Research On Low-voltage Ride-through Of Doubly-fed Induction Generator Based On Equivalent Inductance Strategy

With the increasingly serious situations of environmental pollution and energy crisis, the new clean energy has received more and more attention with its advantages of clean, low-carbon and sustainable utilization. As one of the most widely used renewable energy, the wind energy and its penetration in power system is developing very rapid in recent years, with more greater impact on grid. To ensure the stability of power systems, the grid codes of various countries require the grid-connected wind power generators having the abilities of Low Voltage Ride-Through(LVRT), which means the wind energy systems has to remain connected to the grid and even provide reactive power during certain grid faults. Because of the relatively small capacity of converter, the low cost and the active and reactive power can be controlled independently, Doubly-Fed Induction Generator(DFIG) has been the mainstream generator in wind power. However, voltage fluctuations during grid faults can be easily coupled to DFIG due to the structure that the stator directly connected to the power grid, which will influent the safety and stability of power systems. Therefore, the LVRT issues which safety, reliability and efficiency must be solved from the basic theory and key technical levels.The existing LVRT methods focus on two aspects: to change the control strategies and to change the circuit configuration which based on hardware protection. Magnetic control and improved vector control are usually used as common control strategies, and in terms of hardware protection, crowbar is commonly used to short pass the converter as a practical method.The mathematical model of DFIG is built in this paper, and on the basis of the transient process, the reason of over-voltage and over-current are been analyzed. Based on the summary of the advantages and disadvantages of the above two LVRT technologies, a control strategy based on active crowbar method is proposed, which means that the port of electromotive force(EMF) is considered as an equivalent impedance. Through analysis, the DFIG can be in the most conducive condition to achieve LVRT when the impedance is a pure inductance. Then, the port characteristics of some mainstream control strategies are calculated and analyzed from the perspective of voltage-current characteristics. It is shows from the results that the ports of EMF in these kinds of strategies are all small inductances, and based on this, a novel fault control strategy based on equivalent inductance is proposed.The effect of equivalent inductance control strategy is verified using simulation models built with Matlab/Simulink, and a 2.5kW DFIG presentation system has been built, which is equipped with an equipment to simulate the voltage faults. The feasibility and effectiveness of the strategy are verified through simulation and experiment. Compared with control strategies based on magnetic control and hardware protection based on crowbar resistance, This method contains the following characteristics:(1) Specific physical concept;(2) Simple to control, without complex separation of flux and the switch of strategies process;(3) Torque ripples are closed to zero;(4) Since the value of the equivalent inductance can be regulated during grid faults with different types and degrees, this method has better robustness and flexibility compared with control strategies with fixed magnetic coefficient and tracking coefficient;(5) Compared with the hardware protection with fixed resistance, this method does not require additional hardware, has no need to block rotor-side converter, and does not to consider the questions of the value and duration of the resistance;(6) The equivalent inductance of the port can be seen as a standard to judge whether a method can achieve LVRT.