Turbine Inertia Research Articles

An experimental analysis of wind and power fluctuations through time-resolved data of full scale wind turbines

Addressing short-term wind and wind turbine power fluctuations is fundamental in order to understand the nature of turbulence and of the mechanical loads to which wind turbines are subjected. This work is an experimental study of wind and power fluctuations at on onshore wind farm in Italy. Four wind turbines having 2 MW of rated power each are studied through time-resolved data. The sampling frequency is of the order of the Hz. This wind farm has been selected because there are two orders of magnitude of inter-turbine distance (3 and 7 rotor diameters) and therefore it is possible to study different levels of wake interactions recovery. The power curve at short time scales is studied and the inertia of the wind turbines, with respect to the wind fluctuations, is observed in the form of hysteresis of the power curve. Subsequently, the distribution of the wind and power variations is studied on several time scales and different features of the distributions are observed for downstream wind turbines with respect to upstream ones. The two-point statistics of power and wind-power is shown to be responsive to the wake regime to which wind turbines are subjected. This can suggest new approaches for wake control strategies.

Coordinated Control for Low Voltage Ride Through in PMSG Wind Turbines

Abstract This work proposes a control strategy for permanent magnet synchronous generator (PMSG) based wind energy conversion systems (WECSs) to improve low voltage ride through (LVRT) capabilities. The nonlinear relationship between the generator speed and DC-link voltage is considered by using feedback linearisation. To support the grid during LVRT, the DC-link voltage is controlled by the machine side converter (MSC). The kinetic energy stored in the wind turbine inertia is used to compensate for lower voltage. Further, pitch angle control is used to ensure stable operation, and to guarantee the rotor speed does not exceed the rated value. A simulation of a 2MW PMSG wind turbine is conducted to validate the proposed control strategy.

LABORATORY STAND FOR SMALL WIND TURBINE SIMULATION

The article presents a concept of the laboratory stand for wind turbine simulation, built on the basis of cage induction motor, supplied from a frequency converter. The model assumes control of the engine torque, using simple constant Volt/Hertz control with slip regulation, taking into account the moment of inertia of the simulated turbine. Performance of the system for different wind and generator load conditions has been shown. Test results proved, that the turbine torque is well tracked by the wind motor emulator.

Control and Emulation of Small Wind Turbines Using Torque Estimators

Soft-stall control of small wind turbines is a method to protect the generation system and/or load from excessive wind speeds and wind gusts without discontinuing power generation. Soft-stall can be activated due to an excess of the power and/or torque/current. This paper proposes a method to improve the existing soft-stall methods for over torque/current protection using a turbine torque estimator. In addition, this paper also proposes two methods to emulate the wind turbine inertia without communications between the load drive (wind turbine emulator) and the generation system controller. This will allow the evaluation of the proposed methods in working conditions.

Design and Analysis of a MPC-based Mechanical Hardware-in-the-Loop System for Full-Scale Wind Turbine System Test Benches

In this contribution a mechanical-level Hardware-in-the-Loop system based on a Model Predictive Controller, for a 4MW system test bench is proposed. An experimentally validated model of this test bench is introduced and the basis of the mechanical-level Hardware-in-the-Loop system. The main purpose of that system is to emulate the inertia and eigenfrequency of the wind turbine’s drive train at the system test bench, where the rotor is dismounted and hence the eigenfrequencies are altered.The results show that, with realistic motor torque constraints being considered and the original wind turbine controller activated, the proposed concept allows to reproduce the first two eigenfrequencies accurately. Beyond that the transmission behavior from pitch angle to generator speed of the wind turbine at the test bench and of the unmodified, real wind turbine match up to a frequency of 9 Hz in the nominal case. Even with parameter deviations of up to 20% and time-delay of 15 ms the frequency response match up to 7 Hz.

A numerical study on the aerodynamic performance and the self-starting characteristics of a Darrieus wind turbine considering its moment of inertia

Self-starting characteristics is a challenging issue in the field of Darrieus-type vertical axis wind turbines. Traditionally, the numerical simulations were performed at several constant rotational velocities for the turbine and the generated torques were reported for each rotational velocity, neglecting the effects of the turbine inertia on the transient start-up motion of the turbine. In the current study, a numerical method is proposed to study the self-starting characteristics of a Darrieus wind turbine, considering the turbine moment of inertia. The simulation starts from the initial stationary state and continues until the final steady-periodic condition. At each time step, the instantaneous rotational velocity of the turbine is computed based on the Newton’s second law, according to the instantaneous aerodynamic and mechanical forces acting on the turbine. Results indicate that as the rotor inertia increases, it takes a longer time for the turbine to reach its final velocity, in a manner that, the turbine might even fail to reach the final condition and the rotation halts. Results also show that as the rotor inertia decreases, the oscillations amplitude of the turbine rotational velocity increases. This can enable the turbine to pass higher resistant torques than those computed based on the traditional method.

