Research On Balance Method Of Three-phase Load In Low Voltage Distribution Network Based On The Voltage Source Converter

With the development of science and technology,electrical equipment is becoming more and more compllex.The non-linear,impact and unbalanced loads are heavily connected to the low-voltage distribution network,which has a serious impact on the power quality of the low-voltage distribution network.SVG and APF can match the reactive power and harmonics of the power grid are well compensated.The three-phase voltage and current imbalance caused by the low-voltage three-phase unbalanced load will increase the distribution network loss and reduce the utilization of power distribution equipment.Unbalanced governance has become an important direction in the application research of power electronics technology.In this thesis,the three-phase four-wire active unbalance compensation device is studied.The three-phase unbalance current detection technology,current control strategy and the third harmonic current problem caused by unbalance compensation are studied.For the wiring mode of the low-voltage distribution network side in China,in order to compensate the negative sequence and zero-sequence current at the same time,the three-phase four-bridge voltage source converter is selected as the converter topology of the active unbalance compensation device.Discourse the principle of three-phase unbalance compensation,the mathematical model of the active unbalance compensation system is established,and the main circuit parameters of the unbalance compensation device are designed.The unbalanced current detection method based on instantaneous power theory and instantaneous symmetrical component method are compared.From the aspects of detection accuracy and detection algorithm complexity,the ip-iq method based on instantaneous power theory is selected.Because it is applicable to the detection of three-phase unbalanced current and is verified by algorithm simulation.Aiming at the characteristics of power frequency periodicity of unbalanced compensation current,the current control strategy based on repetitive controller is studied.The working principle of the repetitive controller and the stability conditions of the control system are analyzed.The controller parameters of the active unbalance compensation device are designed.The reason why the DC voltage fluctuation when the active unbalance compensation device compensates for the three-phase unbalanced current and the propagation mechanism of the third harmonic component of the compensation current is analyzed.The measure that add double frequency notch to DC voltage feedback channel in order to eliminate the extra third harmonic component of the command current is proposed.The simulation model of the three-phase four-bridge arm unbalance compensation system is established.The simulation result verifies the effectiveness of the repetitive control and third harmonic elimination methods.An active unbalance compensation experimental prototype with a capacity of 66kVA/380V was developed.The unbalanced current detection method,current loop repetitive control strategy and third harmonic elimination method studied in this thesis were used to complete the relevant experiments.The experimental results show that the effectiveness of the above theory.