A frequency domain approach to wind turbines for flicker analysis

Wind turbines may have an important impact on power quality. Flicker is a more serious issue for fixed-speed wind turbines because these turbines produce electric power following the variations of the incident wind. During continuous operation, wind variations will result in power fluctuations and consequently in voltage fluctuations. It is necessary to evaluate wind turbines flicker emission level, and traditionally time domain simulations have been used to perform the analysis. This paper presents a complete frequency domain model to study flicker produced during wind turbines continuous operation. The model includes a realistic wind speed model as observed by the wind turbine and also a frequency domain induction generator model is presented. The frequency domain model has been compared with a time domain model. The frequency domain approach, as shown in the paper, may be very useful for flicker analysis in electric networks.     

基于时频分析法的智能电网电压闪变分量运算

为获得电网电压闪变治理中设备参数调整和控制策略制定过程中所需的准确可靠电压闪变数据,将一种非线性和非平稳信号时频分析法(HHT变换)成功引入电压闪变低幅调制信号瞬时相位、瞬时幅值、瞬时频率等特性参数的检测中,可从时域和频域两方面对信号进行同时分析。将电压闪变信号调制获得的低频信号,先用EMD分解算法分解成若干IMF分量,再做HHT变换。算例仿真分析表明,该优化算法提取电压闪变信号数据可行有效,数据准确,能满足工程需求。     

Assessment of flicker limits compliance for wind energy conversion system in the frequency domain

Fixed-speed wind turbines produce voltage flicker during switching operations and they also produce flicker during continuous operation. In order to avoid power quality problems to consumers, it would be important to predict flicker emission from wind turbines at a certain site previously to installation. This paper focuses on a method to perform a fast flicker analysis caused by grid-connected fixed-speed wind turbines in continuous operation. The method has been developed completely in the frequency domain, including wind turbines. This method predicts the P_st value for each node system. Applying this method there would be no need to perform measurements in every node, which would be nearly impossible or to simulate the whole model in the time domain, demanding an enormous computational effort and storage capacity. The performance of the frequency domain method is applied to analyze different wind energy generation scenarios to assess their influence in a real power system.     

提取电压闪变参数的一种新方法

基于低幅值单频或多频叠加调制的电压闪变信号经过分解后,在频域内总是在工频两侧呈对称分布的特点,并考虑到实际低幅调值制信号的频率范围,提出将电压闪变信号映射到频域内,实现电压闪变信号的调制,只保留对求取调制信号参数有用的低频部分———调制闪变信号,并运用Hilbert-Huang变换提取调制闪变信号瞬时参数。在此基础上再运用推导公式便能实现电压闪变信号中调制信号参数的有效提取。仿真和实例分析结果均表明了本方法的可行性、有效性,为电压闪变参数提取提供了一种新方案。     

Impact assessment of using wind power

This paper discusses a methodology to accurately determine the impact of using wind-energy-conversion systems, WECS, on the operation of the central power system. To do so, the power-generation-simulation code PROMIX is used to simulate the operation of the power system on an hourly basis and on a power-plant level, including the technological restrictions of every plant. The actual impact assessment of the WECS is done by comparing the output of two scenarios; a base case and an alternative scenario which includes the WECS.To demonstrate the method, several case studies for WECS in different locations in Belgium are worked out. The impact is quantified by looking at the greenhouse-gas-emission reductions that can be obtained by using WECS. In these case studies, we closely look at the influence of the variability of the WECS power output, the geographical spread of the wind farms, the capacity factor and the capacity credit of the WECS and the effect of the power-generation mix.     

考虑风资源影响的风电场电压波动和闪变评估

风电并网后引起的电压波动和闪变水平可能超出国家有关标准,造成严重的电能质量问题,因此,在风电并网之前需对这两者进行评估。采用了一种新的评估方法。区别于国际电工标准(IEC61400-21)中电压波动与闪变的评估,此方法考虑了风电场的风资源情况对这2个指标的影响。对风电场在不同出力下由阵风引起系统的电压波动进行计算,并用IEC闪变仪计算短时间闪变值Pst。用所提方法和IEC标准对我国某一新建的风电场进行评估。结果表明,所提方法不仅能有效地进行电压波动与闪变评估,而且能更好地考虑风速变化对风电场带来的潜在影响。     

Controller design for an induction generator driven by a variable-speed wind turbine

