Direct-drive Permanent Magnet Synchronous Generator Research Articles

Comparative life cycle assessment of electricity generation by different wind turbine types

Wind turbines produce electricity with hardly any emissions during operation. Most environmental impacts are associated with their manufacture. This work performs a comparative life cycle assessment (LCA) to evaluate the environmental impacts of different 3 MW power class wind turbines at a fictive onshore site in Germany. The three most frequently installed onshore turbine types are considered: geared converter with doubly-fed induction generator (DFIG), direct driven synchronous generator (DDSG) electrically excited and direct drive permanent magnet synchronous generator (DDPMSG). LCA reveals that environmental impacts are concentrated during manufacturing of fundament, tower and nacelle, which account for up to 19%, 30% and 99% of single impacts, respectively. Main drivers are the use of copper, steel and in case of DDPMSG also the rare earth permanent magnets. The DDSG shows higher impacts than the other wind turbines in 14 out of all 15 categories, due to the higher weight of its nacelle. Impacts due to operation add up to 3%.Four sensitivity analyses are conducted to estimate the effects of component replacement, recycling, origin of rare earths for permanent magnet production for DDPMSG and electricity yield. Possible recycling options show the highest improvement potential and even change turbine types ranking.

Low‐voltage ride‐through enhancement with the ω and T controls of PMSG in a grid‐integrated wind generation system

This study deals with the formulation of a simple control strategy for the enhancement of the low-voltage ride-through (LVRT) capability of the grid-integrated permanent magnet synchronous generator (PMSG)-based wind generation system (WGS). The control warrants satisfying minimum required reactive current support to the ac grid as first priority with minimum loss of generation and maximum utilisation of the converter current capacity during LVRT compliance as per E.ON Netz German grid code requirements. The reference control variables in coordinated control of the stator-side and grid-side converters are altered so as to support the reactive current to the ac grid and, to store the excess active power in the system inertia for keeping the dc-link capacitor voltage in the safe limit during voltage dips at the grid side. The investigations have been made on the proposed strategy of LVRT enhancement with the speed (ω) and torque (T) controls of the direct-drive PMSG in grid-integrated WGS exploiting commonly used tip-speed-ratio and optimum-torque control algorithms of peak power tracking, respectively. Simulation results have been presented to validate the proposed strategy. The performances of the proposed strategy of LVRT enhancement has also been compared to the conventional method like dc-link braking chopper.

Harmonic compensation capability‐based coordinated control for hybrid wind farms under distorted grid voltage conditions

This article presents a harmonic compensation capability-based coordinated control strategy for a hybrid wind farm including doubly fed induction generator (DFIG)-based and direct-driven permanent-magnet synchronous generator (PMSG)-based wind turbines operation under distorted grid voltage conditions. First, the maximum harmonic current compensation capabilities of the DFIG and PMSG systems are analysed in detail when the grid voltage is harmonically distorted. Combining the maximum harmonic current compensation capabilities with the harmonic current requirements of different control targets, the controllable operating regions of each control target of the DFIG and PMSG systems during network voltage distortions are presented. Furthermore, the effects of the voltage total harmonic distortion (THD) and the system operating conditions on the controllable operating regions are analysed. Based on the analysis, the coordination strategy is proposed for a hybrid wind farm without communication equipment between the different wind power generators. The proposed strategy can improve the interoperability and the output power quality of the entire hybrid wind farm during grid-voltage harmonic distortions. Finally, the effectiveness of the proposed coordination strategy for the hybrid wind farm is validated by simulation and experimental results.

An analytical method suitable for revealing the instability mechanism of power electronics dominated power systems

In view of the instability phenomenon of power electronics dominated power systems, an analytical method suitable for revealing the instability mechanism of power electronics dominated power systems is proposed. The proposed analytical method divides power electronics dominated power systems into two subsystems: electrical subsystem and control subsystem. First, the weak links of the system are screened out by eigenvalue analytical method, then the linearized relation of state variables in the weak links is derived from the perspective of the electrical subsystem. Through the study of the interactions between the weak links of the electrical subsystem and the related control subsystem, the mechanism of the system instability can be explained by “positive feedback effect” formed by the interactions between the electrical subsystem and the control subsystem. The proposed analytical method can reveal the dynamic interactive process of state variables compared to impedance analytical method and eigenvalue analytical method when the system is unstable. Meanwhile, a linearized model of weak links in the electrical subsystem can be set up specially, helping to reduce the workload. Finally, the analytical method is verified by three study cases when VSC-HVDC is connected to the weak AC power grid under two different operation conditions and when direct-drive permanent magnet synchronous generator system is connected to VSC-HVDC system.

