Showing papers in "Iet Renewable Power Generation in 2008"

Overview of different wind generator systems and their comparisons

Abstract: With rapid development of wind power technologies and significant growth of wind power capacity installed worldwide, various wind turbine concepts have been developed. The wind energy conversion system is demanded to be more cost-competitive, so that comparisons of different wind generator systems are necessary. An overview of different wind generator systems and their comparisons are presented. First, the contemporary wind turbines are classified with respect to both their control features and drive train types, and their strengths and weaknesses are described. The promising permanent magnet generator types are also investigated. Then, the quantitative comparison and market penetration of different wind generator systems are presented. Finally, the developing trends of wind generator systems and appropriate comparison criteria are discussed. It is shown that variable speed concepts with power electronics will continue to dominate and be very promising technologies for large wind farms. The future success of different wind turbine concepts may strongly depend on their ability of complying with both market expectations and the requirements of grid utility companies.

Integration of large-scale wind power and use of energy storage in the netherlands' electricity supply

Abstract: The use of energy storage for increased operational flexibility is commonly regarded as a logical complement for systems with large amounts of wind power. The authors explore, the opportunities for energy storage for the integration of large-scale wind power into a future lay-out of the Dutch generation system, for which minimum-load problems are foreseen with high wind power penetrations. A central unit commitment and economic despatch model is extended with models for three large-scale energy storage technologies: pumped hydro accumulation storage (PAC), underground PAC and compressed air energy storage. Furthermore, an alternative solution is investigated, comprising the installation of heat boilers at selected combined heat and power locations (CHP) in order to increase the operational flexibility of these units. Results are shown for different wind power penetrations and scenarios. A cost-benefit analysis shows that the operation cost savings from energy storage increase with the amount of wind power installed. Taking into account the large investment costs, energy storage units are however unlikely to have a profitable exploitation. The installation of heat boilers at CHP locations is found to be more efficient and a promising solution for the integration of large-scale wind power in the Netherlands. A notable result is that for the Dutch system, the use of energy storage increases the system-s overall CO 2 emission levels because energy storage allows storing power from cheap coal plants for substitution of expensive gas during peak. Even though often proposed as a solution for wind power integration, energy storage in fact partly annuls CO 2 emission savings by wind power.

An improved fault ride-through strategy for doubly fed induction generator-based wind turbines

Abstract: Keeping the generators operating during transient grid faults becomes an obligation for the bulk wind generation units connected to the transmission network and it is highly desired for distribution wind generators A proposed scheme is implemented to keep the wind-power DFIG operating during transient grid faults Challenges imposed on the generator configuration and the control during the fault and recovering periods are presented A comprehensive time domain model for the DFIG with the decoupled dq controller is implemented using Matlab/Simulink software Intensive simulation results are discussed to ensure the validity and feasibility of the proposed fault ride through technique The scheme protects the DFIG components, fulfills the grid code requirements and optimises the hardware added to the generator

Lifetime prediction and sizing of lead-acid batteries for microgeneration storage applications

Abstract: Existing models of microgeneration systems with integrated lead-acid battery storage are combined with a battery lifetime algorithm to evaluate and predict suitable sized lead-acid battery storage for onsite energy capture. Three onsite generation portfolios are considered: rooftop photovoltaic (2.5 kW), micro-wind turbine (1.5 kW) and micro combined heat and power (1 kW). With no embedded energy storage, the dwelling exports energy when the microgeneration system generates excess power leading to a high level of generated export throughout the year. The impact that the size of installed battery has on the proportion of the generated export that is reserved onsite, along with the annual energy discharged per year by the energy store is assessed. In addition, the lifetime algorithm is utilised to predict corresponding lifetimes for the different scenarios of onsite generation and storage size, with design tables developed for expected cost and weight of batteries given a predicted generated export and lifetime specification. The results can be used to indicate optimum size batteries for using storage with onsite generation for domestic applications. The model facilitates the choice of battery size to meet a particular criteria, whether that be optimising size, cost and lifetime, reducing grid export or attempting to be self-sufficient. Suitable battery sizes are found to have lifetimes of 2-4 years for high production microgeneration scenarios. However, this is also found to be highly variable, depending on chosen microgeneration scenario and battery size.

Condition monitoring benefit for onshore wind turbines: sensitivity to operational parameters

Abstract: The economic case for condition monitoring (CM) applied to wind turbines is currently not well quantified and the factors involved are not fully understood. In order to make more informed decisions regarding whether deployment of CM for wind turbines is economically justified, a refined set of probabilistic models capturing the processes involved are presented. Sensitivity of the model outputs with respect to variables of interest are investigated within the bounds of published data and expert opinion. The results show that the levels of benefit are dependent on a variety of factors including wind profile, typical downtime duration and wind turbine sub-component replacement cost.

