Direct-drive Permanent Magnet Synchronous Generator Research Articles

Supplementary control based on current source coupling for improving dynamic characteristics of active distribution network

Supplementary control (SC) technology is widely leveraged by power supply companies in active distribution networks (ADNs) to improve their stability and dynamic characteristics. Yet, the existed SCs are generally implemented from inside the converter controllers of distributed generators (DGs) or active loads, so there is a need to redesign the internal physical structure of the existing controller, resulting in the increasing work amount of assembling and workability. This paper studies the specific R & D process of a novel external coupling type SC (ECSC), which is based upon current source injection (CSI-ECSC) for improving the dynamic characteristics of ADN. The SC current signals are coupled to the current sampling loop from outside the converter controller. And the employment of the existing current sample makes it unnecessary to redesign the internal physical structure of the existing controller. As a result, the SC assembling is simplified and its workability is improved. In this paper, a detailed exemplary ADN with direct-drive permanent magnet synchronous generator (PMSG) is firstly set up in math for full eigenvalue analysis. Then, the CSI-ECSC is designed with its control loop, interface circuit, and parameter setting. Furthermore, by using PSCAD/EMTDC, groups of case studies are conducted in ADNs where photovoltaics (PVs) and energy storage (ES) are included. Finally, the real-time hardware-in-the-loop (HIL) testing validates the functionality of the realized CSI-ECSC in RTDS.

Method to Evaluate the Impact of Integration of PMSG on Stability of Synchronous Generator Shaft Under Weak Grid Connection

With the continuous growth in the amount of wind power accessed by the AC grid, the impact of the grid connection of wind-power generators with the power system has gradually increased. In this study, the subsynchronous oscillation of a synchronous generator (SG) shaft caused by the integration of direct-drive permanent-magnet synchronous generators (PMSGs) was investigated. The mechanism governing the effect of the connection strength between the PMSG and AC power system on the stability of the generator shaft system was analyzed based on the complex torque coefficient method. When the connection strength between the PMSG and AC power system weakens, the same voltage variation that occurs at the point of common coupling of the PMSG stimulates more intense power fluctuations in the PMSG, and the electrical damping injected by the PMSG into the SG increases considerably. This may cause the oscillation mode dominated by the generator shaft system to move to the right half of the complex plane, thereby reducing the stability of the generator shaft system. In addition, the evaluation process of the influence of the PMSG on the SG shaft system was summarized, and the proposed method can determine the stability of the AC power system after the integration of the PMSG. Finally, the effectiveness of the proposed method was validated via study cases, and conclusions were drawn. This method is expected to serve as a useful tool for the risk assessment of subsynchronous oscillations in wind farms.

Integrated Optimal Design of Permanent Magnet Synchronous Generator for Smart Wind Turbine Using Genetic Algorithm

In recent years, the investment in the wind energy sector has increased in the context of producing green electricity and saving the environment. The installation of small wind turbines (SWTs) represents an actual strategy for meeting energy needs for off-grid systems and certain specialized applications. SWTs are more expensive per kilowatt installed as compared to large-scale wind turbines. Therefore, the main objective of this study is to produce an economical technology for the wind power market offering low-cost SWTs. The idea consists of considering a simple structure of the wind turbine using direct-drive permanent magnet synchronous generator (DDPMSG). DDPMSGs are the most useful machines in the wind energy field thanks to several advantages, such as elimination of noise and maintenance cost due to suppression of the gearbox and absence of the rotor circuit excitation barriers by the presence of the permanent magnets (PMs). Their major downside is the high cost of active materials, especially the PMs. Thus, the improvement of the generator design is treated as being the main component of the considered chain to assure active materials’ mass and cost reduction. The methodology studied aims to explain the approach of the design integrated by optimization of the considered system. It is based on the elaboration of analytical models to find a feasible structure for the system, taking into account the multi-disciplinary analysis. The relevance of these models is validated by the finite element method using 2D MATLAB-FEMM simulation. The models are integrated to elaborate the optimization problem based on a genetic algorithm to improve the cost of the proposed generator by minimizing the mass of its active constructive materials. As an outcome, an optimal solution is offered for the wind generators market, providing a 16% cost reduction.

