Dewei Xu

A Novel Control Scheme for Current-Source-Converter-Based PMSG Wind Energy Conversion Systems

A novel control scheme for permanent-magnet synchronous generator is proposed in this paper, where a current-source converter is employed as the bridge between the generator and the grid for high-power wind energy conversion systems. In these medium voltage (2.3-13.8 kV) level applications, current-source converters not only have inherent advantages, but also present some challenges for controller design due to the DC link choke and filter capacitors. The control strategy is developed to achieve better performances with improved dynamic response. By maintaining the grid-side converter modulation index at the highest possible level, the proposed control scheme reduces the DC link current to a minimum value to reduce converter conduction loss. The systempsilas dynamic performance is further improved by adopting generator-side power feedforward. Simulation and experimental results are provided to verify the proposed control scheme.

High-Step-Up and High-Efficiency Fuel-Cell Power-Generation System With Active-Clamp Flyback–Forward Converter

A high-efficiency fuel-cell power-generation system with an active-clamp flyback-forward converter is presented in this paper to boost a 12-V dc voltage into a 220-V 50-Hz ac voltage. The proposed system includes a high-efficiency high-step-up interleaved soft-switching flyback-forward converter and a full-bridge inverter. The front-end active-clamp flyback-forward converter has the advantages of zero-voltage-switching performance for all the primary switches, reverse-recovery-problem alleviation for the secondary output diodes, large voltage-conversion ratio, and small input-current ripple. Furthermore, there are two coupled inductors in the proposed converter. Each coupled inductor can work in the flyback mode when the corresponding main switch is in the turn-on state and in the forward mode when it is in the turnoff state, which takes full use of the magnetic core and improves the power density. In addition, the full-bridge inverter with an LC low-pass filter is adopted to provide low-total-harmonic-distortion ac voltage to the load. Therefore, high-efficiency and high-power-density conversion can be achieved in a wide input-voltage range by employing the proposed system. Finally, a 500-W prototype and another 1-kW converter are implemented and tested to verify the effectiveness of the proposed system.

Active Damping for PMSG-Based WECS With DC-Link Current Estimation

An active-damping strategy is proposed for the suppression of speed and torsional oscillations in permanent-magnet synchronous generator (PMSG)-based wind-energy conversion systems (WECSs). Direct-driven configuration with PMSG is an attractive choice for WECS because of the gearbox elimination and cost reduction due to small pole-pitch design. However, speed and torsional oscillations appear when the generator is directly connected to the wind turbine without any assistant damping device. Based on small-signal analysis, a low-bandwidth design for the power or generator torque controller of PMSG can help to reduce the oscillation amplitude, but the system dynamic performance is thus sacrificed. From the power-flows point of view, the oscillation is reflected in the dc-link current. With the help of switch function modeling based on the space-vector-modulation scheme, the average dc-link current can be estimated and applied to the compensation strategy, which provides positive damping resulting in stability improvement. The simulation and experiment results verify the theoretical analysis and the validation of the proposed strategy.

Unified Power Control for PMSG-Based WECS Operating Under Different Grid Conditions

A unified power control strategy is proposed for the permanent magnet synchronous generator-based wind energy conversion system (WECS) operating under different grid conditions. In the strategy, the generator-side converter is used to control the dc-link voltage and the grid-side converter is responsible for the control of power flow injected into the grid. The generator-side controller has inherent damping capability of the torsional oscillations caused by drive-train characteristics. The grid-side control is utilized to satisfy the active and reactive current (power) requirements defined in the grid codes, and at the same time mitigates the current distortions even with unsymmetrical grid fault. During grid faults, the generator-side converter automatically reduces the generator current to maintain the dc voltage and the resultant generator acceleration is counteracted by pitch regulation. Compared with the conventional strategy, the dc chopper, which is intended to assist the fault ride through of the WECS, can be eliminated if the proposed scheme is employed. Compared with the variable-structured control scheme, the proposed strategy has quicker and more precise power responses, which is beneficial to the grid recovery. The simulation results verify the effectiveness of the proposed strategy.

Unified DC-Link Current Control for Low-Voltage Ride-Through in Current-Source-Converter-Based Wind Energy Conversion Systems

The increased penetration of wind power into utility grid brings challenges to power converter design in wind energy conversion systems (WECSs). Among all, low-voltage ride-through has been enforced in the field, which is one of the major challenges for WECS. It is necessary to design an integrated controller to protect the converter from overvoltage/overcurrent and to support the grid voltage during faults and recoveries. In this paper, a unified dc-link current control scheme for current-source-converter-based WECS is proposed. The controllers for generator- and grid-side converters are coordinated to provide fault ride-through capability. In normal operations, the proposed control scheme can also smooth the real power flow while keeping the fast dynamic performance of the dc-link current control. Simulation and experimental results are provided to verify the proposed control scheme.

