Study Of Sliding Mode Control And Synchronized Grid Integration Operation Of Doubly Fed Induction Generator

As one of the most significant renewable energies,wind energy has critical importance for resolving the upcoming energy crisis in the future.In recent years,the installation volume of wind turbine generator around the globe has been skyrocketing.In the meantime,most countries are gradually propagating the utilization of wind energy in the commercial and residential scenrios with strategic and political measures.However,due to the inherent properties of randomness,uncertainty and instability,the harvesting and conversion of wind energy is extremely challenging.Meanwhile,the grid integration of wind energy conversion systems(WECS)will deteriorate the stability and the overall performance of the power systems,including the frequency regulation capability,the loss of inertia and the dropping power quality.To resolve the aforemention control and operation issues of doubly fed induction generator(DFIG)based WECS,this dissertation proposed a series of sliding mode control approaches to control DFIG wind turbine and analyzed the grid integration operation of DFIG.Meanwhile,this dissertation proposed an optimal virtual inertia planning scheme to optimize the frequency regulation of DFIG in large scale power system as well as an approach to optimize the participation of DFIG in primary frequency control by altering the synthetic inertia and droop slopes of wind turbines.The detailed contents of this dissertation are given as follows:(1)The mechanism of the chattering phenomeon in the conventional first order sliding mode control is analyzed and it is found that the happening of the chattering phenomenon is caused by the discontinuous component of the control input.Thus,an improved exponential reaching law is proposed,which is capable of adoptively changing the gain in front of the sign function,thus accerlerating the reaching speed and minimizing the chattering amplitude.Subsequently,the novel exponential law based sliding mode control is implemented in the control of output active and reactive power of DFIG wind turbine under the direct power control framework,wherein the errors between the reference active and reactive powers and the actual active and reactive powers are treated as the control system state variable to construct the sliding surface,and robust control of the output active and reactive powers is achived.In the meantime,the stability of the controlled system is analyzed with Lyapunov function method and the asymptotic stability of the controlled system is guaranteed.Ultimately,the effectiveness and superior performance of the proposed method comparing with conventional first order sliding mode control is validated with four simulation and experimental case studies.(2)The fractional order sliding mode control(FOSMC)is proposed for direct power control of DFIG wind turbine.In FOSMC,the concept of fractional calculus is introduced to replace the integer order integral of conventional sliding manifold with fractional order integral,and a novel sliding manifold is constructed.Comparing with conventional sliding surface,the fractional order sliding surface is approximated with high order transfer function,thus the final control input contains no integer order discontinuous function and the chattering phenomenon is avoided naturally.Subsequently,a stability analysis is conducted with the Lyapunov function method,where the system asymptotic stability is guaranteed.Ultimately,the effectiveness and the performance of the proposed method is validated with five simulation and experimental case studies.(3)A grid synchronization and power optimization approach for DFIG wind turbine based on high order sliding mode control(HOSMC)is proposed.Conventional first order sliding mode control is not capable of eliminating the chattering phenomenon due to its inherent nature,and the FOSMC approach needs to be approximated with high order transfer function in practical implementation.Thus,both methods show their weaknesses in practical scenarios.Hence,it is necessary to implement the HOSMC in DFIG wind turbine.The HOSMC approach takes the high order derivative of the original control input as the actual control input.Thus,it is naturally free from the chattering phenomenon.The HOSMC method is firstly implemented to control the rotor angular velocity and the rotor current to achive optimal energy capturing in the wind turbine.In the meantime,the conventional first order sliding mode control is employed to achive grid synchronization of the stator voltage.Ultimately,the effectiveness of the proposed method is validated with three simulation case studies.(4)An optimization approach for adjusting the synthetic inertia and droop slopes of the wind turbine to participate in power system primary frequency control is proposed.A second order model of a large scale power system with high penetration of wind turbine is firstly introduced,where the load-frequency charateristics of the reduced order system is analyzed.Subsequently,the grid integration process of wind turbine is analyzed,where the concept of stability margin is proposed to evaluate wind turbine's capability of enduring system frequency drops.Ultimately,an optimization problem based on the stability margin and generation cost of wind turbines is proposed,which tries to lower down the generation cost and maintain the stability of the grid-integrated wind turbines by altering the synthetic inertia and droop slopes.The case studies validated the effectiveness of the proposed method.(5)An optimal virtual inertia planning scheme for power systems with high penetration of wind turbines is proposed.By analyzing the internal dynamics of wind turbines,the concept of critical frequency drop and stability margin is proposed.Considering the coherency of the stability margin of wind turbines in the power system,the problem of stability margin coherency minimization is proposed and resolved to improve the frequency stability of power systems with high penetration of wind turbines.The case studies fully validated the effectiveness of the method.