Shuai Xiao

An LVRT Control Strategy Based on Flux Linkage Tracking for DFIG-Based WECS

For doubly fed induction generator (DFIG)-based wind energy conversion systems (WECSs), large electromotive force will be induced in the rotor circuit during grid faults. Without proper protection scheme, the rotor side of DFIG will suffer from overcurrents, which may even destroy the rotor-side converter (RSC). To mitigate this problem, a new flux-linkage-tracking-based low-voltage ride-through (LVRT) control strategy is proposed to suppress the short-circuit rotor current. Under the proposed control strategy, the rotor flux linkage is controlled to track a reduced fraction of the changing stator flux linkage by switching the control algorithm of RSC during grid faults. To validate the proposed control strategy, a case study of a typical 1.5-MW DFIG-based WECS is carried out by simulation using the full-order model in SIMULINK/SimPowerSystems. In the case study, a comparison with a typical LVRT method based on RSC control is given, and the effect of the control parameter on the control performance is also investigated. Finally, the validity of the proposed method is further verified by means of laboratory experiments with a scaled-size DFIG system.

Modeling and Implementation of an All Digital Phase-Locked-Loop for Grid-Voltage Phase Detection

In this paper, a novel all digital phase-locked-loop (ADPLL) is proposed for the phase detection of power grid voltage. The proposed ADPLL features wide track-in range and fast pull-in time, and it can be easily integrated into the digital controller with low cost. The nonlinear model of such digital system is derived from the operation principle of the ADPLL and the linearized model is then obtained to evaluate the steady and dynamic performance due to the nonlinear and discrete property. Compared with the conventional DPLL, the proposed ADPLL has almost no steady state phase error when the frequency of the input signal deviates from the center frequency. Moreover, the tracking speed is highly improved. In contrast with other ADPLLs, the proposed one employs the loop filter with proportional-Integral (PI) structure, which can help to eliminate the steady state error excited by high-order disturbances or noise. As a result, a low system clock or sampling frequency is enough to get a satisfactory performance, which is an attractive advantage for the low cost applications. Simulation and experimental results verify the analysis and the effectiveness of the ADPLL.

A LVRT control strategy based on flux tracking for DFIG-based wind power systems

For doubly-fed induction generator (DFIG)-based wind power systems, when a grid voltage dip happens, a large electromotive force (EMF) will be induced in the rotor circuit. Without proper control for protection, over-current will appear in the rotor circuit, and it may lead to the destruction of the rotor-side converter (RSC). To mitigate this problem, a new low voltage ride-through (LVRT) control strategy based on flux tracking is proposed. With the proposed control strategy, once the voltage dip is detected, the rotor flux is controlled to track the changing stator flux by controlling the RSC. The validity of the proposed method is verified using the full order model of a typical 1.5 MW DFIG in the SIMULINK/SimPower Systems simulation environment. The proposed control strategy is proven to be able to effectively reduce the rotor current during both symmetrical and asymmetrical grid faults without any additional hardware circuit.

Nonlinear dynamic inversion approach applied to pitch control of wind turbines

This paper presents a dynamic inversion approach for nonlinear pitch control of the WTs operating in the above-rated wind speed region, which will improve the generator rotor speed control for power generation and reduce the tower loads for the vibration reduction. The proposed controller consists of a rotor speed regulator, a tower load damper and a lead compensator. The rotor speed regulator and the load damper are designed independently using dynamic inversion approach to deal with the non-affine nonlinearity of the model, and the lead compensator is used to compensate for the phase lag of the pitch angle command caused by the pitch actuator. The simulation results with FAST verify the superiority of the proposed control scheme. Compared to the gain scheduled PI (GSPI) controller, the proposed nonlinear controller is able to improve the rotor speed control performance and reduce the tower loads effectively. Moreover, the proposed controller can work well for the whole above-rated wind speed region, and is simple to implement for application.

Modeling and design of a new bi-directional buck-boost cascade inverter

This paper concentrates on the modeling and design of a new bi-directional buck-boost cascade inverter. The proposed inverter can be seen as the cascade of a Buck converter and a Boost converter, both with bipolar outputs. First, The operation principle of the proposed inverter is explained. With detailed analysis, the switching function model is established. Then, the averaged model for control purpose is given. Afterwards, a decoupled control scheme is presented. Device-level simulations and experiments verify that the proposed system can not only perform bi-directional operation with bipolar buck/boost output voltage, but also achieve good steady-state and dynamic performance.

Linear interpolation model predictive control of large wind turbines for blade asymmetric fatigue loads mitigation

To reduce the blades asymmetric loads of large wind turbines operated in the above-rated wind speed region, this work proposes a model predictive individual pitch controller with the linear interpolation model. Results show a significant reduction of 1P (once-per-revolution), 2P, 4P blade asymmetric loads within the limits of pitch rate. The proposed model of wind turbine can describe the characteristic of blade bending moment up to 4P (once-per-revolution) frequency. The MPC method with this model can reduce 1P, 2P, 4P blade asymmetric loads significantly within the limits of pitch rate.

Nonlinear pitch control design for load reduction on wind turbines

As the modern wind turbines (WTs) become larger and larger in size, the structural loads experienced by WTs increase dramatically. Such loads will shorten the working life of WTs and increase the maintenance cost. To mitigate such loads, this paper proposes a nonlinear pitch control strategy for WTs operating in the above-rated wind speed region. With the proposed strategy, nonlinear dynamic inversion method is employed to transform the non-affine nonlinear WT model into a pseudo linear one. Then the linear control theory can be applied for control design. The simulation results based on the aero-elastic model of Fatigue, Aerodynamics, Structures, and Turbulence (FAST) verify the superiority of the proposed nonlinear controller over the tranditional gain-scheduled proportional-integral (GSPI) controller. The proposed nonlinear controller can work well in the whole above-rated wind speed region, and easy to implement in the real application.