Study On Control Strategies Of Doubly Fed Induction Generators Low Voltage Ride Through

In recent years, with the rapid development of wind power, the power ratio of wind power is increased year by year. The widen utilization of wind power in some Europen and American countries, such as Denmark and the United States, the ratio is more than 20% and 10% separately in the end of 2009. Some domestic regional power grid, such as Inner Mongolian Autonomous Region, now the ratio has reached to 12.8%. The influence of wind power on the connected system is more and more obvious. In order to maintain stable operation of power system, every country has issued wind power operating norms in series, and requests wind generators have some low voltage ride through(LVRT) capability. Hence, to study this issue has important theoretical and practical significance.This paper firstly reviews the current research on LVRT of doubly fed wind generators(DFIGs) at home and abroad, and relative analysis of the advantages and disadvantages of various research methods. Then the paper establishes a relatively complete dynamic mathematical models of DFIGs, and makes vector control strategy based on such models. This control strategy contains power decoupling control strategy of stator flux oriented and voltage decoupling control strategy of stator voltage oriented. A simulation results show that LVRT capability of DFIGs is very limited. In order to further enhance the capability, this paper proposes two novel LVRT control strategies.One method is applying nonlinear control method of input-output feedback linearization, designs a LVRT control strategy based on dynamic mathematical models of DFIGs. This method firstly makes control target to select output function and becomes input-output nonlinear system, which transforms to be linear controllable system by state feedback linearization and appropriate coordinate transformation. Finally, the design of system control strategy can be obtained by linear optimal control method. Such control strategy is only related to its parameters of wind turbine, which also improves robustness of system parameters. Another method is applying sliding mode control of variable structure to design a LVRT control strategy. This method can make system movement process into two phases by choosing sliding surface, then combining function Lyapunov and stability theory to design control strategies of every phase respectively to be the final 5-order LVRT control strategy, at last by ignoring stator magnetic flux it designs the simplified 3-order control strategy. In the process of designing control strategy, it takes outside disturbance and uncertainty of system parameters into account for the effect on modelling system, thus strengthening control robustness. Simulation analysis is carried out to each method, and results show that those control strategies have excellent control performance and enhance effectively the LVRT capability of DFIGs.