An Improved Adaptive-Torque-Gain MPPT Control for Direct-Driven PMSG Wind Turbines Considering Wind Farm Turbulences

Maximum power point tracking (MPPT) plays an important role in increasing the efficiency of a wind energy conversion system (WECS). In this paper, three conventional MPPT methods are reviewed: power signal feedback (PSF) control, decreased torque gain (DTG) control, and adaptive torque gain (ATG) control, and their potential challenges are investigated. It is found out that the conventional MPPT method ignores the effect of wind turbine inertia and wind speed fluctuations, which lowers WECS efficiency. Accordingly, an improved adaptive torque gain (IATG) method is proposed, which customizes adaptive torque gains and enhances MPPT performances. Specifically, the IATG control considers wind farm turbulences and works out the relationship between the optimal torque gains and the wind speed characteristics, which has not been reported in the literature. The IATG control is promising, especially under the ongoing trend of building wind farms with large-scale wind turbines and at low and medium wind speed sites.

Research on super capacitor energy storage system and its control of wind power system based on virtual synchronous technology

High penetration of wind power in power grid would seriously affect the frequency safety. Synchronous generator can adjust system power and stabilize grid voltage and grid frequency. The inverter can be equivalently regarded to be synchronous generator with the help of super capacitor. Not only the wind turbines can participate in the frequency modulation and voltage regulation, but also the inertia of the system will be improved. On the basis of the analysis of virtual synchronous generator (VSG), a method of calculating the capacity of energy storage system (ESS) is proposed and a control strategy of the bi-directional converter is researched to improve the virtual inertia of wind turbines and suppress the fluctuation of power and frequency when load or wind speed is changed. This method makes the wind farm behave like a virtual synchronous generator, and effectively enhance the grid-connected performance of wind turbines. The simulation results based on Simulink show the feasibility of the control strategy.

Development details and performance assessment of a Wind Turbine Emulator

This paper describes an experimental platform to emulate the static and dynamic behavior of real Wind Energy Conversion Systems (WECS). The Wind Turbine Emulator (WTE) consists of two coupled 1.5 kW squirrel-cage induction machines. The WTE calculations are performed on a PC and a DSP, both connected via USB. The total torque applied to the WTE motor is the difference of the scaled wind mechanical torque calculated on the PC, and the turbine inertia torque calculated on the DSP. An improved WTE shaft speed derivative has been used for the inertia torque calculation, which provides better results in terms of attenuation of the high-frequency harmonics present in the generator speed acquisition. The implemented WTE is used to compare different MPPT algorithms and to test power overrating and low voltage ride through procedures.

An Integrated Power Smoothing Control for a Grid-Interactive Wind Farm Considering Wake Effects

This paper presents a power smoothing control scheme for a permanent-magnet-synchronous-generator-based wind farm considering wake effects. The integrated power smoothing method consists of a pitch angle control system, a kinetic wind-energy turbine inertia control system, and a dc-link voltage control system. Low frequency range power fluctuations caused by wind speed variations are smoothed by the pitch angle control system and the kinetic wind-energy turbine inertia control system. Power fluctuations in the high frequency range are smoothed by the dc-link voltage control system. This power smoothing approach is cost-effective because the power smoothing is made possible without additional energy storage devices. In addition, the wake effects of the wind farm are considered for power smoothing with different tower spaces. The proposed method is compared with the conventional maximum-power-point-tracking method. Simulation results confirm the proposed method efficacy in smoothing the power injected by a wind farm.

Impact of emulated inertia from wind power on under-frequency protection schemes of future power systems

Future power systems face several challenges. One of them is the use of high power converters that decouple new energy sources from the AC power grid. This situation decreases the total system inertia affecting its ability to overcome system frequency disturbances. The wind power industry has created several controllers to enable inertial response on wind turbines generators: artificial, emulated, simulated, or synthetic inertial. This paper deals with the issues related to the emulated inertia of wind turbines based on full-converters and their effect on the under-frequency protection schemes during the recovery period after system frequency disturbances happen. The main contribution of this paper is to demonstrate the recovery period of under-frequency transients in future power systems which integrate wind turbines with emulated inertia capability does not completely avoid the worse scenarios in terms of under-frequency load shedding. The extra power delivered from a wind turbine during frequency disturbances can substantially reduce the rate of frequency change. Thus it provides time for the active governors to respond.