This paper presents the modeling, controller design and a steady-state analysis algorithm for a wind-driven induction generator system. An output feedback linear quadratic controller is designed for the static synchronous compensator (STATCOM) and the variable blade pitch in a wind energy conversion system (WECS) in order to reach the voltage and mechanical power control under both grid-connection and islanding conditions. A two-reference-frame model is proposed to decouple the STATCOM real and reactive power control loops for the output feedback controller. To ensure zero steady-state voltage errors for the output feedback controller, the integrals of load bus voltage deviation and dc-capacitor voltage deviation are employed as the additional state variables. Pole-placement technique is used to determine a proper weighting matrix for the linear quadratic controller such that satisfactory damping characteristics can be achieved for the closed-loop system. Effects of various system disturbances on the dynamic performance have been simulated, and the results reveal that the proposed controller is effective in regulating the load voltage and stabilizing the generator rotating speed for the WECS either connected with or disconnected from the power grid. In addition, proper steady-state operating points for an isolated induction generator can be determined by the proposed steady-state analysis algorithm. Constant output frequency control using the derived steady-state characteristics of the isolated induction generator is then demonstrated in this paper.     

Modeling and simulation of multi‐scale transients for PMSG‐based wind power systems

In a wind energy conversion system (WECS), multiple‐time‐scale transients that cover a wide frequency range from low‐frequency transient stability up to high‐frequency switching events are observed. This paper presents a methodology of modeling diverse transients for a permanent magnet synchronous generator (PMSG)‐based WECS within the same study. Multiple physical areas of the PMSG‐based WECS are given depending on the appearance of carriers contained in the considered waveforms. In order to eliminate different carrier frequencies, the PMSG and generator‐side voltage source converter (VSC) are modeled in the dq0‐reference frame. On the other hand, the grid‐side VSC and utility grid are dealt with in the multi‐scale model of the network in which the shift frequency is available. The switching‐function and average‐value models of the VSC are selected depending on the carrier shifted. In addition, interface between the control and electrical subsystems is redesigned to offset the computation error caused by one time‐step delay. Two test cases are performed to study the wind power fluctuations and faults ride‐through. The results show that the proposed multi‐scale model is able to simulate slow‐changing dynamic responses up to high‐frequency transients accurately while decreasing the simulation burden. In comparison with the results obtained from the EMTP (electromagnetic transients program) type simulators, the effectiveness and accuracy of the multi‐scale model are verified. Copyright © 2017 The Authors Wind Energy Published by John Wiley & Sons Ltd.     

Study and experimental verification of control tuning strategies in a variable speed wind energy conversion system

This paper analyzes and compares different control tuning strategies for a variable speed wind energy conversion system (WECS) based on a permanent-magnet synchronous generator (PMSG). The aerodynamics of the wind turbine (WT) and a PMSG have been modeled. The control strategy used in this research is composed of three regulators, which may be based on either linear or nonlinear controllers. In this analysis, proportional-integral (PI) linear controllers have been used. Two different tuning strategies are analyzed and compared. The main goal is to enhance the overall performance by achieving a low sensitivity to disturbances and minimal overshoot under variable operating conditions. Finally, the results have been verified by an experimental WECS laboratory prototype.     

Classical flutter analysis of composite wind turbine blades including compressibility

For wind turbine blades with the increased slenderness ratio, flutter instability may occur at lower wind and rotational speeds. For long blades, at the flutter condition, relative velocities at blade sections away from the hub center are usually in the subsonic compressible range. In this study, for the first time for composite wind turbine blades, a frequency domain classical flutter analysis methodology has been presented including the compressibility effect only for the outboard blade sections, which are in the compressible flow regime exceeding Mach 0.3. Flutter analyses have been performed for the baseline blade designed for the 5‐MW wind turbine of NREL. Beam‐blade model has been generated by making analogy with the structural model of the prewisted rotating thin‐walled beam (TWB) and variational asymptotic beam section (VABS) method has been utilized for the calculation of the sectional properties of the blade. To investigate the compressibility effect on the flutter characteristics of the blade, frequency and time domain aeroelastic analyses have been conducted by utilizing unsteady aerodynamics via incompressible and compressible indicial functions. This study shows that with use of compressible indicial functions, the effect of compressibility can be taken into account effectively in the frequency domain aeroelastic stability analysis of long blades whose outboard sections are inevitably in the compressible flow regime at the onset of flutter.     

