High-power Wind Energy Conversion Systems Research Articles

A Comprehensive Overview of Power Converter Applied in High-Power Wind Turbine: Key Challenges and Potential Solutions

The increasing penetration of offshore wind power generation promotes the revolution of wind turbine toward high power application. The development of high-power wind turbine undoubtedly poses new technical challenges. This paper presents a comprehensive overview for high-power wind energy conversion system (WECS) from key technique aspects including topologies, stability, reliability, and ancillary service capability, and further investigates the key challenges and potential solutions. The various topologies of power converter applied in high-power wind turbine are first reviewed and discussed. The different semiconductor technology, including recent wide bandgap (WBG) devices, and their potential contributions for offshore high-power converters are discussed. Furthermore, the potential stability issues of high-power WECS are investigated and reviewed. Also, the stabilization control strategies are discussed. Further, the reliability issue and enhancement strategies of high-power WECS are discussed, including condition monitoring, active thermal control strategy and fault-tolerant operation method. In addition, the ancillary service capability of high-power WECS including frequency-active power control capability and voltage-reactive power control capability is reviewed. Finally, the future trends and potential solutions are discussed in this paper.

Direct power control using space vector modulation strategy control for wind energy conversion system using three-phase matrix converter

Introduction. Wind energy conversion system is getting a lot of attention since, they are provide several advantages, such as cost competitive, environmentally clean, and safe renewable power source as compared with the fossil fuel and nuclear power generation. A special type of induction generator, called a doubly fed induction generator is used extensively for high-power wind energy conversion system. They are used more and more in wind turbine applications due to the advantages of variable speed operation range and its four quadrants active and reactive power capabilities, high energy efficiency, and the improved power quality. Wind energy conversion systems require a good choice of power electronic converters for the improvement of the quality of the electrical energy produced at the generator terminals. There are several power electronics converters that are the most popular such as the two stage back-back converter. Because of the disadvantage of these converters to produce large harmonics distortions, we will choose using of three-phase matrix converter. Purpose. Work presents a direct power control using space vector modulation for a doubly fed induction generator based wind turbine. The main strategy control is to control the active and reactive powers and reduce the harmonic distortion of stator currents for variable wind speed. The novelty of the work is to use a doubly fed induction machine and a three pulses matrix converter to reduce the low cost, volume and the elimination of the grid side converter controller are very attractive aspects of the proposed topology compared to the conventional methods such as back-to-back converters. Simulation results are carried out on a 1.5 MW of wind energy conversion system connected to the grid. The efficiency of the proposed system has been simulated and high results performances are evaluated to show the validity of the proposed control strategy to decouple and control the active and reactive power for different values of wind speed.

Development of converter configuration and corresponding control strategy for wind turbines using permanent magnet synchronous generator: A Case study

This study aims to investigate a feasible converter architecture and corresponding control method for Wind Turbine (WT) systems using permanent magnet synchronous generators (PMSG). The converter configuration is designed based on the AC/DC/AC converter, including the Diode Bridge Rectifier (DBR) and Pulse Width Modulated Current Source Inverter (PWM-CSI), Buck-Boost Converter (BBC), and Bypass Chopper (BC). The control strategy for the proposed converter is developed to enhance the operating performance of WT-PMSG, which must satisfy four requisitions. Firstly, it proposes the control approach for the pitch angle to control the output power of the WT when the wind speed is over the rated value. The selected control variables are the generator speed and active power. Secondly, the Maximum Power Point Tracking (MPPT) is archived to the satisfaction of the full-range operation through the control strategy for the BBC. The control strategy is applied by the Proportional Integral (PI) controller. The control variables are the generator speed and the diode rectifier's output DC current. Thirdly, the control strategy for PWM-CSI controls the voltage at the connection point and the frequency of the inverter. Fourthly, the DC-link voltage is controlled to the constant value at various operating conditions. Simulation of a 3MW and 0.69 kV WT-PMSG was carried on in PSCAD software to verify under considering the variable wind speed and the three-phase fault. The obtained results prove the feasibility of the proposed WT-PMSG system that serves as an alternative for a high-power wind energy conversion system.