A second-order harmonic circulating current injection method for MMC capacitance reduction in offshore DC wind turbine

The offshore DC wind turbine, consisting of direct-drive permanent magnet synchronous generator (PMSG) and modular multilevel converter (MMC), has become a promising candidate for the large-capacity wind energy conversion system. However, due to the low-frequency output current of PMSG, submodule (SM) capacitors suffer from large voltage ripple especially at rated wind speed. Second-order harmonic circulating current (SHCC) injection is effective for ripple reduction, but conventional techniques only suppress the fundamental component, leading to large SHCC at low wind speed, where the ripple is naturally small. The main contribution of this paper involves that a SHCC injection method, considering all voltage ripple components, is proposed to maintain the ripple within a reasonable limit, which decreases the SHCC amplitude. The proposed method not only mitigates the voltage ripple, but also reduces the MMC power loss over the entire wind speed range. Firstly, the voltage ripple is modeled with SHCC. Then, the injected SHCC for defined ripple limit is derived, which facilitates capacitance selection. Finally, a case study of MMC connected with 10-MW and 10-kV PMSG is conducted, based on which the voltage ripple and power loss without SHCC, with conventional and proposed injection methods, are analyzed and compared. The results demonstrate that, the maximum voltage ripple of conventional method can be more than 2 times that of the proposed method. Moreover, the SM capacitance can be reduced by 40% with the proposed method, which also features power loss reduction over the entire wind speed range.

Hybrid PSO-GSA algorithm-based optimal control strategy for performance enhancement of a grid-connected wind generator

<span>Due to the great level of wind energy penetration in the existing network, huge efforts have been directed to enhance the grid-connected wind generator performance. This paper shows an application of a hybrid algorithm of the particle swarm optimization and the gravitational search algorithm (PSO-GSA) to enhance the transient stability of the grid-tied wind energy conversion system. The variable-speed wind turbine (VSWT) direct-drive permanent-magnet synchronous generator is connected to the network through a full-scale converter. The generator- and grid-side converters are controlled by utilizing an optimum proportional-integral (PI) controller. The criterion of the integral squared error is utilized as an objective function. The PSO-GSA based-PI controller efficacy is validated by comparing its results with that are obtained by utilizing the genetic algorithm (GA)-based-PI controller. The performance of the suggested control scheme is checked during various fault conditions. The control scheme quality is legalized by the simulation results that are obtained using MATLAB/Simulink program</span>

Analysis and Performance Of Grid Connected Straight Driven Pmsg Based Wind Turbines

Permanent-magnet synchronous generator (PMSG) are used widely in wind turbine applications. The performance analysis of PMSG can be enhanced by adopting a number of control mechanisms with the benefit of advanced optimization techniques. Renewable energy is harnessed from continuously replenishing natural processes. Some commonly known are sunlight, water, wind, tides, geothermal heat and various forms of biomass. The focus on renewable energy has over the past few decades intensified greatly. This paper presents modeling and control strategy for the grid connected wind turbine system based on Permanent Magnet Synchronous Generator (PMSG). The considered system is based on back-to-back converter topology. The Grid Side Converter (GSC) achieves the DC bus voltage control and unity power factor. The Machine Side Converter (MSC) assures the PMSG speed control. The PMSG is used as a variable speed generator and connected directly to the turbine without gearbox. The pitch angle control is not either considered in this study. Further, Optimal Tip Speed Ratio based MPPT control strategy is used to ensure the most energy efficiency whatever the wind speed variations. A filter (L) is put between the GSC and the grid to reduce current ripple and to improve the injected power quality.During grid faults, a hierarchical coordinated control scheme for the generator-side converter, main grid-side converter and auxiliary grid-side converter, depending on the grid voltage sags, is presented to enhance the low voltage ride through (LVRT) capability of the direct-driven PMSG WT.

A new robust control scheme: Application for MPP tracking of a PMSG-based variable-speed wind turbine

This paper proposes a novel Improved Maximum Power Point Tracking (IMPPT) algorithm to extract the maximum available power from a direct-drive Permanent-Magnet Synchronous Generator (PMSG) based standalone Small-scale Variable Speed Wind Turbine (VSWT). The proposed control scheme consists of an IMPPT algorithm to effectively improve the extracted power under various regimes of wind speed. The IMPPT constructs the reference value of the DC voltage for the DC bus. Moreover, a composite low-cost controller (LCC) is also proposed in order to improve the DC voltage tracking based on a new designed nonlinear state observer. The proposed nonlinear robust controller accounts for the overall system dynamics and nonlinear behavior. The objective is to improve the dynamic performance and ensure a good balance of energy conversion efficiency, robustness, cost efficiency, and a simple structure for practical implementation in wind energy conversion systems. Furthermore, the stability of the closed-loop system is analyzed and guaranteed through Lyapunov stability theory. Moreover, two scenarios are used for validation in Matlab/Simulink, including step change and stochastic profiles of wind speed. Simulation results verify the effectiveness and superiority of the IMPPT-LCC approach whereas comparisons with other techniques prove its superiority.