Predicting the technical impacts of high levels of small-scale embedded generators on low-voltage networks

Abstract: The anticipated high penetrations of small-scale embedded generators (SSEGs) on public low-voltage (LV) distribution networks are likely to present distribution network operators (DNOs) with a number of technical impacts relating to power quality, distribution system efficiency and potential equipment overloads. Impact studies need to be performed using suitable case study networks in order to evaluate the effects of SSEGs on LV distribution networks and quantify allowable SSEG penetration levels. The aim is to propose a methodology for predicting the technical impacts of SSEGs on LV networks without the need for developing a detailed computer-based model of the power system and simulating a range of operating scenarios. This methodology is drawn from an analysis of the key electrical characteristics that determine the response of LV networks to the addition of SSEGs, focusing on the following technical aspects: (i) voltage regulation, (ii) voltage rise, (iii) voltage unbalance, (iv) cable and transformer thermal limits and (v) network losses. The analysis is carried out on a UK generic and a European generic LV network and simulation results for both networks are presented and discussed. The proposed methodology is then applied to an existing public UK LV network operated by E.ON UK Central Networks, indicating a good agreement between predicted and simulation results.

Structural mass in direct-drive permanent magnet electrical generators

Abstract: Direct-drive permanent magnet (PM) synchronous machines for wind turbines are large, heavy and thus expensive to build, transport and install. The structure that is required to maintain the airgap clearance is often heavier than the electromagnetically active material. It is shown that the structural material can be estimated early on in the design process for axial-flux and radial-flux machines. By moving to a design without iron in the stator, the forces that the machine structure must deal with can be significantly reduced. The mass of iron-cored and air-cored axial-flux PM synchronous machines for a range of multi-MW wind turbines is compared. Scaling laws for PM generators in direct-drive wind turbines are also developed.

Modelling adequacy of the doubly fed induction generator for small-signal stability studies in power systems

Abstract: The dynamics of the doubly fed induction generator (DFIG) with a closed-loop control is analysed. The analysis provides explanations of the impact of the closed-loop control on the DFIG dynamics and relevant modelling requirements for power system stability studies. The discussion considers generic PI controllers for the regulation of rotor speed, reactive power and pitch angle. It is shown that for the closed-loop-controlled DFIG, a simplified model, whereby both stator and rotor dynamics are neglected, is adequate. In such model, stator and rotor variables are algebraic, that is, they change instantaneously and the modelled dynamics are those of the controllers and mechanical parts. The observations and conclusions are obtained from eigenvalue, participation factor and time-domain analysis.

Stability of doubly-fed induction generator under stator voltage orientated vector control

Abstract: The stability of a doubly-fed induction generator (DFIG) under vector control in stator voltage orientation (SVO) is investigated. Prior art has tended to assume that the inner current loop dynamics can be neglected when an SVO is employed. As a result, the poorly damped poles of the DFIG system were considered unaffected by the inner current loop tuning. The state-space model of the machine including the inner current closed loop dynamics is developed for schemes where different feed-forward compensation terms are used. The interaction between inner current loop dynamics and damping of the critical poles of the system is illustrated through analysis and simulation. The main outcome of the analysis is that the stability of the machine system in an SVO depends solely on the parameters of the proportional-integral controllers. Erroneous tuning can lead to instability, irrespective of the particular feed-forward compensation scheme, which could cause the disconnection of the machine as a result of rotor current oscillations of unacceptable magnitude in an actual case. The main contribution is to provide the necessary methodology in order to ensure the stable operation of a DFIG under SVO vector control.

Advanced grid requirements for the integration of wind farms into the Spanish transmission system

Abstract: The main purpose of this paper is to present the grid requirements for the integration of wind farms into the Spanish transmission system. In the particular case of voltage support capability, the Spanish transmission system operator (TSO), Red Electrica de Espana (REE) has defined a set of conditions to be met at the point of common coupling of a wind farm, not at terminals of a single wind turbine. A Verification, Validation, and Certification Procedure (VV&CP) to check compliance at terminals of a wind farm has been developed with the contribution and common agreement of all different wind power players and distribution system operators, leaded by the Spanish Wind Energy Association and supervised by REE. The VV&CP solves the problem defining a "particular procedure" based on testing or a "general procedure" based on the simulation of the wind farm behaviour. It is the scope of Energy to Quality in this paper, to discuss and confirm the VV&CP reliability and validity, supported by field testing, validated models of wind turbines and simulation of wind farms. Conclusions are drawn on our efforts towards worldwide harmonization, on the development of a common VV&CP based on the Spanish and German experience.