Comparative Study between Flatness-Based and Field-Oriented Control Methods of a Grid-Connected Wind Energy Conversion System

Wind energy is an alternative to meet the growing energy demand. Control of wind turbines should help the reliability and stable operation of the power grid. Furthermore, they should respect the technical requirements according to the grid codes to inject the wind energy into the grid. In this paper, a well-known field-oriented control (FOC) method and a new control method based on the flatness properties (FBC) are presented and compared. These control methods are applied to a wind energy conversion system (WECS), which connects a variable-speed wind turbine (WT) based on a permanent magnet synchronous generator (PMSG) to the grid via a back-to-back converter. The main aim of both control methods is to extract the maximum power from the wind. For this purpose, the mathematical model of each subsystem, i.e., WT, PMSG, and electrical grid, is presented. To evaluate and to compare the dynamic behavior of the high-power wind energy conversion system, it is modeled and the control strategies are developed using SimPowerSystems Toolbox in MATLAB. The simulation results obtained in the time domain show that the FBC performs better at managing the energy in the studied system. Indeed, the proposed FBC is a high bandwidth control method with only one closed-loop control, which leads to a high dynamic performance. Therefore, that strategy can be used to provide ancillary services such as frequency control and spinning reserves according to the grid codes.

Techno-economic evaluation of five-level nested neutral point clamped converter topology for transformer-less connection of high-power wind energy conversion systems

Developers and operators are interested in improving the reliability and reducing the associated costs of wind power plants (WPPs) because of the continuous increase in the power capacity of wind energy conversion systems (WECSs) and the increasing development of WPPs. The electrical subsystem of the WPP experiences the highest failure rate and constitutes a significant proportion of its total cost. Reliability of the WECS can be increased and its cost reduced by eliminating the wind turbine transformer from the electrical subsystem. This study gives a techno-economic evaluation of a five-level nested neutral point clamped (NNPC) converter topology for transformer-less connection of high- power WECSs. The approach entailed the calculation of reliability of five-level NNPC converter topology deployed in the grid-side of a WECSs. This method presents a mathematical formula for deriving the reliability of a five-level NNPC converter topology by using the reliability block diagram and reliability estimation-based models in the military handbook (MIL-HDBK-217F). The cost analysis model shows that the total cost of the five-level diode clamped converter topology was higher than the five-level NNPC converter topology. The study could be extended by carrying out accurate modelling of the mission profile of the presented converter by using multi-domain simulation technique.

Analysis of the Medium-Frequency Oscillation Issue in a Medium-Voltage High-Power Wind Energy Conversion System

One promising solution for future large wind turbine power conversion is to use medium-voltage power conversion systems for high efficiency and high power-density, e.g., by using a cascaded structure presented in this paper. However, a medium-frequency (MF) oscillation issue was observed while testing the experimental prototype of the above system. Given the complex multivariable coupling models of the multiwinding transformer and multiple inverters, conventional single-variable theories are not suitable. Therefore, this paper presents a new decoupling analysis method. Using a proposed linear transformation matrix, the output currents of multiple inverters can be transformed to a summation (grid) current and difference currents, where the former is the summation of inverter currents and the latter are the differences between currents of any two inverters. This transformation has enabled the use of single-variable theories, e.g., Bode diagram to analyze the summation current and difference current separately. It was found that the MF oscillation issue is caused by the equivalent impedance of the summation current being much larger than that of the difference current. Specifically, using small controller parameters cannot ensure the grid current quality and using large parameters may cause oscillation in the difference current. The proposed method can provide guidelines to design the parameters of current regulators and filters in multiple-inverter systems. Experimental results of a downscaled wind energy conversion system are presented to verify the proposed method.

High-power medium-voltage three-phase ac–dc buck–boost converter for wind energy conversion systems

This paper proposes three topologies of single-stage three-phase ac–dc buck–boost converters suitable for medium-voltage and high-power wind energy conversion systems (WECS). The proposed converters draw sinusoidal input currents with nearly unity power factor from wind generators over wide range of ac voltages and frequencies. The proposed converters reduce the voltage and current stresses on the self-commutated switching device compared to existing solutions; thus, promising for new generation of relatively cheap WECSs. The proposed converters operate in Discontinuous Condition Mode (DCM), with their ac terminals connected to three-phase generator with open ended windings, or to ac supply using three single-phase transformers with isolated windings at converter side. The validity of the proposed converters and its detailed theoretical analysis and discussions are confirmed using PSCAD/EMTDC simulations, and further corroborated by experimentations, considering different operating conditions.

