Cascaded Observers Based Adaptive Control Of Nonlinear Power Systems:Theory And Applications

This thesis proposes a novel type Ⅰ and type Ⅱ cascaded observer for high-order systems in input-output canonical form and high-order systems having lower-triangular structure,respectively.The proposed observers are utilized to design a novel adaptive output feedback control strategy for stability and tracking control of high-order input-affine nonlinear multi-input multioutput systems.The designed control strategy is also applied to state estimation and control of torque motor,transient stability control of traditional multi-machine power systems,and coordinated control of DFIG-based wind farm penetrated power systems.The major contributions of this thesis are summarized as follows:First of all,the thesis proposes a type Ⅰ cascaded sliding-mode observer for systems in input-output canonical form and equipped with high relative degree.The proposed observer consists of multiple lower-order conventional(high-gain parameter embedded)sliding-mode observers connecting in cascade.The basic working principle,parameter design rules,and performances(including ultimate bounds and convergence speed of estimation errors,and peaking phenomenon)of the proposed observer are discussed in detailed.In particular,the convergence of estimation errors of the proposed observer is rigorously proved.Compared with conventional high-gain observers and high-gain parameter embedded sliding-mode observers,the proposed type Ⅰ cascaded sliding-mode observer has smaller gain coefficients and less severe peaking phenomenon,but similar convergence speed of estimation errors.Besides,unlike existing cascaded high-gain observers,the dimension of each block of the proposed observer need not to be restricted to 1 or 2.Simulation studies are carried out on a fifth-order system in input-output canonical form to test the performances of the proposed observer.Secondly,the thesis proposes another type Ⅱ cascaded sliding-mode observer for highorder systems having lower-triangular structure.The proposed type Ⅱ cascaded observer can be regarded as an improvement of the above-mentioned type Ⅰ cascaded sliding-mode observer.Note that for a typical type Ⅱ cascaded observer,the communication between its blocks is bidirectional.The basic working principle,parameter design rules,and performances(including ultimate bounds and convergence speed of estimation errors,and peaking phenomenon)of the proposed type Ⅱ cascaded observer are clarified.More importantly,the convergence of estimation errors of the proposed type Ⅱ cascaded observer is also rigorously proved.In comparison to the above mentioned type Ⅰ cascaded sliding-mode observer,the peaking phenomenon and gain coefficients of the proposed type Ⅱ cascaded sliding-mode observer are further reduced.Moreover,the convergence speed of estimation errors of the proposed type Ⅱ cascaded observer is identical to that of classical high-gain parameter embedded sliding-mode observers.Simulation studies are carried out on a fifth order system having lower triangular structure to further demonstrate the features of the proposed observer.Besides,the proposed type Ⅱ cascaded observer is used in rotor position adjustments of torque motor to unveil its potential applications in the field of adaptive output feedback control.Thirdly,the thesis proposes a cascaded observers based adaptive output feedback control strategy for input-affine nonlinear multi-input multi-output systems endowed with external subsystems having high relative degree.The control strategy requires the use of type Ⅰ or typeⅡ cascaded observers to provide fast and accurate estimates on the states and perturbations of external subsystems for feedback controllers.The stability of the entire closed-loop system consisting of plant,cascaded observers and feedback controllers,with the proposed control strategy adopted,is thoroughly analyzed.Moreover,the adaptivity of the proposed control strategy to external disturbances and model uncertainties is briefly discussed.Simulation studies are carried out on a modified multiple inverted pendulums model to demonstrate the features of the proposed control strategy,especially its performances on output tracking control.Besides,the proposed strategy is utilized in stability control of the IEEE 4-11 power system to further confirm its advantages on transient stability control of traditional multi-machine power systems.Last but not least,the thesis proposes a new method,which is based on the above-mentioned cascaded observers based adaptive output feedback control strategy,for coordinated stability control of DFIG-based wind farm penetrated power systems.The proposed method requires cascaded observers based feedback controllers to be implemented in different controllable devices of the power systems.Particularly,for synchronous generators,a cascaded observer based excitation controller is designed.And,for DFIG-based wind farms,a cascaded observers based four-loop control system is constructed.Compared with traditional control strategies of DFIGbased wind farm penetrated power systems,the proposed method does not rely on accurate system parameters.In addition,the proposed method enables coordination between controllers of different devices to ensure the stable operation of the entire DFIG-based wind farm penetrated power systems.Furthermore,the proposed method is adaptive to external disturbances,such as the fluctuation of wind speed,and model uncertainties of the power systems.Simulation studies are carried out on a one DFIG-WF infinite-bus power system and a 10 machines 48 buses power system to verify the performances of the proposed method,and its advantages over traditional control strategies of DFIG-based wind farm penetrated power systems.