Optimized Demagnetization Control Strategy Of Double Fed Induction Generator In Fault Ride Through Period

With the development of wind power generation,the proportion of wind farms in the power system is gradually increasing.Among many types of wind turbines,Double Fed Induction Generator(DFIG)has the advantages of small size and low cost,and the active power and reactive power of DFIG can be decoupled.It can run at a variable speed in a large wind speed range and achieve maximum wind energy capture,so it is widely used in large scale wind farms.For the stator side of DFIG is directly connected to the power grid,when faults happen to the power grid,the DFIG is easily to be affected.After the grid fault,there is a transient flux in the stator of DFIG,it is easy to cause the rotor winding overcurrent and DC bus overvoltage.To ensure the safe operation of DFIG and grid,experts and scholars have proposed many fault ride through strategies.Demagnetization control is an effective strategy to accelerate the attenuation of the stator transient flux.In the traditional demagnetization control,the command value of the rotor transient current is inverse to the stator transient flux.But analysis found that the effect of this strategy is not ideal,and the optimal demagnetization can’t be achieved.And the capacity of the converter can’t be used efficiently with this strategy.In this paper,the differential equation of the stator transient flux is derived based on the DFIG model in the synchronous speed rotating coordinate system.The study found that the attenuation of the stator transient flux linkage is influenced by the stator impedance parameters and the rotor transient current.Controlling the rotor transient current can affect the attenuation of the transient stator flux and can achieve demagnetization.Then,this article introduces the basic idea of traditional demagnetization control,and establishes a DFIG transient model using traditional demagnetization control.Fault ride through is divided into low voltage ride through and high voltage ride through.Based on the transient model,in this two cases,this paper theoretically analyzes the performance characteristics of traditional demagnetization control and verifies it through simulation.The analysis results show that the performance of the traditional demagnetization control is not ideal,it occupies more rotor converter capacity,but the demagnetization effect is not ideal.With this reason,this paper proposes an optimized demagnetization control.Furthermore,this paper analyzes the transient process of low voltage ride through and high voltage ride through,and analyzes the characteristics of the two processes.The analysis proves that the transient processes in the two cases are similar and can be applied to the transient model in this paper.The optimized demagnetization control based on the transient model is also applicable to both cases.Based on traditional demagnetization control,this paper proposes an optimized demagnetization control,introduces the specific working principle,working process and specific value of each parameter in demagnetization control,and improves the transient model of traditional demagnetization control.Theoretical analysis proves that the optimized demagnetization control has more excellent performance.Finally,this paper builds a 2MW DFIG model in DIgSILENT/Power Factory software,and simulates the process of low voltage ride through and high voltage ride through.In the simulation software,this paper verifies the correctness of the transient model with optimized demagnetization control firstly.To verify the performance of the optimized demagnetization control through simulation,this paper applies traditional demagnetization control and optimized demagnetization control at the same time.The simulation results show that,the optimized demagnetization control occupies a small rotor capacity and has an excellent demagnetization effect compared with the traditional demagnetization control.So,the optimized demagnetization control can expand the application range of the demagnetization control,and improve the fault ride through capability of DFIG.