Intelligent Commutation Control System For Three-phase Unbalanced Low Voltage Distribution Grid

In recent years,in the three-phase four wire system of the low voltage side for the distribution grid,the phenomenon of asymmetrical and uneven single phase load are very common,causing power unbalance in certain network.When unbalance exists,the system will have a larger line loss and the unbalanced loads can result in reducing of efficiency of power energy and affecting the voltage quality.And then the safety and stabilization of power system in the low-voltage distribution grid will be directly affected.Therefore,based on the problems above,combined with the characteristic of the low voltage distribution grid in the three-phase four wire system,a three phase load unbalance automatic regulating system for low voltage distribution grid is designed.The three-phase unbalanced intelligent commutation control system of the low-voltage distribution grid is completed by the intelligent commutation terminal installed at the outlet side of the transformer and the intelligent commutation switch installed at the user side.The intelligent commutation terminal detects the three-phase current of the transformer in real time through an external current transformer,and analyzes and processes the data to determine whether it is in an unbalanced state.All the commutation switch current information dispersed on each user side is sent to the intelligent commutation terminal per several seconds.If the system is in an unbalanced state,all the information are used to determine the equilibrium data needed for each phase change value.The control signals are sent to a commutation switch and driving thyristor and magnetic latching relay.In order to achieve a relative balance,we need to move unbalanced current from the current phase transfer to small current phase.Under the background of analyzing the causes of three-phase unbalance and the harm caused to the power grid,this dissertation firstly briefly summarizes the research status of improving three-phase unbalance suppression methods at home and abroad.Then starting from the basic concept of low-voltage three-phase unbalance,this dissertation compares the advantages and disadvantages of several existing three-phase unbalance calculation methods,and proposes an average symmetric component method to calculate the three-phase unbalance.Based on the above basis,the overall design scheme of the three-phase unbalanced intelligent commutation control system for low-voltage distribution network is proposed.The overall composition framework of the intelligent commutation control system is constructed.According to the functional requirements and technical index requirements of the system,the intelligent commutation terminal control strategy based on the load distribution prediction algorithm and the compound switch commutation strategy in the intelligent commutation switch are proposed.The hardware and software of the intelligent commutation switch and the intelligent commutation terminal are designed respectively.In terms of hardware,DSPIC digital signal controller is adopted as the core hardware system.Hardware system mainly deal with the stable voltage power supply module,clock circuit,data acquisition circuit,communication module design,liquid crystal display circuit,temperature measurement circuit,phase change switch drive circuit and other parts.In terms of software,the development of programs is completed under the MPLAB compilation environment with C language and assembly language.Software mainly composed with the design of main program,data acquisition and processing program,copy design of intelligent phase changing terminal,three-phase unbalanced phase changing control strategy program design,voltage and current detection processing program design,intelligent phase changing program design and communication module part program design,etc.The most critical modules are the AC sampling of three-phase voltage and current,the real-time calculation of the phase of voltage and current,and the real-time coordination between the calculated phase and the trigger time of thyristor.At the end of the design,the whole system is put into operation.Through the experiment and the field operation data in a village,the system can reasonably adjust the three-phase load imbalance of the low-voltage power grid.The system makes low voltage grid as close as balance state.It reduces the distribution network loss and improves the power quality,and has a good promotion and application value.