Impact of wind generation control strategies, penetration level and installation location on electricity market prices

Abstract: Integration of wind facilities into power system grids have several impact on power system related issues including; transmission congestion, optimum power flow, system stability, power quality, system economics and load dispatch. Consequently, wind farm control strategy, location across the distribution network and its penetration level could have an impact on electricity market prices. This paper addresses these issues, by developing a single auction market model. An optimal power flow problem was formulated for determining the close to real time electricity market-clearing price and the total cost of generation. Simulation results, considering different operational cases, are presented to highlight the impact.

Optimal gain-scheduled control of fixed-speed active stall wind turbines

Abstract: For a smooth integration of large wind farms into the utility grids, the individual wind turbines must be able to achieve various power control objectives. In this context, the authors focus their attention on the control of fixed-speed active stall wind turbines. This sort of turbine includes a pitch servomechanism to induce stall on the blades, thereby having control on the output power. The authors develop a methodology to design optimal gain-scheduled pitch controllers valid for the whole operating region of the wind turbine. The proposed solution uses concepts of linear parameter-varying system theory. In addition to providing a formal framework for the control design, this theory guarantees stability and performance. Further, because of the similarities with ℋ∞ control, the tools developed for the controller design are very familiar to the control community. The main features of the proposed controller are assessed by means of numerical simulations obtained for realistic wind speed profiles and power production demands.

Initial study of using rechargeable batteries in wind power generation with variable speed induction generators

Abstract: There is a possibility of having a small rechargeable battery bank consisting of one or two groups of batteries inside a wind turbine tower. This paper investigates such a possibility for wind turbines using double fed induction generators (DFIGs). Two schemes are proposed. The first scheme, BS1, uses rechargeable batteries as an energy source/sink for the control of generator rotor currents. The second scheme, BS2, in addition to the batteries under BS1, has another group of batteries to smooth wind turbine power output and to provide power balancing and emergency support to the grid. A case study is used to evaluate these two schemes. The study is based on the statistics of the wind speeds measured at Dunstaffnage, Scotland, and the manufacturer's data for the turbine generator. It shows that if the proposed BS2 scheme is adopted and some mild assumptions are used in simulation: (1) variation of the turbine power output to the grid is reduced ' the variation of the output of a wind farm is within 13' of the combined ratings of all turbines in the farm; (2) less variation in power also means smoother currents and reduced I 2 R loss - for some cables in the wind farm plant this is a reduction of more than 53-; (3) requirements or ratings of some equipment can be reduced - the capacitor and filtering circuits for the DC bus are no longer required and the ratings of some transformers can be reduced; (4) some complicated control can be simplified - the control for the grid-side converter under the current -DFIG scheme- is replaced by a simple battery charging/discharging algorithm; (5) the wind turbine plant will be more supportive to the grid; and from the grid point of view, the performance of the wind farms adopting the proposed BS2 scheme will be similar to and in certain aspects even better than that of power plants using fossil fuels; and (6) all the above will bring some economic benefits at the expense of some additional costs associated with using batteries. Some practical issues, including available space in a typical turbine tower, are also discussed. The simulation results presented, however, are limited to steady-state models. Further research will include the type of batteries to be used, more detailed dynamic studies and quantified analysis of economic benefits and costs.

Small-signal model and dynamic analysis of variable speed induction machine wind farms

Abstract: A unified, modular, small-signal dynamic model for an induction machine-based wind farm is presented. The proposed model can represent an arbitrary number of fixed-speed, partially variable-speed (doubly fed) and variable-speed induction machine-based wind units in a wind farm. The model represents the dynamics of (i) each wind turbine-generator electromechanical system, (ii) each converter system and the corresponding controls and (iii) the host electrical grid, within the subsynchronous frequency range (0.1–60 Hz). In contrast to the widely used state-space formulation, the proposed approach is based on developing electrical and mechanical transfer-function matrices to formulate the overall system as a multivariable feedback system. Thus, it provides significant flexibility to represent a large number of wind units and their controllers, identical or non-identical, within a wind farm.Based on the developed model, a frequency response-based method is also introduced, as an alternative to the eigen analysis approach, for small-signal subsynchronous dynamic analysis of the wind farm and its host electrical grid. The proposed method provides a new performance robustness criterion for performance evaluation and also a design tool based on the ‘size’ of the system transfer-function matrix. The concept of induced norm is adopted here as a measure of matrix size. As an example, applications of the model and the analysis approach to a two-unit wind farm is presented and the results are validated based on time-domain simulation studies in the PSCAD/EMTDC environment.