Characterisation of flicker emission and propagation in distribution networks with bi-directional power flows

With the increasing penetration levels of intermittent and fluctuating energy sources such as wind generating systems in the electricity grid, resulting voltage fluctuations and flicker can be expected to become an important power quality considerations. Due to significant bidirectional power flows resulting from large renewable power generation systems connected to downstream, voltage fluctuations may propagate from downstream to upstream. The work presented in this paper investigates and characterises flicker emission and propagation resulting from fluctuating generating sources connected to a distribution network. Mathematical models are developed for flicker emission under different generator control strategies and flicker propagation to upstream network. These emission and propagation characteristics are investigated and verified using a test network comprised of a wind farm. The study has revealed that flicker emission characteristics are influenced in a detrimental manner by the reactive power control strategy of the generator and the flicker attenuation characteristics are influenced by the various load types connected to the distribution feeder.     

Considering load-carrying capability and wind speed correlation of WECS in generation adequacy assessment

Wind power is an intermittent energy source that behaves quite differently from conventional energy sources. The reliability impact of this highly variable energy source is an important aspect that needs to be assessed as wind power penetration becomes increasingly significant. Generation adequacy assessment including wind energy conversion systems (WECS) at multiple locations is described in this paper. Effective load-carrying capabilities (ELCC) obtained using the loss of load expectation (LOLE) and the loss of load frequency (LOLF) for a power system containing WECS are illustrated and compared. The results show that ELCC obtained using the LOLF and obtained using the LOLE for WECS can be considerably different, while they are similar for a conventional generating unit. The impact on the system reliability indices of wind speed correlation between two wind farms is also examined. The studies show that the degree of wind speed correlation between two wind farms has a considerable impact on the resulting reliability indices. The sequential Monte Carlo simulation approach is used as this methodology can facilitate a time series modeling of wind speeds, and also provides accurate frequency and duration assessments. An autoregressive moving average time series model is used in this study to simulate hourly wind speeds.     

A sequential simulation technique for adequacy evaluation ofgenerating systems including wind energy

A wind energy conversion system (WECS) has a different impact on the reliability performance of a generating system than does a conventional energy conversion system. This is due to the variation of wind speeds and the dependencies associated with the power output of each wind turbine generator (WTG) in a wind farm. In this paper, a sequential Monte Carlo simulation technique is proposed for adequacy evaluation of a generating system including WECS. The method is based on an hourly random simulation to mimic the operation of a generating system, taking into account the auto-correlation and fluctuating characteristics of wind speeds, the random failure of generating units and other recognized dependencies. The hourly wind speeds are simulated utilizing autoregressive and moving average time series models that are established based on the F-criterion. A small reliability test system designated as the RBTS is used to illustrate the proposed method     

Energy and reliability benefits of wind energy conversion systems

The electrical energy production and reliability benefits of a wind energy conversion system (WECS) at a specific site depend on many factors, including the statistical characteristics of the site wind speed and the design characteristics of the wind turbine generator (WTG) itself, particularly the cut-in, rated and cut-out wind speed parameters. In general, the higher the degree of the wind site matching with a WECS is, the more are the energy and reliability benefits. An electrical energy production and reliability benefit index designated as the Equivalent Capacity Ratio (ECR) is introduced in this paper. This index can be used to indicate the electrical energy production, the annual equivalent utilization time and the credit of a WECS, and quantify the degree of wind site matching with a WECS. The equivalent capacity of a WECS is modeled as the expected value of the power output random variable with the probability density function of the site wind speed. The analytical formulation of the ECR is based on a mathematical derivation with high accuracy. Twelve WTG types and two test systems are used to demonstrate the effectiveness of the proposed model. The results show that the ECR provides a useful index for a WTG to evaluate the energy production and the relative reliability performance in a power system, and can be used to assist in the determination of the optimal WTG type for a specific wind site.     

风电并网引起闪变的测试系统仿真

风电场并网引起的周期性电压波动频率位于IEC闪变仪的低频段，由于高通滤波器设计困难，按照IEC推荐方法设计的闪变仪测试此频段电压波动引起的闪变时就有可能带来误差。我们对IEC闪变仪进行了仿真，并增加校正环节对闪变测试系统在低频段的输出进行了校正。结果表明校正后的系统在低频段具有较高的精度，能够满足风电并网引起的闪变的测试要求。     

An adaptive feedback linearization strategy for variable speed wind energy conversion systems

This paper presents a control strategy based on adaptive feedback linearization intended for variable speed grid‐connected wind energy conversion systems (WECS). The proposed adaptive control law accomplishes energy capture maximization by tracking the wind speed fluctuations. In addition, it linearizes the system even in the presence of turbine model uncertainties, allowing the closed‐loop dynamic behaviour to be determined by a simple tuning of the controller parameters. Particularly, the attention is focused on WECS with slip power recovery, which use a power conversion stage as a rotor‐controlled double‐output induction generator. However, the concepts behind the proposed control strategy are general and can be easily extended to other WECS configurations. Copyright © 2000 John Wiley & Sons, Ltd.     