Component-Level Reliability Assessment of a Direct-Drive PMSG Wind Power Converter Considering Two Terms of Thermal Cycles and the Parameter Sensitivity Analysis

The availability and efficiency of a wind power system are highly affected by the failure or the unreliable operation of its power converter. This article investigates the failure rate and annual consumed lifetime for a 2-MW direct-drive permanent magnet synchronous generator based full-scale converter in a wind power generation system. The reliability assessment mainly focuses on the component level, namely, diodes and IGBTs, in both of the machine-side converter and the grid-side converter. Annual damages and power cycles for semiconductors are calculated separately under long-term thermal cycles (several seconds to hours) and short-term thermal cycles (dozens to hundreds of milliseconds). Experiments regarding thermal stress measuring for different semiconductors under short-term thermal cycles are affiliated. A comparison between different thermal cycles is given and discussed in detail. To ensure a visualized time-to-failure evaluation, a Monte Carlo method is used to generate the lifetime distributions and entire unreliability functions for power semiconductors. Final B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> and B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> lifetimes can easily be observed from the cumulative distribution functions. Moreover, different standard deviations are assumed for parameters in the Bayerer's lifetime model, and by using several parallel Monte Carlo algorithms, parameter sensitivity to the final lifetime evaluation is analyzed.

Novel Switching Frequency FCS-MPC of PMSG for Grid-Connected Wind Energy Conversion System with Coordinated Low Voltage Ride Through

The integration of wind energy systems (WECS) into the power grid through power electronic converters should ensure the high performance of the control system. In spite of several advantages of conventional Finite control set-model predictive controller (FCS-MPC), variable switching frequency and high computational burden are considered its main drawbacks. In this paper, a fast FCS-MPC of a machine side converter (MSC) of direct-driven permanent magnet synchronous generator (PMSG) based wind turbines for wind energy conversion system is proposed. The wind energy conversion system has been realized using a direct driven PMSG and a full-scale back-to-back power converter. The proposed controller is designed to reduce the required calculations in each horizon. In addition, the performance of conventional FCS-MPC is compared with the proposed method, and an improvement in total harmonic distortion spectra and simulation time required even when imposing a lower sampling frequency was found. To overcome the variable switching frequency problem, a modulation algorithm is introduced in the minimization process of modulated FCS-MPC. To keep the proposed system attached to the utility during a fault, a coordinated pitch angle control and low voltage-ride through (LVRT) algorithm is designed and inserted in the vector control of the grid side converter (GSC) to supply reactive power to the grid during fault for ensuring safe operation of the inverter and meeting the grid code requirements. The effectiveness of the proposed controller is illustrated using simulation results under different operating conditions.

STFT Analysis of Broadband Oscillation in VSC-HVDC Connected to Offshore Wind Farms

The transmission technology of offshore windfarms (OWFs) through VSC-HVDC system is developing rapidly. The impact factor of broadband oscillation is complex, it often occurs in VSC-HVDC and direct drive permanent magnet synchronous generator (DDPMSG). In this paper, based on short time fourier transform (STFT), factors that cause broadband oscillation are studied. Firstly, the primary structure of DDPMSG and VSC-HVDC is established. Then the correctness of small signal method (SSM) is verified. Based on eigenvalue analysis, instability caused by controlling parameter is analysed. Finally, time-frequency characteristic of oscillation waveforms in PSCAD is extracted using STFT, asynchronous characteristics of broadband oscillation is verified. It is of guiding significance, for the developing technology of offshore wind farm integrated into grid through VSC-HVDC system.

Wind Generator Impedance Matching in Small-Scale Passive Wind Energy Systems

In this paper, the improved turbine power matching of passive wind energy systems for dc-connected battery storage applications with an impedance matching method is investigated. The passive system uses a direct-drive permanent magnet synchronous generator and is directly connected via a diode rectifier to the dc fixed-voltage battery storage. To improve power matching, an external inductance is added to the passive system between the generator and the diode rectifier. A static finite element based solution method is proposed to accurately calculate the necessary external inductance to achieve near maximum power point matching. It is shown that the proposed finite element based calculation method is computationally efficient and excellently suited for generator design optimization, which is critical for this application. It is also shown that by rewinding existing machines for the correct cut-in speed and adding the external inductance for improved power matching, existing machines can be effectively recycled for passive wind energy systems. The proposed static finite element solution method's accuracy and improved power matching are confirmed with measured results on a sub 5 kW power level.

