Research On LVRT Of BDFIG Wind Energy System Converter

This thesis presents the brushless doubly fed induction generator (BDFIG) system for the wind energy conversion. The system is proposed as an advanced solution to the traditional doubly fed induction generator (DFIG) to reduce the maintenance cost and improve the system reliability of the wind turbine system. The dynamic response control of grid fault and LVRT control research is the key research of DFIG system, especially in the offshore wind power system. Althrough DFIG Can be a certain reference of BDFIG, but because of the different structure and model, the equivalent impedance is different and so on a number of factors, it is need a depth comparative analysis of DFIG and BDFIG. The main work of this paper includes:(1) According to the system model of BDFIG, the BDFIG mathematical model is build and compared with DFIG, the equivalent circuit of BDFIG suitable for LVRT analysis is also established.Based on the steady state model of BDFIG, the BDFIG wind power system control strategy is build.(2) The grid fault characteristics of BDFIG wind generation system are analyzed, based on the different fault type and grid LVRT guideline.the transient model and electromagnetic transient process is analysised, and the difference with DFIG is also analysised.According to the shallow grid fault, this paper proposes the power converter LVRT control strategy with the power stator currents detection and power stator flux measurement. According to the deep grid fault, this paper proposes the power converter LVRT control strategy with crowbar. The power control of BDFIG wind power converter in balance and unbalance grid conditions is also analysised.(3) According to the LVRT capability comparison of BDFIG and DFIG. The Pontryagin's minimum principle is employed to analysis the optimum value of the BDFIG motor-side control voltage without extro control and with crowbar. To get the Limit controlable range of whole system in LVRT.After compared with the DFIG system, the results shows that the BDFIG has more excellent LVRT capability. In addition, the optimization algorithm is used to verify the positive sequence reactive current maximum value of bdfig wind power generation system positive sequence reactive current maximum value.Finally, this paper build the semi physical simulation of BDFIG wind generation system simulation platform, it is used for the verification of MW level BDFIG converter control strategy and the limit controlable range of both DFIG and BDFIG.