Variable Parameter Nonlinear Control for Maximum Power Point Tracking Considering Mitigation of Drive-train Load

Since mechanical loads exert a significant influence on the life span of wind turbines, the reduction of transient load on drive-train shaft has received more attention when implementing a maximum power point tracking(MPPT) controller.Moreover, a trade-off between the efficiency of wind energy extraction and the load level of drive-train shaft becomes a key issue. However, for the existing control strategies based on nonlinear model of wind turbines, the MPPT efficiencies are improved at the cost of the intensive fluctuation of generator torque and significant increase of transient load on drive train shaft. Hence, in this paper, a nonlinear controller with variable parameter is proposed for improving MPPT efficiency and mitigating transient load on drive-train simultaneously. Then,simulations on FAST(Fatigue, Aerodynamics, Structures, and Turbulence) code and experiments on the wind turbine simulator(WTS) based test bench are presented to verify the efficiency improvement of the proposed control strategy with less cost of drive-train load.

Optimal Decreased Torque Gain Control for Maximizing Wind Energy Extraction Under Varying Wind Speed

Optimal torque(OT)control is a widely used method for maximum power point tracking(MPPT)due to its simplicity.In order to overcome the adverse impacts of turbulent wind speed variations on MPPT,in several methods,modification factors have been proposed to dynamically modify the ideal power curve for OT control.However,this paper finds that the update cycles used in existing methods to adjust power curve modification factors are very long and hence these factors are difficult to be updated in a timely manner along with the wind speed variations.This thereby may deteriorate the effectiveness of wind energy extraction.Therefore,an optimal decreased torque gain(DTG)control method is proposed in this paper.Based on the persistence method,an offline mapping from the wind speed and rotor speed to optimal modification factors is established via optimal control theory.The power curve can be periodically modified online through the mapping relationship.In this method,the update cycles for these power curve modification factors are shortened from tens of minutes to seconds.The simulations and experiments show that the proposed method is more efficient than others in terms of energy extraction under varying wind speeds,especially for turbulent wind cases.

Torque Limit-based Inertial Control Method Based on Delayed Support for Primary Frequency Control of Wind Turbines

To avoid the secondary frequency dip caused by the steep drop of the electrical power of wind turbines(WTs)at the end of frequency support stage,the torque limit-based iner-tial control(TLIC)method sets the power reference as a linear function of rotor speed,rather than the step form for the step-wise inertial control.However,the compensation effect on the frequency nadir(FN)caused by the load surge is weakened as the TLIC power is no longer in the step form.Specifically,the maximum point of the frequency response component(FRC)contributed by TLIC occurs earlier than the minimum point of FRC corresponding to the load surge,so that the FN cannot be adequately raised.Therefore,this paper first investigates the relation between the peak and nadir times of FRCs stimulated by the TLIC and load power.On this basis,with the compensation principle of matching the peak and nadir times of FRCs,the improved TLIC method based on delayed support is proposed.Finally,the effectiveness of the proposed method is validated via the experiments on the test bench of wind-integrated power system.

Optimized dispatch of wind farms with power control capability for power system restoration

As the power control technology of wind farms develops,the output power of wind farms can be constant,which makes it possible for wind farms to participate in power system restoration.However,due to the uncertainty of wind energy,the actual output power can’t reach a constant dispatch power in all time intervals,resulting in uncertain power sags which may induce the frequency of the system being restored to go outside the security limits.Therefore,it is necessary to optimize the dispatch of wind farms participating in power system restoration.Considering that the probability distribution function(PDF)oftransient power sags is hard to obtain,a robust optimization model is proposed in this paper,which can maximize the output power of wind farms participating in power system restoration.Simulation results demonstrate that the security constraints of the restored system can be kept within security limits when wind farm dispatch is optimized by the proposed method.

Inertia compensation scheme for wind turbine simulator based on deviation mitigation

Wind turbine simulator(WTS) is an important test rig for validating the control strategies of wind turbines(WT). Since the inertia of WTSs is much smaller than that of WTs, the inertia compensation scheme is usually employed in WTSs for replicating the slow mechanical behavior of WTs. In this paper, it is found that the instability of WTSs applying the inertia compensation scheme,characterized by the oscillation of compensation torque, is caused by the one-step time delay produced in the acceleration observation. Hence, a linear discrete model of WTS considering the time delay of acceleration observation is developed and its stability is analyzed. Moreover, in order to stably simulate WTs with large inertia, an improved inertia compensation scheme, applying a first-order digitalfilter to mitigate deviation response induced by the time delay, is proposed. And, the criterion for selecting the filter coefficients is established based on the stability condition analysis. Finally, the WTS with the proposed scheme is validated by simulations and experiments.