Improvement Research On Transient Stability And Power Quality Of Distributed Wind Power Integrated System

With people placing more attention on sustainable development and reducingenvironmental pollution, distributed generation (DG) technology based on renewableenergy for its unique environmental protection and economical efficiency is attractingmore and more attention. But wind energy among DG is affected by natural conditionsso as to can not output power continuously and steadily, so wind energy generationincorporated into the electric power system will bring a series of issues for its steadyoperation, especially when wind power integrated system is fault. Because of windfarms mostly are in remote areas and often are at the most endings of the grid, it willinevitably make power flow of wind power integrated system change, and has aninfluence on electrical energy quality and stability of the grid.Therefore, several solutions are proposed to improve transient stability andpower quality of wind power integrated system in this dissertation. Distributed windpower integrated system is analyzed systematically and deeply in this dissertation,respectively from these aspects including different type of wind generator, energystorage system, differential evolution (DE) method, simultaneous perturbationstochastic approximation (SPSA) algorithm, electric power system stabilizer (PSS),automatic voltage regulator (AVR) and flexible AC transmission systems (FACTS)equipment, the main research results are as follows:（1）The combined scheme of super capacitor energy storage system (SCESS)based on fuzzy logic control and doubly-fed induction generator (DFIG) is proposed.In order to improve energy efficiency and stability and reliability of system, usuallyenergy storage device is used in DG, using SCESS is able to control active power andreactive power at the same time, so it can compensate power fluctuations; The fuzzylogic controller can be used in the stable system, DFIG can also be used to adjustreactive power and improve transient stability of power system. For this reason, thecombined scheme of fuzzy logic control based on DC-DC step-down/boost converterand voltage source inverter control based on PWM is put forward, modeling andsimulation analysis of the wind power integrated system are done by usingMatlab/Simulink software. The study results show that this scheme can improvestability and power quality of wind power integrated system. (2) The maximum wind power point tracing control strategy based on SPSAalgorithm is proposed. In order to make wind power generation system being freefrom outside interference, and easily operating at maximum power point underfluctuant wind speeds, using SPSA algorithm to track system maximum power isproposed in this dissertation. SPSA algorithm indirectly controls wind turbine speedby adjusting duty cycle of power converter, makes system operate at the maximumpower point under any wind speed, and avoids using anemometer and tachometer inpractical applications. As shown in the simulation results, no matter how wind speedchanges, SPSA algorithm can effectively improve power output of wind powergeneration system, and make tip speed ratio and coefficient power be near optimumvalue.(3) The combined scheme of static synchronous compensator (STATCOM) basedon voltage source converter (VSC) and PSS based on synchronous generator (SG) isproposed. STATCOM is in parallel with the power grid, it is equivalent to acontrollable reactive current source, the reactive current can quickly vary with theload reactive current, it can automaticly compense the required reactive power ofsystem and realize dynamic reactive power compensation for the grid. But when thesystem is without excitation controller, even increases STATCOM, the system may notbe able to remain stable, so it is considered that PSS is installed in SG and itsparameters is adjusted appropriately. In order to ensure that transient stability andpower quality of wind power integrated system is not decreased, usingSTATCOM-PSS scheme to improve power quality and fault through ability of theintegrated wind farm is studied in this dissertation.The research results show thatSTATCOM-PSS can effectively restore voltage after fault at the point of commoncoupling in wind farm, enhance the fault through ability of wind farms, improvetransient stability and power quality of wind power integrated system.(4) The AVR-PSS coordinated control strategy based on DE algorithm isproposed. For solving the stability problem of wind power integrated system, PSSsare installed in SGs from the consideration of economics, but AVR provides transientstability at expense of robust oscillation stability; PSS provides damping andoscillation stability at expense of AVR. So it is necessary through the synchronousadjustment of AVR and PSS to obtain good voltage regulation and oscillation damping,using DE algorithm to solve the optimal regulation problem of AVR and PSSparameters in SG is presented. Under the situation of with or without PSS andwhether using differential evolution algorithm, the stability of wind power integrated system is studied in this dissertation. The research shows that wind power integratedsystem has a good damping effect using AVR-PSS coordinated tuning based on DEalgorithm, it can reduce oscillation of rotor angle difference and improve voltagestability.（5）The control scheme based on DFIG-FMAC-PSS is proposed. For the systemwith a lot of wind power, the solution of PSS being installed in synchronousgenerators may no longer provide additional damping, if synchronous generatorsequipped with these devices are cut off to adjust wind power, and the dampingobviously is lost. Therefore PSS being installed in DFIG is considered, anothercontrol scheme of PSS along with flux magnitude and angle controller (FMAC) usedin DFIG is proposed in this dissertation. Stator power of DFIG is chose as PSS inputsignal, through adjusting rotor flux vector magnitude and phase angle of DFIG tocontrol output power and voltage of generator. Under the situation of largedisturbance, the system stability is researched, this research shows that the proposedscheme is economic and feasible, this scheme is superior to the PSS control schemebased on the conventional synchronous generator for providing voltage control,contributing to damping and improving the stability of wind power integrated system.