Evaluation of electricity generation and energy cost of wind energy conversion systems (WECSs) in Central Turkey

The negative effects of non-renewable fossil fuels have forced scientists to draw attention to clean energy sources which are both environmentally more suitable and renewable. Although Turkey enjoys fairly high wind energy potential, an investigation and exploitation of this source is still below the desired level. In this study which is a preliminary study on wind energy cost in Central Anatolian-Turkey, the wind energy production using time-series approach and the economic evaluation of various wind energy conversion systems (WECSs) enjoying the 2.5, 5, 10, 20, 30, 50, 100 and 150 kW rated power size using the levelised cost of electricity (LCOE) method for the seven different locations in Central Turkey were estimated. In addition, effects of escalation ratio of operation and maintenance cost and annual mean speed on LCOE are taken into account. The wind speed data for a period between 2000 and 2006 years were taken from Turkish State Meteorological Service (TSMS). According to the result of the calculations, it is shown that the WECS of capacity 150 kW produce the energy output 120,978 kWh per year in the Case-A (Pinarbasi) for hub height 30 m and also the LCOE varies in the range of 0.29–30.0 $/kWh for all WECS considered.     

FPGA-based reconfigurable control for switch fault tolerant operation of WECS with DFIG without redundancy

This paper deals with reconfigurable back-to-back converter topology and control orders in Wind Energy Conversion Systems (WECS). A typical WECS with Doubly Fed Induction Generator (DFIG) in balanced conditions is concerned. Based on the classical topology, a fault tolerant converter without any redundancy has been studied. The presented fault tolerant topology allows a “five-leg” structure with converters reconfiguration after switch failure detection. Furthermore, the control strategy for classical topology can no longer be applied after fault occurrence. Thus, a “five-leg” control strategy has also been proposed. The validation of the reconfigurable digital controller for the studied WECS with DFIG topology has been performed using a Hardware-in-the-Loop (HIL) reconfigurable platform including a Field Programmable Gate Array (FPGA) chip. HIL simulation results in both healthy and fault conditions have been presented to show simultaneously the viability of the studied converters topology and the reconfigurable control.     

A fuzzy control strategy for power smoothing and grid dynamic response enrichment of a grid‐connected wind energy conversion system

This paper concentrates on the output power smoothing and the grid dynamic response enhancement of a grid‐interactive MW‐class permanent magnet synchronous generator‐based wind energy conversion system (WECS). A simple fuzzy controller method is applied to improve the overall performance of the WECS. The proposed method can retrieve the storing kinetic energy from the inertia of a wind turbine, perfectly. As a result, it can ensure a proficient power smoothing of the variable speed WECS. On the other hand, the grid side inverter is controlled by the fuzzy controller. This approach can reduce the fluctuation of DC link voltage and can deliver a smooth power to the power grid. The proposed method is compared with two other methods such as the maximum power point tracking control method and the without fuzzy controller method. A simple shunt circuit also includes in the DC link circuit. Therefore, during the system fault condition, the WECS can perform a stable operation. Effectiveness of the proposed method is verified by numerical simulations. Copyright © 2013 John Wiley & Sons, Ltd.     

Maximization of generated power from wind energy conversion system using a new evolutionary algorithm

In this paper, a grid-connected Doubly Fed Induction Generator controlled by a Sliding Mode Controller (SMC) is used to maximize the Wind Energy Conversion System (WECS) output power. A SMC is implemented using a PID controller that is tuned using a new algorithm based on hybrid Differential Evolution with a Linearized Biogeography-Based Optimization (LBBO-DE). Biogeography-Based Optimization (BBO) is an evolutionary optimization algorithm based on a mathematical model of organism distribution. BBO permits a recombination of the solutions features by migration. A new migration model based on the sigmoid function is proposed. An analysis of the LBBO-DE is conducted using six different models, including the sigmoid model. Their performance were tested with 23 benchmark functions. The comparison reveals that the sigmoid model has the best performance. Therefore, the LBBO-DE with a sigmoid model is used to optimize the controller parameters to maximize the WECS output power. The LBBO-DE with the sigmoid model is compared with the Tyreus-Luyben tuning method, Genetic Algorithm (GA) and Linearized BBO (LBBO). The results showed that the LBBO-DE has the best performance. The proposed algorithm is verified using an experimental setup for the maximization of the generated power from the WECS and reducing power loss.