Robust Predictive Control of Three-Level NPC Back-to-Back Power Converter PMSG Wind Turbine Systems With Revised Predictions

A direct-drive permanent magnet synchronous generator (PMSG) with a three-level neutral-point-clamped back-to-back power converter is an attractive configuration for high-power wind energy conversion systems. For such a topology, finite-control-set model-predictive control (FCS-MPC) has emerged as a promising alternative. However, due to its fully model-based concept, variation of system parameters (in particular, the stator and grid filter inductance and rotor permanent-flux linkage) will (seriously) affect the system control performances when using the classical FCS-MPC. In this work, a robust FCS-MPC method with revised predictions is proposed and validated for such a system. With the proposed solution, not only the system robustness against parameter variations is improved, but also the control variable ripples are evidently reduced. The proposed method has been implemented with a fully field-programmable-gate-array-based real-time hardware. Its performance improvements in comparison with the conventional solutions are validated with experimental data.

Adaptive Control Strategy for Improving the Efficiency and Reliability of Parallel Wind Power Converters by Optimizing Power Allocation

Parallel converters are widely applied in high-power wind energy conversion system because of the limited power capacity of a single converter and demands for improved reliability. The handling power of these converters remarkably varies because of the fluctuating wind power. Using conventional power sharing control results in low operation efficiency of parallel converters at low wind speed and when reliability is not considered. Thus, an adaptive control strategy to improve the efficiency and reliability of parallel wind power converters based on optimizing power allocation is proposed. The efficiency-optimized power allocation between parallel converters is calculated by an exhaustive optimization algorithm, according to the efficiency curve of a single converter. The proposed adaptive control is realized by off-line optimization and real-time application, and can rapidly adapt to the fluctuating wind power. The handling power of the parallel converters is redistributed based on the accumulated power factors of these converters to balance total power stresses and increase lifetime. In addition, power hysteresis control is applied to optimize the transient process. An RT-LAB experiment validates the feasibility and effectiveness of proposed adaptive control. The operation efficiencies and failure rates of parallel converters with different control strategies under distinct wind profiles are compared. Comparative results demonstrate that the proposed adaptive control can improve the operation efficiency and reduce failure rate of parallel system under fluctuating wind.

Control of High-Power Wind Energy Conversion System Fed by Multi-level Converters

The progress of Variable Speed Wind Turbine Systems (VS-WTS) is currently focused in reaching higher power ratings (8MW). Also, medium voltage operation is at this power level, particularly at electrical network, a required feature. Consequently, high power medium-voltage multilevel converters for VS-WTS have been attracting more and more attentions. Nevertheless, most modern high-power VS-WTS consist of a large number of Wind Turbine Generators (WTGs) connected and operating simultaneously to reach the multi-megawatt level. This study presents a control strategy for a grid-connected onshore wind farm based Permanent Magnet Synchronous Generator (PMSG), where cascaded multilevel converters are employed on the generators and grid side. The generator-side converters provide speed control of the turbine-generator units to implement Maximum Power Point Tracking. The grid side Neutral Point diode Clamped (NPC) converter system regulates reactive and real power flow to the electrical network. So, it regulates the dc bus voltage and the ac side power-factor. The modulation technique for the proposed topology is developed from Space Vector Modulation. The overall control approaches of a rectifiers and an NPC are also described. Nonlinear Sliding Mode technique (NL-SM) with speed control and Vector Oriented Control are developed. To validate the proposed approaches, simulations are carried out on a high-power wind energy conversion system with multi-level converters and MATLAB/Simulink software. The simulation results demonstrate a good performance in diverse scenarios.