Novel Integration of a PV-Wind Energy System With Enhanced Efficiency

A novel integration scheme of solar photovoltaic (PV) with a large capacity doubly excited induction generator-based wind energy system is described. The proposed scheme uses both the grid- and rotor-side power converters of doubly fed induction generator to inject PV power into the grid. Thus, it renders a cost-effective solution to PV-grid integration by obviating the need for a dedicated converter for PV power processing. The system is able to feed significantly large PV power into the grid compared to an equivalent rating inverter used in the conventional PV-grid system. The proposed scheme prevents circulating power during subsynchronous operation during the availability of solar radiation. All these features enhance system efficiency. System stability is also augmented due to turbine inertia, facilitating high PV penetration into the power grid. The intermittent but complementary nature of solar PV and wind energy sources considerably improves the converters' utilization. Besides, the proposed scheme does not hamper maximum power point tracking of PV and wind sources except during very rarely occurring environmental glitches, which the PV power control algorithm is suitably geared to handle. A comprehensive system model is presented and used for designing the control strategy. The proposed scheme is supported by analysis, simulations, and experiments on a laboratory prototype.

Blending HVDC-Link Energy Storage and Offshore Wind Turbine Inertia for Fast Frequency Response

This paper explores the benefits of combining the dc-link energy storage of a voltage source converter-based high-voltage dc (VSC-HVDC) link and the kinetic energy storage from wind turbines to facilitate in fast primary frequency control and system inertia to an ac network. Alongside physical and analytical justifications, a method is proposed which blends the energy stored in the HVDC link with the power control capabilities of the wind turbines to provide frequency response that is fast while not requiring excessive volume of capacitance nor demanding performance requirements on the wind turbines.

A novel MPPT method for enhancing energy conversion efficiency taking power smoothing into account

With the increasing capacity of wind energy conversion system (WECS), the rotational inertia of wind turbine is becoming larger. And the efficiency of energy conversion is significantly reduced by the large inertia. This paper proposes a novel maximum power point tracking (MPPT) method to enhance the efficiency of energy conversion for large-scale wind turbine. Since improving the efficiency may increase the fluctuations of output power, power smoothing is considered as the second control objective. A T-S fuzzy inference system (FIS) is adapted to reduce the fluctuations according to the volatility of wind speed and accelerated rotor speed by regulating the compensation gain. To verify the effectiveness, stability and good dynamic performance of the new method, mechanism analyses, small signal analyses, and simulation studies are carried out based on doubly-fed induction generator (DFIG) wind turbine, respectively. Study results show that both the response speed and the efficiency of proposed method are increased. In addition, the extra fluctuations of output power caused by the high efficiency are reduced effectively by the proposed method with FIS.

Reproduction of five degree-of-freedom loads for wind turbine using equispaced electro-hydraulic actuators

Several improvements are proposed to enhance the capability of conventional wind turbine simulators since such simulators can only simulate the turbine shaft torque, or can partially generate the other five degree-of-freedom turbine loads in addition to the turbine shaft torque. In the improved wind turbine simulator, the real wind rotor and blades are represented by an equivalent rotating disc to replicate the turbine inertia effects. Twenty-four electro-hydraulic loading actuators are symmetrically distributed around this rotating disc and are controlled independently to reproduce the additional five degree-of-freedom turbine loads that can be described in the blade and hub coordinate systems. A loading decomposition strategy is also proposed to decompose such five-degree-of-freedom turbine loads into reference loading forces for each loading actuator by incorporating additional seven-degree-of-freedom dummy loads. A typical axial loading actuator is taken as an example for in-depth dynamic modeling and analysis. Two intrinsic pressure-sensed closed control loops and different pressure modes of this actuator are presented and analyzed in detail. The dynamic model of this typical loading actuator has been thoroughly validated by using experimental measurements. Simulation results have also demonstrated that the five degree-of-freedom turbine loads can be accurately reproduced by using such electro-hydraulic loading actuators. Therefore, the improved wind turbine simulator can fully simulate steady-state and dynamic turbine loads with good accuracy and can expedite the system-level loading tests that are critical in increasing turbine reliability and offering high confidence system design.

A Dynamic Control Strategy for Wind Turbines in Wind Farms

SUMMARYThis paper presents a new control method for wind turbine groups by use of dynamic control including pump‐up operation. Under conditions of oscillating wind below the rated speed of the wind turbine, maximum power point tracking (MPPT) operation is critical to improving efficiency. Even though, owing to the huge inertia of a wind turbine, the operating point of the wind turbine produces an unavoidable delay at the maximum power point, if dynamic control is used wind turbines respond to the changes in the wind speed more rapidly. The wind turbines hence reach the maximum power point quickly and capture energy from the wind more efficiently. On the other hand, as a result of dynamic operation, the output from a wind turbine fluctuates more severely than during normal operation. In the proposed solution, by taking advantage of the kinematic energy stored in other wind turbines that belong to the same wind farm, not only the efficiency of output power but also the severity of the fluctuations is improved. In the results of simulations, group dynamic operation enables wind farms to improve their efficiency and smooth the output fluctuations from the wind farm.

