Study On Reactive Power Compensation Of Urban Low-voltage Distribution-network Load

With the rapid construction of city,the distribution of power load becomes more and more complex,which has a serious impact on the power supply quality of the system.The application of non-linear load on the user side increases the reactive power flow of power grid and causes the three-phase imbalance.Therefore,it is necessary to introduce reactive power compensation device to optimize power quality of low-voltage power grid.STATCOM has the advantages of flexible compensation,high efficiency and energy saving,which can meet the requirements of modern power system.First,the main circuit topology selection of STATCOM is analyzed,and three-phase fourleg inverse bridge STATCOM is selected for comprehensive consideration of economy,practicability and efficiency,and its working characteristics and mathematical modeling are briefly introduced.Then,phase-locked loop is introduced in coordinate transformation of compensation current detection method adopted by STATCOM,while traditional phase-locked loop(PLL)can’t fast track of voltage phase change of low voltage power grid.Improve to the phase-locked loop of decoupling synchronous coordinate transformation in order to enhance the real-time and precision of calculation process,and introduce the positive sequence,negative sequence and zero sequence component of load current that is calculated by the sequence separation method and coordinate transformation as the input of the current control ring.STATCOM compensation control strategy is an important part of its function realization.The design adopts the voltage and current double closed loop control strategy,the current loop control realizes its compensation function,the voltage loop control maintains its stable operation.Direct current control method is adopted for three-phase four-bridge arm STATCOM.Three groups of three-phase bridge arms adopt the positive and negative sequence current synchronization compensation strategy based on feedforward decoupling under rotating coordinates for the separated positive and negative sequence currents,which can simultaneously realize three-phase reactive power and imbalance compensation.The fourth bridge arm uses hysteresis comparison control strategy to compensate the separated zero sequence component current,which meets the requirement of the Central-Line operation of the system.Due to the poor anti-disturbance performance of STATCOM DC-side voltage control,a Fuzzy-PI voltage closed-loop control method based on genetic algorithm is proposed,and the controller is introduced into the voltage control to improve the operation efficiency of the device.The current loop control can realize the compensation function of reactive current and unbalanced current,and the voltage loop control proposed can realize DC-side voltage stable and improve the anti-interference ability.The dual-loop control not only achieves the real-time reactive power compensation function,but also eliminates the three-phase imbalance and maintains the three-phase symmetry,the anti-interference ability is also enhanced.Finally,A model of STATCOM compensation system is constructed for low voltage distribution network on MATLAB simulation platform,fast track dynamtic reactive power compensation in the random switching of capacitive reactive power and inductive reactive power,and quickly implement unbalanced three-phase compensation by simulation results.The real-time and precision requirements of the system is satisfied by the proposed compensation current test and control strategy.