System-level reliability assessment for a direct-drive PMSG based wind turbine with multiple converters

System-level reliability of wind power converter has an essential effect on the operation performance and lifespan of a wind turbine system. In this paper, a wind turbine equipped with a 2 MW direct-drive permanent-magnet synchronous generator (PMSG) serves as a case study. Considering the maximum stator current limitation of the PMSG, several multiple-converter structures and their reliability block diagrams (RBDs) are constructed for the machine side converter (MSC). To investigate the reliability influence caused by the amount of semiconductor components and the current for each component, the structures with four and five bridges in parallel are both configured. Reliability evaluation between two major parallel structures, namely, bridges in parallel and converters in parallel are also compared. In addition, the effect of different wind classes on the MSC system-level reliability has been investigated. A detailed discussion regarding the system reliability cumulative distribution function (CDF) is presented, which could serve as reference for future MSC structure design. It is concluded that the component current dominates in the system-level reliability analysis of the MSC and the standby structure can also improve the reliability under the same current level. Besides, compared with the converter structure changing, various wind classes have a minor impact on the system-level reliability.

P.300 Neurocognitive endophenotypes for bipolar disorder identified by visual backward masking and wisconsin card sorting test

System-level reliability of wind power converter has an essential effect on the operation performance and lifespan of a wind turbine system. In this paper, a wind turbine equipped with a 2 MW direct-drive permanent-magnet synchronous generator (PMSG) serves as a case study. Considering the maximum stator current limitation of the PMSG, several multiple-converter structures and their reliability block diagrams (RBDs) are constructed for the machine side converter (MSC). To investigate the reliability influence caused by the amount of semiconductor components and the current for each component, the structures with four and five bridges in parallel are both configured. Reliability evaluation between two major parallel structures, namely, bridges in parallel and converters in parallel are also compared. In addition, the effect of different wind classes on the MSC system-level reliability has been investigated. A detailed discussion regarding the system reliability cumulative distribution function (CDF) is presented, which could serve as reference for future MSC structure design. It is concluded that the component current dominates in the system-level reliability analysis of the MSC and the standby structure can also improve the reliability under the same current level. Besides, compared with the converter structure changing, various wind classes have a minor impact on the system-level reliability.

Analysis of Forced SSOs Excited by Subsynchronous Interharmonics From DPMSG-Based Wind Farms

The subsynchronous oscillations (SSO) caused by the dense integration of direct-drive permanent magnet synchronous generator (DPMSG) based wind farms have threatened the secure and stable operation of the local Hami power grid in Xinjiang Region, China. The DPMSG-involved SSO events show three new characteristics: random occurrence, time-varying frequency and amplitude, and wide involvement of all types of power electronics devices. To reveal the mechanism of the three new characteristics shown in the real SSO events, this paper develops the forced SSO (FSSO) theory and its method of use. Firstly, the generation and the time-varying performance of subsynchronous interharmonics (SSIH) from the DPMSG converters are analyzed based on the modulation theory. It is discovered that the SSIHs from the DPMSGs can be the disturbance source that excites the positive-damping subsynchronous mode to cause the FSSO. Further, the three occurrence conditions of the FSSO are proposed, and the three characteristics of the FSSO are revealed. It is concluded that the characteristics of the FSSO coincide with those of the real SSO events in the local Hami power grid. Finally, the theoretical analyses are validated by simulations in case studies.

Small-Signal Stability Analysis of The Hydrokinetic Energy Harnessing connected to The Grid

This paper presents the modelling of the hydrokinetic system for the small-signal stability analysis under the small disturbances due to variation and fluctuation of water velocity in the river or marine. The complete modelling of the hydrokinetic system consists of vertical axis H-Darrieus turbine, direct-drive permanent magnet synchronous generator (PMSG), back-to-back converter and the grid network. By linearising all the equation around the steady-state value, the dynamic equation of the hydrokinetic system is derived. The stability of the system is tested with and without the PI controller. The eigenvalues analysis-based approaches have been used to investigate the stability of the system under the small disturbances. The findings show, the stability of the hydrokinetic system with PI controller is improved up to 57.82% by reducing the oscillation frequency at the Rotor Side Converter (RSC).

