Research On Low Voltage Traversing Technology Of Concept - Fed Wind Power System

With the rapid development of wind power technology, the installed capacity and the scale of grid-connected of wind turbines has been increasing. The large-scale off-grid Wind Turbines result from the gird voltage drops seriously affect the stability of the power system. In order to meet the requirements of grid-connected standard for wind power, effective measures of low voltage ride-through (LVRT) should be taken to keep the wind power system under uninterrupted operation and provide reactive power support to the grid. Compared with the research of LVRT for Permanent Magnet Synchronous Generation (PMSG) wind turbines and the Fixed Speed Induction Generator (FSIG), doubly-fed induction generator (DFIG) is particularly sensitive to grid failure. So, this thesis mainly deals with the LVRT for doubly-fed induction wind systems. The main contents are as follows:Firstly, the model of the doubly fed induction generator is established according to its mechanical, electrical and physical properties, on the base of which, the suitable dynamic model of low voltage operation research is obtained. The control strategy based on sinusoidal pulse width modulation (SPWM) of rotor flux orientation control generator-side converter and the SPWM of the grid-side converter are confirmed. The effectiveness of DFIG wind power system dynamic model is verified by simulations under different working conditions.Secondly, according to the basic law of circuit, the electromagnetic transient characteristics of DFIG wind power systems are analyzed, especially for the change of the stator flux, rotor current features and DC voltage during voltage drops. The transient characteristics are simulated in PSCAD/EMTDC when three-phase symmetrical fault happens.Thirdly, the coordinated protection measures based on the stator of DFIG series resistor and super capacitor energy storage system is proposed, and the reactive power compensation of gird-side converter was put forward by the adjustment ability of DFIG itself. The simulation result in power system computer aided design and electromagnetic transient including (PSCAD/EMTDC) show that coordinated protection measures can effectively help to maintain the DC-link voltage and inhibit the over-current, which improve the ability of low-voltage operation of DFIG.Finally, the active low voltage protection measure was put forward to compensate for the lack of the above scheme. The simulation result in PSCAD/EMTDC show that the protection of the gird-connect point series resistor without adding other protective measures can effectively improve the ability of LVRT for DFIG.