A Low-Cost Rectifier Topology for Variable-Speed High-Power PMSG Wind Turbines

A novel rectifier topology consisting of two three-phase diode bridges and three thyristors is proposed in the paper for variable-speed high-power permanent-magnet synchronous generator (PMSG) wind energy conversion systems (WECSs). The proposed rectifier has several prominent features such as low cost, low power loss, and simple control. Its ability to cascade the input voltages allows it to properly regulate generator speed even when the wind velocity drops to half of the rated value. Consequently, maximum power-point-tracking algorithms can be applied to optimize power capture in a wide range of wind velocities. The operating principle of the rectifier is elaborated. Its use and control in the WECS is presented. The converter and control are verified by simulation and experimental results.

Hybrid PWM for High-Power Current-Source-Inverter-Fed Drives With Low Switching Frequency

In this paper, a hybrid pulsewidth modulation (PWM) scheme is proposed to suppress the LC resonance for the high-power current-source inverter (CSI) fed drives with low switching frequency. In CSI drive systems, the selective harmonic elimination (SHE) is selected due to the better performance of harmonics, but the LC resonance cannot be effectively damped due to the lack of control flexibility of SHE. In this paper, SHE is only used for the steady-state operation. The modulation scheme is then switched to space vector modulation with dynamic capacitor voltage control when the transient event occurs. The smoothed transitions between different modulation schemes are proposed. The proposed hybrid method effectively solves the resonant issues in CSI drives, while maintaining the power quality. Both simulation and experiments verify the performance of the proposed hybrid PWM scheme.

A Novel Hardware-Based All-Digital Phase-Locked Loop Applied to Grid-Connected Power Converters

For grid-connected power converters, the frequency and phase angle of the grid voltage, which are essential to the system operations, must be quickly and accurately obtained even if the utility voltage is distorted or unbalanced. In this paper, a novel hardware-based all-digital phase-locked loop (ADPLL) is proposed for grid interface converters to detect the frequency and phase angle based on the voltage zero crossings. The proposed ADPLL features wide track-in range and fast pull-in time, and it can easily be integrated with the digital controller for grid-connected power converters. A discrete small-signal model is presented to investigate the performance and parameter dependence of the ADPLL. As expected, the output phase error and pulse jitter are minimized by selecting a high clock frequency and proper regulator parameters. With additional voltage sensors, the ADPLL can be readily extended into applications with grid disturbances. Experimental results verify the analysis and the effectiveness of the ADPLL.

An Integrated AC Choke Design for Common-Mode Current Suppression in Neutral-Connected Power Converter Systems

This paper presents an integrated ac choke that incorporates the common-mode (CM) suppression function into a three-phase differential-mode (DM) inductor. The proposed choke, together with other filter components, provides a transformerless solution to issues caused by the switching CM voltage in neutral-connected power converter systems. The required high CM-to-DM inductance ratio is generated by different magnetic paths in the novel structure. An approach based on magnetic circuit calculation can be applied to the weight-minimized design in which a reasonably small core loss confirms the chokes heat dissipation capability. The existence and uniqueness of the optimization are proven by a design example and analysis. Comparison to separate DM and CM inductors demonstrates the benefits of integration: the iron and copper material savings, the volume and weight reduction, and overall system efficiency improvement. The finite element method is used to tune the design parameters with which a prototype is implemented. The simulation and experimental results of the prototype in a voltage source converter verify the feasibility and effectiveness of the integrated choke.

A Multisampling SVM Scheme for Current Source Converters With Superior Harmonic Performance

The device switching frequency of current source converters (CSCs) in high-power medium-voltage applications is usually several hundred Hz. Selective harmonic elimination (SHE) has been the dominant modulation scheme because of its capability to eliminate unwanted low-order harmonics at low switching frequencies. Conventional space vector modulation (SVM), as another CSC modulation scheme, provides variable modulation index control but its output contains low-order harmonics with high magnitudes. In this paper, a multisampling SVM (MS-SVM) is proposed to substantially suppress the low-order harmonics in practical CSC-based drives. Investigation of the switching frequency of the proposed modulation method is carried out and methods to reduce additional switchings are developed. The proposed MS-SVM method exhibits a superior low-order harmonic performance comparable to that of SHE and provides the same fast and flexible control capability as the conventional SVM. Simulation and experimental results are provided to verify the proposed methods.