Research Of Control Strategies For Converters Used In Doubly-fed Wind Generators

With the increase of energy consumption, energy crisis has become the most important problem that restricts the development of human society. Human have to search new energies to replace the traditional fossil energy, for it's unrenewable and environmental problems when it's consumed. Among the new energies, wind power has equipped to a large-scale wind-capacity for its advantages in cost and technology, it has gradually developed to large capacity of generating unit and offshore wind projects.In the wind turbine generators, variable-speed constant-frequency (VSCF) wind power technology can be changed by wind speed with doubly-fed induction generator (DFIG) to keep output in constant frequency, the capacity of rotor-side convector is small(about 30% of the full power) and has nice control characteristic, so wind energy generation control system with DFIG is widely used in wind turbines. However, China's independent research and developing capabilities in power unit should be enhanced in the future. So in this paper, it designs the wind energy generation control system based on doubly-fed induction machine.In this paper, it models of natural wind speed simulation and analyzed the relation between wind power generator's output power and wind power's coefficient, with this and succeeds in tracing maximum wind-energy; discussed the four states of wind energy generation system running in different wind speeds, and the conclusion provides foundation for the rotor-side convector's control.Mathematical model of DFIG under static three-phase coordinates (STC) and Synchronous Reference Frame (SRF) is also analyzed in this paper, it designed the control system for rotor-side convector based on stator magnetic field's vector control technology. In the system, it gets the rotor-side convector's reference value in the structure of double close-loop of power and revolving speed, then it succeeded in active and inactive powers' decoupling control using hysteresis current PWM control technology.This paper analyzed network-side converter topology and the mathematical model, designed the control system with phase stator voltage oriented stator magnetic field's vector control technology to rotor-side convector. Current and voltage dual-loop structure were used in the system, to keep the DC voltage constant, succeeded achieving running under unity power factor. Also, the grid voltage sag's impact on the system and several rotor side protection circuit topology of low voltage ride through is discussed in this paper; it uses the topology of IGBT plus diode Rectifier Bridge and resistance, then it'll real-time detect the sag of the grid voltage with dq transmission method when three-phase symmetrical voltage fault occurs in the power grid; simulates the dynamic response of the system.In a word, the whole system has been modeled and simulated in PSCAD/EMTDC simulation environment. The results showed that the system can achieve maximum power tracking at different wind speeds; rotor side converter achieved a very good active and reactive power decoupling control; network-side converter maintain a stable DC voltage; the low voltage ride through can be achieved when occurs sharp voltage sag.