Impedance Network Model of D-PMSG Based Wind Power Generation System Considering Wind Speed Variation for Sub-Synchronous Oscillation Analysis

Recently, sub-synchronous oscillation (SSO) issues were observed in direct-drive permanent magnet synchronous generator (D-PMSG) based wind power generation systems (WPGS) comprising multiple wind farms. Impedance model (IM) analysis has become an important method to study such problems. In this paper, an IM of a D-PMSG grid-connected system (GCS) is established considering the relationship between wind speed and wind turbine output power and the effect of voltage outer loop control. An impedance network model (INM) for a multi-wind-farm grid-connected system (MWFGS) is constructed with IMs of each wind turbine/farm and transmission lines according to the interconnection topology of them. Then, based on the IM of the D-PMSG-GCS, the influence of wind speed on the SSO characteristics is quantitatively analyzed, and the accuracy of the model is verified by comparing with the eigenvalue analysis results. Finally, the influencing factors such as distributed wind speed and network topology of MWFGS on SSO characteristics are quantitatively analyzed based on the INM, and the time-domain simulation verifications are performed in PSCAD/EMTDC. The results prove the effectiveness of INM in analyzing SSO problems of a large-scale WPGS. This is of great significance for the planning of large-scale D-PMSG-based WPGSs and the suppression of SSO.

Subsynchronous Oscillation Analysis of Grid-Connected Converter Based on MIMO Transfer Functions

Subsynchronous oscillations (SSOs) induced by the interaction between wind farms based on direct-drive permanent magnet synchronous generators (D-PMSGs) and weak grids have attracted extensive attention. To better understand this SSO phenomenon, this paper derives multi-input multi-output (MIMO) transfer functions between the references of the current controller and the current of a grid-connected converter in the d-q frame, where the dynamic of PLL is considered. The dynamic of PLL lead to positive feedback in the MIMO transfer function, which impairs the stability of the overall system and induces the SSO phenomenon. Using the classical root locus method, the influence of correlated control parameters and different operation conditions on the system stability is substantially analysed. It is found that heavy load, low short circuit ratio (SCR) will intensify the SSO instability of system. Nonlinear electromagnetic transient simulations of a D-PMSG-based weak-grid-connected wind-farm system using PSCAD/EMTDC software effectively verify the correctness of the model proposed in the SSO analysis.

Grasshopper Optimization Algorithm-Based Fuzzy Logic Controller for Performance Enhancement of a Grid-Connected Wind Generator

Due to the direction of the world to diminish pollution production and to enhance the environmental conditions, the penetration of wind energy in the electric grid is appreciably increased nowadays. The variable-speed wind turbine direct-drive permanent-magnet synchronous generator is connected to the electrical network through a frequency converter-based an optimum controller, such as a conventional proportional-integral controller (PI) or the fuzzy logic controller (FLC). The PI has a constant gain but has some drawbacks, such as affected by variable variation. This paper exhibits an implementation of a grasshopper optimization algorithm (GOA) to design FLC. GOA-based FLC is utilized to control the grid- and generator-side converters to enhance the transient stability of the grid tied-wind energy. To ensure the efficacy of the proposed control scheme, a comparison among the results achieved by the GOA-based FLC and Newton Raphson based-PI has occurred. The simulation results describe the capability of the GOA-based FLC to enhance the response of the grid-tied wind energy under various operation cases. Simulation is carried out by MATLAB/Simulink program.

Sensorless control of wind power generator with flywheel energy storage system

For the purpose of sensorless low-speed control of direct-drive permanent magnet synchronous generator–based wind power systems, a technique based on the rotating high-frequency voltage-signal injection is discussed in this article, which improves the efficiency of the method and eliminates the phase shift introduced by traditional filters; band pass and low-pass filters are replaced by an extended complex Kalman filter algorithm. In addition, a flywheel energy storage system based on a squirrel cage induction machine is connected with the wind power generator by a DC bus through two power converters. For sensorless control of the induction machine, a rotor speed estimation method based on the extended complex Kalman filter is established. Finally, network connection interface of the complete system is presented and the efficiency of the proposed sensorless control of the whole chain of wind power generation is shown by numerical simulations using MATLAB/Simulink environment.

Fault-Tolerant Back-to-Back Converter for Direct-Drive PMSG Wind Turbines Using Direct Torque and Power Control Techniques

Fault tolerance in wind turbines is considered crucial to increase their reliability and availability levels. This paper presents a fault-tolerant direct-drive permanent magnet synchronous generator (PMSG) using new direct control techniques, with the ability to handle power semiconductor open-circuit faults in the full-scale back-to-back converter. The fault-tolerant topology consists of a five-leg converter, with a shared leg connected to a generator phase and to its corresponding grid phase, through a triode for alternating current (TRIAC). The main contribution of this paper consists of the development of an alternative direct torque control and direct power control schemes for both machine-side converter and grid-side converter, respectively. Moreover, a reliable fault diagnostics algorithm without requiring additional sensors is also integrated, providing the information required to instantaneously trigger fault-tolerant remedial strategies. Simulation and experimental results are presented to validate the effectiveness of the proposed fault-tolerant PMSG drive.