Analytical determination of steady-state converter control laws for wind turbines equipped with doubly fed induction generators

Abstract: An analytical method for the determination of the steady-state control laws of the doubly fed induction generators (DFIG) used in wind turbines is presented. The analytical model is used to derive the converter control laws of the generator in terms of rotor voltage and control angle (real and reactive power) on the whole operation speed range. The method can be efficiently implemented in a global optimisation CAD environment to design the converters and the generator associated to a given wind turbine. It is used to present some DFIG performances for different grid specifications and discuss the relative size of the rotor and grid side converters for a fixed induction generator and a given wind turbine of 2.5 MW.

Wind farm modelling for reliability assessment

Abstract: Increasing exploitation of wind energy requires the development of adequate models and techniques for reliability assessment of wind farms (WFs) and of electric power systems including wind energy conversion systems. A novel approach to WF modelling is investigated for reliability assessment, which is based on the universal generating functions. Such an approach combines the use of the z-transform and composition operators, allowing to account for all the factors affecting the WF performance. The results, both theoretical and numerical, of the case study, give evidence of the effectiveness of the proposed approach in terms of accuracy as well as of flexibility and efficiency of the solving algorithm.

Specification of rotor side voltage source inverter of a doubly-fed induction generator for achieving ride-through capability

Abstract: The ride-through capability of a doubly-fed induction generator under three-phase balanced voltage sags is examined, under the condition that the machine should have the capability of real and reactive power control during the external fault. Mathematical formulae for the peak rotor fault current and the required rotor voltage output under vector control are derived. Moreover, the DC link dynamics are incorporated into the analysis and it is shown that they can have a decisive impact on fault behaviour of the machine during voltage sags. Combined, a design methodology for the rotor side voltage source inverter aiming to achieve a ride-through capability at the lowest cost is described. Simulation results in PSCAD/EMTDC show very good agreement with the theoretical analysis.

Interaction between proton exchange membrane fuel cells and power converters for AC integration

Abstract: New aspects about steady-state and transient behaviours are considered in the relationships among the model variables for fuel cell (FC) stacks that are of great interest in the scientific environment. In this way, the most important variables are voltage, current, power, heat from cooling system, membrane temperature and hydrogen pressure. The transient model aims at reproducing FC variations of its internal resistance under distinct current levels. This current effect modifies the time response during load turning on/off conditions. Results from a modified FC model are presented and comparisons with real data are made. Additionally, these results are included in an analysis about the electrical interaction between FCs and converters as a cause of great concern among power electronics designers. As the number of such converters has significantly increased in the last few years, FC generation systems are steadily calling attention for operational problems related to their efficiency, stability and durability when DC–DC converters are connected across its terminals. Therefore design proceedings of a DC–DC converter associated to ‘T’ filters to avoid fast current transitions caused by converter connection across the FC stack terminals are included. To deliver the energy produced by the FC system to the grid, it is presented, also, an analysis of a DC–AC converter used to improve power quality when the FC is, simultaneously, supplying load and grid.

Analysis of injected apparent power and flicker in a distribution network after wind power plant connection

Abstract: The maximum apparent power that can be injected in a bus and the flicker emission can represent a significant limit in the total capacity of a wind farm when wind turbines are connected to a radial distribution system. Quantification of these values is important in determining the largest number of wind turbines that can be connected in a network. The calculation of maximum apparent power and flicker of residential and commercial radial distribution feeder with remotely connected wind turbines has been investigated using elaborated specific software denominated winds port program and data from two energy systems. The developed software can calculate the approximate injected maximum apparent power, for a wind farm, in one bus of the network and in two buses of the network, simultaneously, without the need of a load flow program. The various simulations results reveal that wind farm capacities, in each bus, can be limited firstly for flicker and later for bus injected power for a voltage variation value, and the calculation of approximate apparent powers allows a precise and fast adjustment of voltage variation. If a load flow program is used for the determination of the wind farm maximum apparent power in each bus of the distribution system, a lot of simulations with a lot of attempts must be necessary. Therefore the methodology used by the authors is advantageous.

