Simulation On Low Voltage Ride Through And Unbalanced Voltage Operation Of Grid-side Onverter Of Direct-drive Wind Power Generation

With the rapid development of wind power industry, large-scale wind power turbines connect to the grid, their stable operation is very important to the security of grid. Direct-drive wind power system because of its advanced performance of high energy conversion efficiency, high reliability and flexibility of power control becomes one of the mainstream models. In this paper, the grid-side converter of grid-connected direct-drive wind generator is studied. Its control strategies of low voltage fault ride through when system voltage balanced drops and unbalanced voltage occurs are researched. The simulation results show the correctness and effectiveness of the control strategies.First, the development and research status of the wind power industry and wind power generation technology is investigated. A brief analysis of the issues of the low voltage and grid voltage unbalanced fault existing in wind power generation system technology, which identified the study object and the main content of this paper.Second, the structure and principle of direct-drive permanent magnet synchronous wind power system is introduced. The steady-state mathematical models of permanent magnet synchronous generator (PMSG) and grid-side converter are established. Generator-side converter control based on rotor flux orientation of stator zero d-axis current control, grid-side converter based on system voltage oriented, DC voltage control strategy and active and reactive decoupling control strategies have been discussed.; According the mathematical models and control strategies, a simulation model is built based on the PSCAD/EMTDC simulation software, and the maximum power point tracking, DC side voltage control of the grid-side converter, decoupling control of active and reactive power were simulated and analyzed.Then, briefly introduced the domestic and international typical standards of wind power connection to the grid on low voltage fault ride through (LVRT); analyzed the causes and classifications of the voltage drop and its testing methods; analyzed low voltage operation characteristics of direct-drive wind turbines. For the realization of direct-drive wind turbine system through the low voltage fault, respectively studied the control strategies of DC side Crowbar protection circuit and control strategy of grid-side converter operating on the STATCOM model.Finally, The stable operation of the grid-side converter under abnormal condition of unbalanced grid voltage drop is studied. Using the method of symmetrical component, a grid-side converter instantaneous power mathematical model under system unbalanced voltage has been established. DC voltage fluctuation caused by negative sequence voltage is analyzed. On this basis, in order to suppress doubler fluctuations in DC voltage, calculated the reference current according to a given power. With the independent control of the positive and negative sequence current control strategies, grid-side unbalanced voltage control effect of control strategies proposed were simulated. Simulation results show the effectiveness of the proposed control strategy.