Analysis of mechanical power fluctuations in asynchronous WECs

In recent years, wind energy has become an important part of electrical power generation in some countries. Some problems such as small voltage variations and flicker can be caused by wind turbines and wind farms, when wind speed variations produce mechanical power fluctuations. The consequence is that speed changes in the turbine rotor are produced, which lead to variations in their currents and voltages. The purpose of this paper is to study the relationship between fluctuations of mechanical power, rotor speed and currents as a function of their frequencies, in order to establish the models that should be used to analyse slow voltage variations and flicker problems. Simulations are carried out using a dynamic model of the induction machine and assuming sinusoidal waveforms for the fluctuations of the mechanical power     

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.     

Flicker contribution from wind turbine installations

In this paper, the flicker emission from a wind park connected to a grid with a high wind energy penetration is evaluated. The influence of wind speed, turbulence intensity, grid voltage quality, grid types, and number of turbines operating in the same group is measured and analyzed. The investigated wind turbines are of constant-speed stall-regulated type. It is found that the voltage quality of the grid to which the turbines are connected strongly influences the flicker emission of the turbines. Moreover, it is found that the formula used in IEC-61400-21 for determining the flicker contribution from a whole wind park gives too low total flicker value.     

风机并网对电能质量影响因子研究

为了给电网控制策略提供可靠的参考依据,对电网在接入风机后电能质量的变化进行了详尽的分析。风机作为电源同时也作为非线性负载接入配电网中使得电能质量的分析更加复杂。首先对国标中着重介绍的稳态电能质量：电压偏差,三相不平衡度,谐波以及电压波动进行理论介绍,然后在Matlab／Simulink上建立了配电系统与风机并网的模型,得到风机对电能质量不同方面的影响程度,最后采用先进电力电子设备为平稳电压电流输出提供一定的方法,并进行验证。研究结果表明,电力谐波和电压波动闪变是风机对电网的最主要的影响因素。     

Applying power quality characteristics of wind turbines for assessing impact on voltage quality

Injection of wind power into an electric grid affects the voltage quality. As the voltage quality must be within certain limits to comply with utility requirements, the effect should be assessed prior to installation. To assess the effect, knowledge about the electrical characteristics of the wind turbines is needed or else the result could easily be an inappropriate design of the grid connection. The electrical characteristics of wind turbines are manufacturer‐specific but not site‐specific. This means that, having the actual parameter values for a specific wind turbine, the expected impact of the wind turbine type on voltage quality when deployed at a specific site, possibly as a group of wind turbines, can be calculated. The methodology for this is explained and illustrated by case studies considering a 5 × 750 kW wind farm on a 22 kV distribution feeder. The detailed analysis suggests that the wind farm capacity can be operated at the grid without causing unacceptable voltage quality. For comparison, a simplified design criterion is considered assuming that the wind farm is only allowed to cause a voltage increment of 1%. According to this criterion, only a very limited wind power capacity would be allowed. Measurements confirm, however, the suggestion of the detailed analysis, and it is concluded that a simplified design criterion such as the ‘1% rule’ should not be used for dimensioning the grid connection of wind farms. Rather, this article suggests a systematic approach including assessment of slow voltage variations, flicker, voltage dips and harmonics, possibly supported by more detailed analyses, e.g. system stability if the wind farm is large or the grid is very weak, and impact on grid frequency in systems where wind power covers a high fraction of the load, i.e. most relevant for isolated systems. Copyright © 2002 John Wiley & Sons, Ltd.     

Flicker Mitigation by Active Power Control of Variable-Speed Wind Turbines With Full-Scale Back-to-Back Power Converters

Grid-connected wind turbines are fluctuating power sources that may produce flicker during continuous operation. This paper presents a simulation model of a megawatt-level variable-speed wind turbine with a full-scale back-to-back power converter developed in the simulation tool of PSCAD/EMTDC. Flicker emission of this system is investigated. Reactive power compensation is mostly adopted for flicker mitigation. However, the flicker mitigation technique shows its limits, when the grid impedance angle is low in some distribution networks. A new method of flicker mitigation by controlling active power is proposed. It smoothes the 3p active power oscillations from wind shear and tower shadow effects of the wind turbine by varying the dc-link voltage of the full-scale converter. Simulation results show that damping the 3p active power oscillation by using the flicker mitigation controller is an effective means for flicker mitigation of variable-speed wind turbines with full-scale back-to-back power converters during continuous operation.      