Concepts for the improved integration of wind power into the german interconnected system

Abstract: The ongoing increase in wind power in Germany requires the accomplishment of a number of challenges. Most notably, the substantial strain on the transmission grid and the increasing requirement of control reserve capacity need to be tackled. Therefore the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety has commissioned a scientific study, in which novel concepts for optimised integration of wind energy into the German interconnected system are being evaluated. The Institute of Power Systems and Power Economics (IAEW) of RWTH Aachen University and the Research Centre for Electrical Systems and Power Economics cooperatively handles the study. The investigations carried out by IAEW are focused. They comprise a generation management (GM) of wind turbines and an improvement in load forecast, both in terms of a reduction in necessary control reserve, an improved integration of wind power because of the utilisation of intraday markets, a demand-side management in order to provide control reserve and the potentials of modern storage technologies. Primarily, the application of GM proves to be of significant economical benefit. Moreover compressed air energy storages may prove to be a suitable utility for the integration of wind power.

Introducing a coloured fluid stochastic petri net-based methodology for reliability and performance evaluation of small isolated power systems including wind turbines

Abstract: A new approach in reliability evaluation of small isolated hybrid power systems, which include wind turbines and conventional generators, based on fluid stochastic Petri net (PN) modelling is presented. A major novelty of the proposed methodology is that a selection of constant time intervals is adopted, instead of assuming continuous dynamics. The proposed analysis presents similar characteristics with simulation methods, with the additional advantages of graphical representation of system's components and attributes. Moreover, emphasis is given on the parameterisation of input data that describes operational and reliability characteristics of the system, so a global investigation of the examined system can be done. In order to import more precise data in the simulation process modelled by the PN, the concept of database arcs is introduced. The evaluation is based on the calculation of system's basic indices related to reliability estimation and energy production. The results obtained verify the flexibility and the capabilities of the proposed methodology.

Evaluation of the voltage change factor k U for DG equipped with synchronous generators

Abstract: An evaluation of the voltage disturbances caused by the synchronisation to the grid of distributed generation (DG) resources equipped with synchronous generators is presented. The synchronisation procedures and the resulting transients are first discussed. The effect of the synchronisation conditions (frequency, phase, voltage magnitude deviations) on the resulting fast voltage changes is then demonstrated through an extensive set of simulations. The investigation is extended to the effect of other parameters of the grid and the generator. Objective of the analysis is to shed some light on the selection of appropriate values for the voltage change factor kU, as per IEC 61400-21, which is used in practical situations to evaluate the fast voltage changes due to the connection of DG power stations.

An Evaluation of Distributed Generation Constrained Connection Managers

Abstract: This paper presents the technical considerations and economics of a number of solutions that would allow a greater installed capacity of distributed generation to be connected to, and managed within, the distribution network. The paper describes the various solutions and compares their relative energy yield and economics. It was found that a distributed generation constrained connection manager informed by dynamic thermal ratings is the most attractive solution for developers wishing to connect wind generation to this case study network when compared to alternative solutions.

Editorial [selected papers from the european wind energy conference (EWEC) 2007]

Abstract: Wind energy is undergoing a remarkable resurgence, and is now poised to provide an ever increasing share of the world's energy in the coming years. The story of its rise, fall, and rise again is a fascinating one. From roughly 1200 A.D. to the 1800's, wind was one of the major sources of energy for industry. Energy from the wind was used to grind flour, pump water, saw wood, press oil and make paper. With the advent of the steam engine however, the use of wind for mechanical power began to decline. This decline was only partially interrupted by the application of windmills for pumping water in the settlement of the American West during the 19th century. Once electric power generation appeared, late in the 19th century, there were a number of efforts to use wind rotors as the prime movers. Some of the smaller of these wind electric generators, such as the Jacob's wind turbine, had some success. Others, such as the much larger Smith-Putnam turbine of the late 1930's, proved to be ahead of their time.

Selected papers from the European Wind Energy Conference (EWEC) 2007

Abstract: Wind energy is undergoing a remarkable resurgence, and is now poised to provide an ever increasing share of the world's energy in the coming years. The story of its rise, fall, and rise again is a fascinating one. From roughly 1200 A.D. to the 1800's, wind was one of the major sources of energy for industry. Energy from the wind was used to grind flour, pump water, saw wood, press oil and make paper. With the advent of the steam engine however, the use of wind for mechanical power began to decline. This decline was only partially interrupted by the application of windmills for pumping water in the settlement of the American West during the 19th century. Once electric power generation appeared, late in the 19th century, there were a number of efforts to use wind rotors as the prime movers. Some of the smaller of these wind electric generators, such as the Jacob's wind turbine, had some success. Others, such as the much larger Smith-Putnam turbine of the late 1930's, proved to be ahead of their time.