직접 구동형 풍력발전 시스템의 계통연계를 위한 제어 성능 개선

Abstract

These days, the world energy market has been interested in the development of new and renewable energy with the increase in the concern about atmospheric pollution and global warming by fossil fuels and the excessive demand on the energy. Among the new and renewable energies, wind power system has been increased in its extent as the alternative energy technology which the wind energy is converted to the electrical energy. In terms of the electrical problems, since the wind speed which is the energy resource of the wind turbine is changed irregularly according to the meteorological conditions the output of the wind turbine also changes irregularly and the fluctuation of the active power finally influences the power qualities of the connected grid such as the total harmonic distortion, frequency and power factor. The small scale wind power systems connected to the distribution line has been being changed to the large scale wind power systems connected to the transmission line along with the increased penetration of wind energy in the power system. In the case of grid connection to the AC line power transmission system, there could happen a variety of problems such as voltage stability, transient stability and the ripple effect of the faults which have not been considered in the small scale wind power systems. So, on the grid codes of the developed countries in wind power, the wind farms or the individual wind turbines connected to the grid are required to have various control strategies for the active contribution to the stable operation of the power system.The variable speed-variable pitch direct drive wind power system using full rated power converter and multi-poles permanent magnet synchronous generator is the noticeable wind power system due to its high efficiency at the low wind speed region as well as its easier maintenance and reliability. Besides, since the wind turbine and the grid are isolated by the full rated power converter the system has a good capability in the grid faults. Therefore in this thesis, the control capabilities based on the direct drive type wind power system using multi-poles PMSG are secured for the satisfaction with grid codes.Firstly, mechanical-electrical system is modeled to evaluate the power and the control performance of the wind turbine according to the wind speed and the control strategies for the cut-in through cut-out wind speed are explained. Starting from modeling the rotor effective wind speed at the hub height, non-linear aero-dynamic torque, two mass drive train, electrical system including PMSG, and pitch control system are modeled and the maximum power point tracking(MPPT) and pitch control are implemented according to the basic control strategies of the wind turbines.Secondly, controls and design of wind power system including the power converter to meet the grid codes of the advanced countries in wind power such as active-reactive power control, frequency control, an operation of the wind turbines when faults occur, and the power quality are discussed. For the active power and frequency control of wind farms, a concept of a wind farm controller is introduced and some control methods to meet the seven active power control standards and a frequency control standard. Particularly, the low voltage ride through(LVRT) regulation which is the remarkable issue is explained and the LVRT strategies for the grid connected PMSG wind turbines under the distorted-unbalanced grid conditions are introduced.Lastly, the control methods introduced in this thesis are verified by simulation and the characteristics of the wind power system using a full rated power converter are analyzed based on the simulation results. With the analyzed results, control strategies for the direct drive type wind power system using multi-poles permanent magnet synchronous generator and a full rated power converter are summarized and the direction of new researches is established.