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.     

Power quality impact of a sea located hybrid wind park

The power quality impact of a 2.5 MW wind park has been measured and analyzed. The turbines in the park operate at variable speed at low wind speeds and at fixed speed at higher wind speeds. The harmonic current injection is low and the calculated flicker contribution is seldom above 0.25. The wind park also has the potential to maintain the voltage on the supplying grid below a certain limit, in this case 11 kV     

Modeling and experimental verification of grid interaction of a DFIG wind turbine

The response of the doubly fed induction generator (DFIG) wind turbine system to grid disturbances is simulated and verified experimentally. The results are compared to the response that a fixed-speed wind turbine would have given. A voltage sag to 80% (80% remaining voltage) is handled very well, which is not the case for a fixed-speed wind turbine. A second-order model for prediction of the response of DFIG wind turbines is derived, and its simulated performance is successfully verified experimentally. The power quality impact by the DFIG wind turbine system is measured and evaluated. Steady-state impact, such as flicker emission, reactive power, and harmonic emission, is measured and analyzed. It is found that the flicker emission is very low, the reactive power is close to zero in the whole operating range, and the current THD is always lower than 5%.     

Wind farms connected to weak grids in India

This article describes the mutual influence between the wind turbines and the power quality in the Indian power systems. The Indian power systems are weak in general and the wind energy development has been very fast and concentrated in a few rural areas where the existing transmission and distribution grids are very weak. Therefore the mutual influence between wind turbines and power quality is particularly strong in India. The wind farms influence power quality aspects such as steady state voltage, power factor, flicker and harmonic and interharmonic distortion. The power quality of the grid influences the power performance and safety of the wind turbines and the lifetime of mechanical and electrical components. The findings presented are general for wind farms connected to weak grids, but the Indian case is pronounced concerning weakness of grid as well as wind energy penetration level. Copyright © 2001 John Wiley & Sons, Ltd.     

Flicker study on variable speed wind turbines with doubly fed induction generators

Grid connected wind turbines may produce flicker during continuous operation. This paper presents a simulation model of a MW-level variable speed wind turbine with a doubly fed induction generator developed in the simulation tool of PSCAD/EMTDC. Flicker emission of variable speed wind turbines with doubly fed induction generators is investigated during continuous operation, and the dependence of flicker emission on mean wind speed, wind turbulence intensity, short circuit capacity of grid and grid impedance angle are analyzed. A comparison is done with the fixed speed wind turbine, which leads to a conclusion that the factors mentioned above have different influences on flicker emission compared with that in the case of the fixed speed wind turbine. Flicker mitigation is realized by output reactive power control of the variable speed wind turbine with doubly fed induction generator. Simulation results show the wind turbine output reactive power control provides an effective means for flicker mitigation regardless of mean wind speed, turbulence intensity and short circuit capacity ratio.     

并网双馈风电机组低电压穿越能力研究

详细分析了双馈风电机组LVRT功能的实现原理,并在电力系统仿真分析软件PSASP中建立双馈风电机组的LVRT功能模型,采用地理接线图直观地表示风电场外部系统发生短路故障瞬间对风电机组端电压的影响．并以我国某地区电网为例来分析在风电场接入方式不同的情况下系统短路故障对风电机组的影响。根据仿真结果给出风电机组LVRT能力的最低电压限值要求。最后提出了利用串联制动电阻来提高风电机组的LVRT能力的新方法。分析结果表明,串联制动电阻能够可观地提高风电机组的低电压穿越能力。具有较高LVRT能力的风电机组。可以节省一定的投资费用,在一定程度上降低了风电的上网电价。     

风力发电并入微网电能质量分析与检测

风力发电并网引发的电能质量问题不仅是限制风电机组装机规模的重要因素,而且对微网的安全稳定运行产生了很大的影响,因此有必要对风力发电并网引发的电能质量问题进行深入的分析,而分析的前提就是对其进行准确的检测。首先简要介绍微网的定义,并在此基础上给出风力发电机并入微网的结构示意图,其次对风力发电并网引发的电压波动与闪变、谐波以及电压偏差的机理进行分析,得出引发这些电能质量问题的根本原因。最后针对风力发电并网引发的电压偏差现象,提出采用Hilbert-Huang变换的方法对其进行检测。仿真测试结果表明了该方法的有效性。     

