Trong Duy Nguyen

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.

An Efficiency Optimization Scheme for Bidirectional Inductive Power Transfer Systems

Unidirectional inductive power transfer systems allow loads to consume power, while bidirectional inductive power transfer (BIPT) systems are more suitable for loads requiring two-way power flow such as vehicle to grid applications with electric vehicles. Many attempts have been made to improve the performance of BIPT systems. In a typical BIPT system, the output power is controlled using the pickup converter phase shift angle, while the primary converter regulates the input current. This paper proposes an optimized phase-shift modulation strategy to minimize the coil losses of a series-series compensated BIPT system. In addition, a comprehensive study on the impact of power converters on the overall efficiency of the system is also presented. A closed-loop controller is proposed to optimize the overall efficiency of the BIPT system. Theoretical results are presented in comparison to both simulations and measurements of a 0.5 kW prototype to show the benefits of the proposed concept. Results convincingly demonstrate the applicability of the proposed system offering high efficiency over a wide range of output power.

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.

Modeling and Sensorless Direct Torque and Flux Control of a Dual-Airgap Axial Flux Permanent-Magnet Machine With Field-Weakening Operation

This paper presents the modeling and motion-sensorless direct torque and flux control of a novel dual-airgap axial-flux permanent-magnet machine optimized for use in flywheel energy storage system (FESS) applications. Independent closed-loop torque and stator flux regulation are performed in the stator flux ( x-y) reference frame via two PI controllers. This facilitates fast torque dynamics, which is critical as far as energy charging/discharging in the FESS is concerned. As FESS applications demand high-speed operation, a new field-weakening algorithm is proposed in this paper. Flux weakening is achieved autonomously once the y-axis voltage exceeds the available inverter voltage. An inherently speed sensorless stator flux observer immune to stator resistance variations and dc-offset effects is also proposed for accurate flux and speed estimation. The proposed observer eliminates the rotary encoder, which in turn reduces the overall weight and cost of the system while improving its reliability. The effectiveness of the proposed control scheme has been verified by simulations and experiments on a machine prototype.

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.

Design and loss analysis of a high speed flywheel energy storage system based on axial-flux flywheel-rotor electric machines

A novel high speed flywheel energy storage system is presented in this paper. The rated power, maximum speed and energy stored are 4kW, 60,000 rpm and 300Whr respectively. High power density, energy density and efficiency can be obtained in this system with the compact design. In this design, the rotor with composite rim acts as the flywheel of the system and is sandwiched between two disk type stators. Concepts of active magnetic bearings and axial flux PM synchronous machine are adopted in the design to assist the rotor-flywheel to spin and remain in magnetic levitation in the vertical orientation. The other four degrees of freedom in radial directions are constrained by mechanical bearings for simple operation, and the touchdown bearings are not required at the meantime. Three-dimensional FEM analyses are implemented, and the analysis results support the feasibility of the proposed system.

Stiffness analysis and levitation force control of active magnetic bearing for a partially-self-bearing flywheel system

A compact flywheel energy storage system assisted by axial-flux partially-self-bearing permanent magnet motor has been proposed by the authors. The proposed machine combines axial magnetic bearing and motoring functionality into a single magnetic actuator, which not only spins the rotor-flywheel but also generates a levitation force to overcome the gravity. The mathematical model of the levitation force and stiffness of the active magnetic bearing are developed. The relationships between the stiffness and the key motor parameters are studied. High stiffness can be achieved for the special motor design. The derived force model is validated by finite element analysis. The control system of the levitation force is designed and experimental setup is built. The test results prove the correctness of the mathematical analysis and feasibility of the designed control system.

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.