Research Of Nine-switch United Power Quality Conditioner Based DFIG Wind Power System

With the dramatic development of wind power,the proportion of wind turbines gradually increased.The DFIG wind turbine system occupies large scale of market with mature technologies and lower cost.The grid power quality problem,especially the voltage drop,will have obvious negative influence on DFIG,may even cause the system off grid,which brings the potential safety hazard to the safety and stability of power system.In order to reduce affection of power quality problems to DFIG wind turbine system,such as voltage harmonics,swells,drop and unbalanced voltage,the article raised a plan of nine-switch UPQC to be applied to DFIG system.Nine-switch UPQC is the improved structure of back to back converter,and reduces number of switches from 12 to 9 by applying shared switches.To some extend,it helps to reduce the complexity of driver circuit and protect circuit to improve to power density.By using the proper modulation method of drive impulse,it can achieve the independent control of two separated output ports.The plan of UPQC can achieve flexible fault ride through(FFRT)using voltage compensation which gives better performance of transient period and can improve the result of harmonic voltage and current.It has comprehensive function and high components utilization characteristics.This article studied the restrictive relation of nine-switch UPQC when sharing the switching element components,and on the basis of this relation,the article designed two-channel modulation method which can be controlled independently.Then taking nine-switch UPQC as inverter circuit,verified the topology of UPQC including double fed induction generator.On the basis of modeling the operation circuit of voltage and current compensation unit in UPQC,the article also designed the corresponded control algorithm.According to the power grid condition,4 types of working scenario was set to optimize the operation scheme of UPQC,including no compensation,harmonic circuit compensation,mild voltage compensation,severe voltage compensation.Finally,simulation and corresponded experiments were given to verify the validity of the equipment.In order to solve the running high voltage problem on the DC side of nine-switch UPQC,the article raised a plan of dynamic voltage modulation ratio arrangement to ensure the higher priority functions.On the basis of grid voltage condition,dynamically arrange amplitude limitation of modulation ratio on current compensation and voltage compensation side.By doing this,the overall voltage utilization of DC side has been improved,and this has been approved with the simulation result.The final stage experiment includes the researches towards the producing of nine-switch UPQC driving impulse,dead zone arrangement,nine-switch IGBT drivers and investigates the method of nine-switch UPQC in industrial implementation.The result shows that,the nine-switch UPQC can suppress harmonic voltage and current,eliminate voltage deviation and phase jump,which significantly improves powerquality of DFIG side.Therefore,the DFIG system can achieve flexible fault ride through(FFRT).