Smoothing of Wind Power Fluctuations for Permanent Magnet Synchronous Generator-Based Wind Energy Conversion System and Fault Ride-through Consideration

Due to the increment of penetration level of wind power generation, output power fluctuation is one of the most important issue's that can destabilize the power system operation. This article mainly deals with the smoothing of the output power fluctuations of a wind energy conversion system based permanent magnet synchronous generator and fault ride-through enhancement during a grid fault. The concerned wind energy conversion system based permanent magnet synchronous generator adopts an AC-DC-AC converter system. The proposed control method limits the wind energy conversion system output power by adjusting the pitch angle of the wind turbine blades when wind speed is above the rated wind speed. In the grid-side converter, a fuzzy logic controller is used to determine the torque reference for which the kinetic energy stored by the inertia of wind turbine can smooth the output power fluctuations of the permanent magnet synchronous generator. Also, the DC-link voltage, controlled by the grid-side inverter, is adjusted in accordance with the output power fluctuations of the permanent magnet synchronous generator using a voltage smoothing index. Moreover, in this aticle, the proposed method ensures that the wind turbine stays operational during grid faults and provides fast restoration once the fault is cleared. To show the effectiveness of the proposed method, simulations under different conditions have been performed by using MATLAB/Simulink® (The Math Works, Natick, MA, USA).

Maximum power control system for small wind turbine using predicted wind speed

This paper describes a novel approach of maximum power control for small wind turbines by using predicted wind speed data. Because of the moment of inertia of the wind turbine, when using conventional control method, the time lag of control will occur due to turbulence in the environment. Our proposed control system uses future information, which is the predicted wind speed, for wind turbine control. The control algorithm creates a reference trajectory of the rotational speed of the wind turbine. The advantage of using the predicted data is that the controller can operate the wind turbine efficiently so that the rotational speed of the wind turbine catches up with the reference speed at the maximum power point. Simulation results show improvement of generation efficiency compared to the conventional control method. Then we discuss the influence of the prediction error of wind speed on control performance. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Power system frequency management challenges – a new approach to assessing the potential of wind capacity to aid system frequency stability

With the increasing wind penetration level in power systems, transmission system operators have become concerned about frequency stability. The inertia of a variable speed wind turbine is decoupled by power electronic converters from the power network and therefore does not intrinsically contribute to power system inertia. Moreover, as wind plant progressively displaces conventional generation and their inertia, a substantial reduction in power system inertia may occur. Variable speed wind turbines can be controlled to provide synthetic inertial response to compensate for their lack of direct contribution to power system inertia. A probabilistic approach to assessing the collective inertial contributions from wind generation across a power system is proposed and is applied to the Great Britain power system. The impact of the aggregate inertial response on arresting frequency fall is examined for the case of a sudden generation loss of 1.8 GW at the time of minimum load on both a mid-summer and a mid-winter day. The results show that synthetic inertial response from wind can reduce the rate of fall of frequency and the minimum system frequency (nadir) following the loss of generation event.

On the Design and Matching of Turbocharger Single Scroll Turbines for Pass Car Gasoline Engines

With emission legislation becoming more stringent within the next years, almost all future internal combustion gasoline engines need to reduce specific fuel consumption, most of them by using turbochargers. Additionally, car manufactures attach high importance to a good drivability, which usually is being quantified as a target torque already available at low engine speeds—reached in transient response operation as fast as possible. These engine requirements result in a challenging turbocharger compressor and turbine design task, since for both not one single operating point needs to be aerodynamically optimized but the components have to provide for the optimum overall compromise for maximum thermodynamic performance. The component design targets are closely related and actually controlled by the matching procedure that fits turbine and compressor to the engine. Inaccuracies in matching a turbine to the engine full load are largely due to the pulsating engine flow characteristic and arise from the necessity of arbitrary turbine map extrapolation toward low turbine blade speed ratios and the deficient estimation of turbine efficiency for low engine speed operating points. This paper addresses the above described standard problems, presenting a methodology that covers almost all aspects of thermodynamic turbine design based on a comparison of radial and mixed-flow turbines. Wheel geometry definition with respect to contrary design objectives is done using computational fluid dynamics (CFD), finite element analysis (FEA), and optimization software. Parametrical turbine models, composed of wheel, volute, and standard piping allow for fast map calculation similar to steady hot gas tests but covering the complete range of engine pulsating mass flow. These extended turbine maps are then used for a particular assessment of turbine power output under unsteady flow admission resulting in an improved steady-state matching quality. Additionally, the effect of various design parameters like either volute sizing or the choice of compressor to turbine diameter ratio on turbine blade speed ratio operating range as well as well as turbine inertia effect is analyzed. Finally, this method enables the designer to comparatively evaluate the ability of a turbine design to accelerate the turbocharger speed for transient engine response while still offering a map characteristic that keeps fuel consumption low at all engine speeds.