Sensorless-Based Active Disturbance Rejection Control for a Wind Energy Conversion System With Permanent Magnet Synchronous Generator

This paper focuses on the control problem of wind energy conversion systems (WECSs) with direct-driven permanent magnet synchronous generator (PMSG) working in variable power output stage. A novel compound control scheme combined the active disturbance rejection controller (ADRC) and speed sensorless technology is proposed. In order to achieve maximum power point tracking (MPPT), a speed control loop is designed based on ADRC to improve the speed tracking ability and anti-disturbance ability of system. Moreover, considering the large amount of system model information, a model-assisted ADRC is designed on the nominal ADRC to improve the response speed of system and reduce the energy consumption for the system control. Additionally, to solve the contradiction between the requirement of speed sensor precision and the economy of system design, a speed observer based on current model is designed in $\alpha -\beta $ coordinate system. Furthermore, a resistance observer is introduced to improve the convergence rate of the observer. And a position sensorless observer structure based on the speed observer is proposed. Finally, simulation studies are conducted to evaluate power tracking performances of the proposed speed-observer-based model assisted ADRC technology. It is shown that the proposed compound scheme exhibits significant improvements in both control performance and anti-disturbance ability with high observation precision compared with the normal ADRC method.

Probability Assessment of Characteristics of Sub-Synchronous Oscillation in D-PMSG-Based Wind Power Generation System

In recent years, sub-synchronous oscillation (SSO) phenomena occur in direct-drive permanent magnet synchronous generator (D-PMSG) based wind power generation systems (WPGS) in many regions of China. The SSO characteristics, frequency and damping, are all time-varying mostly affected by changes of wind speed and D-PMSGs number. It is the importance for the design of SSO suppression to know the variation ranges of characteristics and the risk of unstable SSO. The small signal model of the D-PMSG-based WPGS is firstly established in this paper, considering the functional relationship between wind speed and wind turbine output power. Based on the small signal model and the probability distribution function (PDF) of wind speed, the SSO probability assessment method with the least square method (LSM) polynomial fitting is then proposed. Therefore, the probability distributions of SSO frequency and damping considering wind speed variation are obtained efficiently. By PSCAD/EMTDC simulations and comparisons, the obtained results with the eigenvalue analysis method based on the small signal model are certificated. Then the probability assessment method is applied to study the influences of different average wind speeds under different number of D-PMSGs on the probability distribution of SSO frequency and damping. The changes of SSO frequency bandwidth and the risk of unstable SSO are depicted relative completely. The variation range of SSO frequency is consistent with the actual accident scenario in Hami area of Xinjiang, China.

A model-based implementation of an MPPT technique and a control system for a variable speed wind turbine PMSG

This paper proposes a model-based control system for a wind energy conversion system (WECS) using a direct driven permanent magnet synchronous generator (D-PMSG). The generator is connected to the grid via a back-to-back pulse-width modulation (PWM) converter with a switching frequency of 10 kHz. The integrated system model is developed in MATLAB/Simulink and is based on the practical setup parameters to fully and accurately mimic the behaviour of the experimental system. A PI controller provides the generator with an optimum speed via an aerodynamic model of the wind turbine and can be readily employed for the practical setup using the designed parameters in the simulation model. A maximum power point tracking (MPPT) algorithm is further developed to ensure the maximum power is captured from a wind turbine. The MPPT algorithm is simplified to reduce the computational time required for the real-time simulation.