Line Filter Design of Parallel Interleaved VSCs for High-Power Wind Energy Conversion Systems

The voltage source converters (VSCs) are often connected in parallel in a wind energy conversion system to match the high power rating of the modern wind turbines. The effect of the interleaved carriers on the harmonic performance of the parallel connected VSCs is analyzed in this paper. In order to achieve low-switching losses, the $60^\circ$ clamp discontinuous pulsewidth modulation (DPWM1) is used to modulate the VSCs. A step-by-step design procedure of the line filter, which ensures the desired harmonic performance under all operating conditions, is presented. The analytical harmonic solution for the two parallel interleaved VSCs is derived in order to obtain the worst case voltage magnitude of the individual harmonic components. The required value of the filter admittance for the specific harmonic component is obtained by using the worst case voltage magnitude and the allowable harmonic injection limit. In order to achieve the desired filter performance with optimal values of the filter parameters, the use of an LC trap branch with the conventional LCL filter is proposed. The expressions for the resonant frequencies of the proposed line filter are derived and used in the design to selectively choose the values of the line filter components. The analysis and design methodology are also verified experimentally.

High-power wind energy conversion systems: State-of-the-art and emerging technologies

This paper presents a comprehensive study on the state-of-the-art and emerging wind energy technologies from the electrical engineering perspective. In an attempt to decrease cost of energy, increase the wind energy conversion efficiency, reliability, power density, and comply with the stringent grid codes, the electric generators and power electronic converters have emerged in a rigorous manner. From the market based survey, the most successful generator-converter configurations are addressed along with few promising topologies available in the literature. The back-to-back connected converters, passive generator-side converters, converters for multiphase generators, and converters without intermediate dc-link are investigated for high-power wind energy conversion systems (WECS), and presented in low and medium voltage category. The onshore and offshore wind farm configurations are analyzed with respect to the series/parallel connection of wind turbine ac/dc output terminals, and high voltage ac/dc transmission. The fault-ride through compliance methods used in the induction and synchronous generator based WECS are also discussed. The past, present and future trends in megawatt WECS are reviewed in terms of mechanical and electrical technologies, integration to power systems, and control theory. The important survey results, and technical merits and demerits of various WECS electrical systems are summarized by tables. The list of current and future wind turbines are also provided along with technical details.

Prolog to, "High-power wind energy conversion systems: State-of-the-art and emerging technologies"

This paper reviews state-of-the-art progress with respect to MW wind generator-converter configurations, wind farm configurations and grid code compliance methods. Four categories of generator-converter configurations have been identified and analyzed based on component count, modularity, reliability, converter/control complexity, device voltage stress, operation voltage, and achievable power levels.

Model-Predictive Control of Grid-Tied Four-Level Diode-Clamped Inverters for High-Power Wind Energy Conversion Systems

In this paper, a model-predictive control of medium voltage, grid-tied four-level diode-clamped inverter for use in high-power wind energy conversion systems is presented. The control objectives such as regulation of net dc-link voltage, reactive power generation to meet the grid operator request, dc-link capacitor voltages balancing, and switching frequency minimization are included in the cost function. The latter objective is an important requirement for high-power converters, and this has been achieved without any modifications to the software or hardware configuration. The future behavior of the grid currents and dc-link voltages is predicted for all the possible switching states using the discrete-time model of the inverter, dc-link, inductive filter, and grid. The switching state that minimizes the cost function has been chosen and applied to the inverter directly. During different operating conditions, the switching frequency is regulated between 750 and 850 Hz with the help of a lookup-table-based weighting factor, and thus, the proposed methodology appears as a promising tool to control the high-power grid-tied inverters. The results obtained through the MATLAB simulations on 4 MVA/4000 V system and dSPACE DS1103-based experiments on 5 kW/208 V prototype are in a close relationship and thus validate the proposed control strategy.

MPPT Control for Current Source Converter Based PMSG Wind Energy Conversion System

Current source converter (CSC) configuration tailored to high-power grid-connected wind energy conversion system (WECS) has been an attractive solution for direct drive of permanent magnet synchronous generators (PMSGs). A maximum power point tracking (MPPT) scheme for a PMSG-based WECS is presented in this paper. On the generator side, a fully controlled current source converter is inserted as a circuit interface to handle a wide range of the variable wind speeds. Rectifier side controller extracts maximum power through closed-loop regulation of generator speed. The available maximum power fed to the grid is adjusted by regulating dc-link output current of CSC according to the variable wind speed. Simulation results obtained from a megawatt PSIM model are provided to verify the proposed concepts.

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.

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.