while ensuring the treatment accuracy.It gives full play to the advantages of themselves,and achieves favorable comprehensive treatment effect.Firstly,this thesis expounds the overall structure of the treatment device,which includes two parts:SVC for compensating reactive power and treating negative sequence current and APF for restraining harmonic and treating zero sequence current.The running principle of SVC is explained in detail,the compensation admittance formula is deduced,and the functional relationship between compensation admittance and thyristor conduction angle is established.Aiming at the harmonics generated by SVC,a type I three-level APF is proposed,and its main circuit is modeled and analyzed,and the carrier modulation strategy is selected.Secondly,this thesis introduces the application ofi_p-i_q current detection algorithm in three-phase four-wire system,and improves its defects.A single-phase current detection algorithm is adopted to avoid the complex matrix calculation.The control strategy of the treatment device is studied.Especially for the problems existing in the traditional PI control,a compound control of quasi proportional resonant(PR)and repetitive control in series is adopted,which broadens the response frequency band and improves the control accuracy and anti-interference ability.Simulink is used to simulate the treatment scheme,and the effectiveness of the detection and control algorithm is verified.Finally,according to the main circuit topology and the algorithm of detection and control,this thesis completes the hardware and software design of power quality comprehensive treatment device.A test platform is built.Through the analysis of the measured data of the system before and after treatment,it is verified that the treatment scheme can effectively improve the three-phase imbalance of the low-voltage distribution network,suppress harmonics,and compensate reactive power,which has practical application value.