Propagation of flicker in electric power networks due to windenergy conversions systems

Wind energy conversion systems (WECS) produce fluctuating output power, which may cause voltage fluctuations and flicker. Flicker assessment in networks may be difficult since its evaluation requires long computing time and special procedures to calculate the flicker severity index, Pst. In this paper, a frequency domain method to study flicker propagation is presented. This method is based on propagation of frequency components from WECS output currents throughout the grid. In this way, a fast flicker analysis in a network of any size can be performed. Also, an algorithm for flicker measurement in the frequency domain, which allows Pst calculation, is proposed. Several study cases have been performed, and results have been compared with time domain simulations, showing good agreement between them     

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.     

Prediction of flicker produced by wind turbines

Electrical flicker is a measure of voltage variations which may cause disturbances to consumers. Flicker is caused by both generators and loads connected to the network. This paper discusses the various issues which affect electrical flicker from wind turbines, and describes the development of a software tool capable of predicting, at the design stage, the flicker which would be produced by a wind turbine, or by a wind farm of similar turbines, on a particular network. The paper describes the modelling of the physical dynamics of the wind turbine and the turbulence in the wind which drives it, the electrical dynamics of the generator, and the network itself with various types of embedded consumer loads. Measurements carried out on two different 1 MW wind turbines have been used to validate the models. Copyright © 1998 John Wiley & Sons, Ltd.     

Difference between grid connections of large-scale wind power and conventional synchronous generation

In China, regions with abundant wind energy resources are generally located at the end of power grids. The power grid architecture in these regions is typically not sufficiently strong, and the energy structure is relatively simple. Thus, connecting large-capacity wind power units complicates the peak load regulation and stable operation of the power grids in these regions. Most wind turbines use power electronic converter technology, which affects the safety and stability of the power grid differently compared with conventional synchronous generators. Furthermore, fluctuations in wind power cause fluctuations in the output of wind farms, making it difficult to create and implement suitable power generation plans for wind farms. The generation technology and grid connection scheme for wind power and conventional thermal power generation differ considerably. Moreover, the active and reactive power control abilities of wind turbines are weaker than those of thermal power units, necessitating additional equipment to control wind turbines. Hence, to address the aforementioned issues with large-scale wind power generation, this study analyzes the differences between the grid connection and collection strategies for wind power bases and thermal power plants. Based on this analysis, the differences in the power control modes of wind power and thermal power are further investigated. Finally, the stability of different control modes is analyzed through simulation. The findings can be beneficial for the planning and development of large-scale wind power generation farms.     

A Novel Scheme to Connect Wind Turbines to the Power Grid

In this paper, a new scheme is proposed to connect wind turbines to the power grid. This scheme helps in limiting the fault currents as well as in voltage unbalance compensation. Voltage is injected in series with the transmission line to limit the fault currents as well as to balance the voltages. This scheme allows wind turbines to remain synchronized to the grid during faults or during low voltages and is useful for doubly fed as well as squirrel cage induction generators.      

基于PC测量仪器的风电机组电能质量测试系统

风电机组的电能质量测试不同于传统负荷的电能质量测试,需要排除电网特性和其他负荷的影响,并考虑不同风速对测试结果的影响．目前常规的电能质量测试仪器无法完成测试。风电机组电能质量测试系统选择高性能的PC测量仪器作为硬件采集平台．通过搭建虚拟电网排除电网结构及其他负荷产生的影响,采用适合风电机组的数字化闪变仪算法实现了风电机组的电能质量测试。测试系统通过对典型风电机组实际测试,证明测试系统完全能够满足风电机组电能质量测试标准IEC61400—21的要求。     

影响风电场电能质量的因素研究

风电场并网时产生的电压波动和闪变、电压偏差以及谐波,直接影响电网运行的稳定性。本文以某风电场为例,探讨以上影响风电场电能质量的因素。通过对相关因素的阐述、计算及分析,结合国家标准规范,明确电场电能质量的分析评估方法。结果表明,风电并网前需进行电能质量评估,对相关参数进行详细计算,其结果必须满足国标要求,才可实现电网稳定运行。