Shao Zhang

Design of a Robust Grid Interface System for PMSG-Based Wind Turbine Generators

A robust and reliable grid power interface system for wind turbines using a permanent-magnet synchronous generator (PMSG) is proposed in this paper, where an integration of a generator-side three-switch buck-type rectifier and a grid-side Z-source inverter is employed as a bridge between the generator and the grid. The modulation strategy for the proposed topology is developed from space-vector modulation and Z-source network operation principles. Two PMSG control methods, namely, unity-power-factor control and rotor-flux-orientation control (Id = 0), are studied to establish an optimized control scheme for the generator-side three-switch buck-type rectifier. The system control scheme decouples active- and reactive-power control through voltage-oriented control and optimizes PMSG control for the grid- and generator-side converters independently. Maximum power point tracking is implemented by adjusting the shoot-through duty cycles of the Z-source network. The design considerations of the passive components are also provided. The performances and practicalities of the designed architecture have been verified by simulations and experiments.

A Novel Axial Flux Permanent-Magnet Machine for Flywheel Energy Storage System: Design and Analysis

This paper presents the design and analysis of a novel axial flux permanent-magnet (AFPM) machine for a flywheel energy storage system (FESS). Its design and control facilitate significant reduction in axial bearing stress and losses. Due to the unconventional flux distribution in this machine, a 3-D finite element method was employed for its design and analysis, including its electromagnetic torque and axial force performances. The effects of the rotor PM skew angle on the cogging torque and the axial force have been studied. It is found that an optimum skew angle is effective in reducing the overall cogging torque with negligible effect on the static axial force. The latter is crucial as it can be utilized to minimize the axial bearing stress in FESS application. The concept, design, and analysis methodology have been validated by experimental results from an experimental AFPM machine prototype.

Advanced Control of Series Voltage Compensation to Enhance Wind Turbine Ride Through

Due to increasing wind power penetration into utility grids, system operators have instituted stringent standards for ride-through compliance of grid-connected wind turbines. Series voltage compensation can be an effective solution to meet the new standards, especially if a low-cost system can be developed. This paper first presents the characteristics of ride-through behavior of doubly fed induction generator (DFIG) using phasor analysis. The analysis is extended to include the effects of generic series voltage compensation. A new control scheme is then described and analyzed, which allows relatively low-energy ratings in the series compensator, thereby significantly lowering the overall system cost. The new controller enables the DFIG with the compensator to provide the reactive power required for system voltage recovery during grid fault, in compliance with the new grid codes. Simulation and experimental results have demonstrated the enhanced performances resulting from the application of the control scheme.

On the modeling and control of a novel flywheel energy storage system

The concept of a novel axial flux permanent magnet machine for flywheel energy storage system is presented. Modeling and control of this novel flywheel energy storage system are given. This flywheel energy storage system is designed to work as a fast-response energy storage device which is planned for use in ride-through applications in wind power. Therefore the flywheel has to store and release energy at high power rating in short period of time to meet such requirements. Motoring torque and axial force of this flywheel are well controlled by a simple control technique. The flywheel is driven by a single power converter. Mathematical model of the proposed flywheel is presented and verified by Matlab simulations. In order to evaluate this novel machine concept, an experimental test-rig was fabricated and tested. Experimental and simulation results are presented and compared.

Novel three-phase AC-AC Z-Source converters using matrix converter theory

This paper proposes a new type of three-phase AC-AC Z-Source converters which are derived from matrix converter (MC) theories. The proposed converters are capable of three-phase AC-AC power conversion and, except for exhibiting the input characteristics of MC: controllable (unity, leading and lagging) input power factor, it does overcome the two main drawbacks of MC: limited voltage transfer ratio and low immunity to grid disturbance by utilizing a Z-Source network. The complete modulation strategy of the proposed converter is developed from Z-Source modulation and space vector modulation (SVM) of MC. And thereby important aspects of the developed AC-AC converter, i.e., switching operation and realization, are presented together with theoretical analysis, which gives an in-depth understanding of the proposed converters. Finally, the MC concept based Z-Source topology is verified by numerical simulation and experimentally using a laboratory prototype.

Model predictive control of a novel axial flux permanent magnet machine for flywheel energy storage system

This paper deals with a fast current control for a novel axial flux permanent magnet machine based on model predictive control. This machine, called flywheel enegy storage system, is controlled to function as a fast-response energy storage device. Fast acceleration or deceleration is required for fast storing and releasing energy respectively. Therefore model predictive control method, which can pre-calculate the system behavior and choose an optimal value of control variables, is applied to control this machine. Performance of MPC for this flywheel is compared with PID control. Both simulation and experimental results point out the validity and potentials of the model predictive control for this flywheel system.

A flywheel cell for energy storage system

A flywheel cell intended for multi-flywheel cell based energy storage system is proposed. The flywheel can operate at very high speed in magnetic levitation under the supports of the integrated active magnetic bearing and a passive magnetic bearing set. 3D finite element analyses were applied to verify various configurations of passive magnetic bearing. The feasibility of PID controllers for the flywheel cell was evaluated by simulation study.

Position sensorless control of a novel flywheel energy storage system

This paper presents a position-sensorless field-oriented control of a novel axial flux permanent magnet machine for flywheel energy storage system. The approach is based on the estimation of the motor induced voltage with the help of measured stator currents and reference voltages. Based on sliding mode observers, rotor position can be accurately estimated. This novel flywheel energy storage system is successfully controlled under position-sensorless scheme which eliminates the hassle of rotary encoder and reduces the systems cost. Both simulation and experimental results are illustrated for verification.

WRIG based wind conversion system excited by matrix converter with current control strategy

This paper presents a current modulation scheme for matrix converter which serves as a power electronics interface in wound rotor induction generator based wind energy conversion system. It not only efficiently decouples active and reactive power of the generation system, but also gives a good grid connection performance. The proposed wind generation system is verified via simulation.

One-cycle controlled three-switch buck-type rectifier

This paper presents a robust control scheme derived from one-cycle control (OCC) for three-switch buck-type rectifier under normal and unbalanced inputs. The developed control scheme is desired to make the rectifier insensitive to the input disturbances. The detailed modulation and implementation of the developed OCC method as well as the conventional space vector modulation (SVM) are presented for the purpose of comparison. The design consideration of the passive components in the topology is also formulated for system integration. The simulation results from normal and abnormal input voltages have demonstrated the feasibility and performance of the developed OCC scheme for the studied rectifier. The comparison between the developed OCC scheme and the conventional SVM has shown that the rectifier is insensitive to the input disturbances by utilizing the developed OCC principle.