Modeling and Robust Control of a Grid Connected Direct Driven PMSG Wind Turbine By ADRC

In this paper, we present the modeling and control of a grid connected Variable Speed Wind Energy Conversion System (VS-WECS) based on a Direct Driven Permanent Magnet Synchronous Generator (DD-PMSG). A new robust control has been proposed and utilized to operate the wind turbine so as to extract the maximum power from the wind energy and to ensure a unit power factor. This control is known as the Active Disturbance Rejection Control (ADRC) and it is based on the Extended State Observer (ESO). It consists in controlling, through the stator currents, the machine side converter in order to adapt the rotational speed of the generator to the different wind speed profiles (Maximum Power Point Tracking MPPT). In addition, it ensures the control of the DC bus voltage and the exchange of active and reactive powers between the wind turbine and the electrical power grid. In order to evaluate the performance of the proposed control a series of simulations are made under the MATLAB/SIMULINK environment. The results obtained by simulation show that the proposed strategy is efficient in terms of stability and precision as well as for the robustness with regard to the internal disturbances when variying the parameters of the machine.

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.

Impedance Modeling and Stability Analysis of Direct Drive Permanent Magnet Synchronous Generators

Sub-synchronous oscillation (SSO) problems exist in wind power gathering areas as northwest China. To make clear mechanism of SSO induced by large capacity direct drive permanent magnet synchronous generators (D-PMSG), linear impedance model of grid side converter (GSC) is built and verified by simulation. Closed-loop transfer functions are deduced, and stability of D-PMSG connected to weak network, together with the influencing factors, are analyzed by impedance method, which helps to analyze and solve SSO problems in wind farms.

Comparison Between PSO and Genetic Algorithms and for Optimizing of Permanent Magnet Synchronous Generator (PMSG) Machine Design

This paper proposes application of Genetic Algorithm (GA) and Particle Swarm Optimization (PSO) in the design of direct-driven permanent magnet synchronous generator machine (PMSGs) for wind turbine applications. The power rating of these machines is in the mega watt (MW) level. The constraints and requirements of the generator are outlined. The proposed design scheme optimizes various PMSG parameters like Pole pair number, Linear current density, Air gap thickness, Rotor outer diameter, Relative width of the permanent magnet etc to achieve certain objectives like maximizing efficiency, increasing Torque, improving power factor etc. The results obtained by GA algorithm and those by PSO algorithm are compared. The performance of Particle Swarm Optimization is found to be better than the Genetic Algorithm, as the PSO carries out global search and local searches simultaneously, whereas the Genetic Algorithm concentrates mainly on the global search. Results show that the proposed PSO optimization algorithm is easy to develop and apply and produced competitive designs compared to the GA algorithm.

Comparative Analysis of Bearing Current in Wind Turbine Generators

Bearing current problems frequently appear in wind turbine systems, which cause wind turbines the break down and result in very large losses. This paper investigates and compares bearing current problems in three kinds of wind turbine generators, namely doubly-fed induction generator (DFIG), direct-drive permanent magnet synchronous generator (PMSG), and semi-direct-drive PMSG turbines. Common mode voltage (CMV) of converters is introduced firstly. Then stray capacitances of three kinds of generators are calculated and compared through the finite element method. The bearing current equivalent circuits are proposed and simulations of the bearing current are carried out. It is verified that the bearing currents of DFIGs are more serious than the two kinds of PMSG, while common mode current (CMC) of the direct-drive PMSG is much greater than the other two types of wind turbine generators.

Coordinated control of a hybrid wind farm with DFIG-based and PMSG-based wind power generation systems under asymmetrical grid faults

Abstract A non-communication-based coordinated control strategy for a hybrid wind farm with doubly fed induction generator (DFIG)-based and direct-driven permanent magnet synchronous generator (PMSG)-based wind farms under severe asymmetrical grid faults is proposed in this paper. Firstly, the in-depth research of the severe asymmetrical fault and its impact on the operation characteristics of the DFIG and PMSG systems are investigated. Secondly, based on the operation characteristics analysis, the control objectives and priorities of the hybrid DFIG and PMSG systems are described first time during severe asymmetrical fault, respectively. In addition, the current allocation principles of each control unit in the DFIG and PMSG systems are investigated in detail according to the converter capacity and the system operation conditions. Furthermore, a coordinated control strategy for the hybrid wind farm is proposed. This strategy make full use of each wind farm's current capability, both the operation performance of the entire hybrid wind farm and the voltage quality of the power grid was greatly improved collectively. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed control strategy for the hybrid wind farm with DFIG and PMSG are validated by the simulation and experimental results.

