Research On Control Scheme Of Low Voltage Ride Through Of Doubly-Fed Induction Generator System By Series Resitor

Energy crisis and environmental pollution are increasing human demands forrenewable energy. Wind power technology has developed rapidly as the mainstreamof new energy in recent years. With the increasing wind turbine capacity andpenetration of wind power, interaction between wind power systems and the gridcauses wide attention. DFIG(doubly fed induction generator), which is one of the twomainstream wind turbine manufactures, may cause trouble when the grid voltage sags,such as overvoltage and overcurrent in armature and the fluctuations in bus voltage.Then wind generators may split and grid collapse occurs. Therefore, low voltage ridethrough control strategies of DFIG for grid faults are appreciated. The paper focuseson DFIG in the existence of grid voltage sags. An external protection device and anoptimized excitation control strategy of DFIG for LVRT (low voltage ride through)are proposed. Simulations and experiments clarify the feasibility.Firstly, the operating principle of DFIG and its vector control strategy areanalyzed based on the model of a doubly fed wind power generation systemconstructed by PSCAD/EMTDC. The theoretical analysis of motor electromagnetictransient characteristics in the case of severe grid voltage sags is presented anddemonstrated by simulation.Secondly, the paper proposes a protection scheme of applying series adjustableresistor for power factor of wide range. The protection device is connected betweenthe step-up transformer of wind turbine and the bus bar in each phase. Each device iscomposed of non-controlled rectifier circuits and resistors paralleled with IGBTs. Thefull amplitude compensation method is used to calculate the initial equivalentresistance at the moment of power grid fault occurs for the transient phenomena ismost serious at that moment. The terminal voltage of the wind generator can be keptstable at the moment of the voltage drop occurs with this control strategy. Besides, anoptimization for low power factor environment is produced to improve the applicationof the protection device.Then, de-excitation control strategy based on sliding mode stator flux observeris proposed. Sliding mode flux observer tracks the stator flux and compares with itsown ac component obtained by using a50Hz band-pass filter to get the dc component. Thereafter, current for excitation of rotor side converter is regulated to establish theopposing leakage to the dc component of the stator flux. Thus the decay of transientDC component accelerates. Simulation results show the effectiveness of de-excitationcontrol strategy proposed.Finally, an experimental platform includes grid side converter of a small windpower system and resistor protection device is built. The protection device usedDSP2812as the core to regulate the duty cycle of PWM signals for the purpose ofadjusting the equivalent value of the series resistor. The symmetrical three-phasevoltage sags are simulated. The experiments show that the terminal voltage ismaintained at the desired range after putting the protection device into operation. Thefeasibility of the designed protection device is validated and the theoretical researchis proved.