Sub‐synchronous oscillation in PMSGs based wind farms caused by amplification effect of GSC controller and PLL to harmonics

Sub-synchronous oscillation (SSO) caused by direct-drive permanent magnet synchronous generators (PMSGs) occurs frequently in Xinjiang Uygur Autonomous Region, China. As a new type of SSO, the mechanism of power oscillation triggered by PMSGs has not been clarified yet. In this study, a small-signal analysis method was used to investigate the response process of the grid-side converter (GSC) controller and a phase-locked loop (PLL) to harmonics. Components with the same frequency of input disturbance signal and an output signal which generated by response process of controller superpose at the outlet of PMSGs could be amplified by positive feedback. Amplified harmonics will cause sustained power oscillation in sub-synchronous frequency. A criterion for judging dangerous frequencies of SSO was deduced based on phase relation between input and output signals, and the form of output signal was predicted under the condition of certain parameters setting. The theoretical analysis is then validated by the simulation results and the on-site data measured by the phasor measurement unit. Furthermore, the impacts of GSC and PLL parameters on SSO were discussed. This study provides the theoretical basis for SSO mechanism and judgement method in practical engineering.

Sensorless Strategy of a Two-Level MPPT for Permanent Magnet Wind Generation System Using SRF-PLL

This paper presents the optimization of the wind power in a variable speed Wind Energy Conversion System (WECS) using a sensorless Maximum Power Point Tracking (MPPT) strategy that avoids mechanical measurement of the wind speed. The proposed system consists of a turbine, a direct-drive Permanent Magnet Synchronous Generator (PMSG) controlled by a Field Oriented Control (FOC) through a three phase controlled rectifier connected to a DC load. The choice of the FOC is justified by a power maximization analysis. The generator rotor speed and position required by the FOC and the sensorless MPPT are estimated using a Synchronous Reference Frame Phase Locked Loop (SRF-PLL). The proposed technique is validated using both simulation in Matlab-Simulink and experimental work based on step changes in wind speed. Results are presented at the end of the paper showing that the estimated angle and speed by the SRF-PLL are approximatively the same as the real ones, also figure of the output power shows that it is maximized confirming the effectiveness of the adopted approach.

The Research on Second-Order ADRC Algorithm of Using Wind Turbine Virtual Inertia to Participate in Primary Frequency Regulation in a Small Stand-Alone Microgrid

In order to improve the transient stability of frequency in a small stand-alone microgrid (SSM), this paper takes a SSM composed of a direct-drive permanent magnet synchronous generator (D-PMSG) and a micro gas turbine (MGT) as the background and uses wind turbine generator (WTG) virtual inertia (VI) to participate in the primary (short-term) system frequency regulation. First of all, this paper constructs a grid-connected model composed of a WTG and a MGT, analyzes the WTG virtual inertia frequency regulation mechanism, and explains the principle of proportional-differentiation (PD) virtual inertia control (VIC) and its shortcomings. Secondly, the paper introduces the structure principle of n-order active disturbance rejection control (ADRC) and deduces the design process of second-order ADRC-VIC. Finally, through the simulation and experimental verification, comparing the frequency perturbation of without-VIC, PD-VIC, and ADRC-VIC, it is concluded that PD-VIC and ADRC-VIC both can use the WTG virtual inertia to participate in the primary frequency regulation. The frequency regulation effect of ADRC-VIC is better than PD-VIC, ADRC-VIC can extend the rotor speed recovery time and avoid overshoot, and its frequency fluctuation amplitude and settling time are obviously improved, and ADRC-VIC can effectively avoid the overshoot phenomenon of the MGT output power.

Direct-Drive Conical-Rotor Permanent Magnet Synchronous Generator for Turbo-Expander, Accounting for Adaptive Equilibrium of Axial Force

The permanent magnet synchronous generator (PMSG) has been widely employed in high-speed direct-drive turbo-expander for distributed energy generation. However, for conventional PMSG based turbo-expander system, expensive mechanical balance devices should be installed on the rotor to restrain the axial force of turbine impeller under high-speed flow, which may cause both complex system structure and higher manufacturing cost. Thus, in this paper, a direct-drive conical-rotor PMSG (CR-PMSG) for turbo-expander is developed, which can generate an axial magnetic force as equilibrant to counteract the axial force of turbine impeller without mechanical balance device, so as to reduce the system complexity, improve the operating performance and extend the operating cycle. First, the structural features of the CR-PMSG are introduced and the basic principle of axial force balance is analyzed. Then, a 1.5 kW, 6000 rpm CR-PMSG with cone angle of 6° is designed. Using 3-D finite element analysis, the axial force of turbine impeller, the axial magnetic force of the rotor, and the electromagnetic performance are simulated. Also, the control strategy of d, q-axis currents is obtained to achieve the adaptive equilibrium of axial force. Finally, a CR-PMSG prototype is fabricated and tested to validate the theory.

A Two-Level Sensorless MPPT Strategy Using SRF-PLL on a PMSG Wind Energy Conversion System

In this paper, a two-level sensorless Maximum Power Point Tracking (MPPT) strategy is presented for a variable speed Wind Energy Conversion System (WECS). The proposed system is composed of a wind turbine, a direct-drive Permanent Magnet Synchronous Generator (PMSG) and a three phase controlled rectifier connected to a DC load. The realised generator output power maximization analysis justifies the use of the Field Oriented Control (FOC) giving the six Pulse Width Modulation (PWM) signals to the active rectifier. The generator rotor speed and position required by the FOC and the sensorless MPPT are estimated using a Synchronous Reference Frame Phase Locked Loop (SRF-PLL). The MPPT strategy used consists of two levels, the first level is a power regulation loop and the second level is an extremum seeking bloc generating the coefficient gathering the turbine characteristics. Experimental results validated on a hardware test setup using a DSP digital board (dSPACE 1104) are presented. Figures illustrating the estimated speed and angle confirm that the SRF-PLL is able to give an estimated speed and angle which closely follow the real ones. Also, the power at the DC load and the power at the generator output indicate that the MPPT gives optimum extracted power. Finally, other results show the effectiveness of the adopted approach in real time applications.

Impact of wind conditions on thermal loading of PMSG wind turbine power converters

Power converter reliability is critical for permanent magnet synchronous generator (PMSG) wind turbines. Converter failures are linked to power module thermal loading but studies often neglect turbine dynamics, control and the impact of wind speed sampling rate on lifetime estimation. This study addresses this using a 2 MW direct-drive PMSG wind turbine model with a two-level converter, and simulating junction temperatures (Tj) using a power module thermal equivalent circuit under various synthetic wind speed conditions. These synthetic wind conditions include constant and square wave profiles representing stable and gusty wind conditions. Responses to square wave wind speeds showed that the lower the gust frequency, the higher ΔTj becomes, demonstrating that low turbulence sites have greater thermal variation in the converter. In contrast, wind speed variations with frequencies >0.25 Hz deliver only small increases in ΔTj. It is concluded that reasonable approximations of Tj profiles can be made with 0.25 Hz wind speed data, but that lower data rate wind measurements miss essential, damaging characteristics.

Implementation and experimental investigation of a sensorless field-oriented control scheme for permanent-magnet synchronous generators

Variable-speed wind energy conversion systems based on permanent-magnet synchronous generators (PMSGs) are typically controlled using the field-oriented control (FOC) principles. Therefore, accurate information of the rotor speed and position are essential to perform the required reference frame transformations. These signals can be obtained by mechanical sensors (e.g., position encoders or speed transducers) or via estimation schemes. This paper proposes a sensorless FOC strategy for direct-driven PMSGs in variable-speed wind turbines. A synchronously rotating reference frame phase-locked loop (PLL) that utilizes a model-based back elector-motive force (back-EMF) estimation is employed to estimate the rotor speed and position of the PMSG. The proposed sensorless FOC strategy is experimentally implemented, and its performance is investigated for all operation conditions and under parameter variations of the PMSG.