Large Offshore Wind Farms Research Articles

Regime-switching modelling of the fluctuations of offshore wind generation

The magnitude of power fluctuations at large offshore wind farms has a significant impact on the control and management strategies of their power output. If focusing on the minute scale, it looks like different regimes yield different behaviours of the wind power output. The use of statistical regime-switching models is thus investigated. Regime-switching approaches relying on observable (i.e. based on recent wind power production) or non-observable (i.e. a hidden Markov chain) regime sequences are considered. The former approach is based on either self-exciting threshold autoregressive (SETAR) or smooth transition autoregressive (STAR) models, while Markov-switching autoregressive (MSAR) models comprise the kernel of the latter one. The particularities of these models are presented, as well as methods for the estimation of their parameters. The competing approaches are evaluated on a one-step ahead forecasting exercise with time-series of power production averaged at a 1, 5, and 10-min rate, at the Horns Rev and Nysted offshore wind farms in Denmark. For the former wind farm, the one-step ahead root mean square error (RMSE) is contained between 0.8% and 5% of installed capacity, while it goes from 0.6% to 3.9% of installed capacity for the case of Nysted. It is shown that the regime-switching approach based on MSAR models significantly outperforms those based on observable regime sequences. The reduction in one-step ahead RMSE ranges from 19% to 32% depending on the wind farm and time resolution considered. The presented results clearly demonstrate that the magnitude of fluctuations of offshore wind power cannot be considered as simply influenced by the generation level only.

Point of Connection Voltage Regulation Strategies for AC Connected Large Offshore Wind Farms

The offshore wind energy generation industry is developing rapidly in the UK. The long submarine cable connection needed between the wind turbine generators and the onshore Point of Connection (PoC) introduces additional difficulties for compliance with the GB Grid Code reactive power requirement. To address this problem, this paper investigates different PoC voltage regulation strategies. The methods are compared in terms of control requirements, fault behaviour of the wind farm and economics. It is shown that for long connection distances to the PoC, additional dynamic reactive compensation equipment and/or a tap-changing transformer at the offshore platform will probably be needed for Grid Code Compliance. It is shown that a strategy where a STATCOM solely regulates the PoC voltage, leads to increased capital expenditure. However, this provides advantages in respect to its simplicity, speed of PoC voltage regulation and wind turbine behaviour for a fault at the PoC.

Offshore Coastal Wind Speed Gradients: Issues for the Design and Development of Large Offshore Windfarms

Simulations, from mesoscale numerical models, and analyses of in-situ and remote sensing data from offshore wind farms in Denmark, are used to examine both horizontal and vertical gradients of wind speeds in the coastal zone. Results suggest that the distance from the coastline over which wind speed vertical profiles are not at equilibrium with the sea surface (which defines the coastal zone) extends to 20 km and possibly 70 km from the coast. Using this operational definition of the coastal zone, these results thus imply the typical width of the coastal zone in northern Europe is between 20 and 70 km. The width of the coastal zone, and the wind's vertical (shear) and horizontal gradients within the coastal zone, depend on atmospheric stability. Although vertical wind speed profiles above 50 m are likely responding to additional factors such as the height of the boundary-layer, using a stability correction improves predictions of wind speed compared with the logarithmic profile. Modelling indicates that within the coastal zone, wind speeds at typical turbine hub-heights can change by 2 m/s over the horizontal extent of a large wind farm, depending on stability. However, if the fetch is sufficiently long (or the windfarm is further from the coast) both horizontal and vertical wind speed gradients over the area of the wind farm appear to be small and negligible.

Modelling of power fluctuations from large offshore wind farms

AbstractThis paper deals with modelling of power fluctuations from large wind farms. The modelling is supported and validated using wind speed and power measurements from the two large offshore wind farms in Denmark. The time scale in focus is from 1min to a couple of hours, where significant power fluctuations have been observed from these wind farms. Power and wind speed are measured with 1s sampling time in all individual wind turbines in almost 1 year, which provides a substantial database for the analyses. The paper deals with diversified models representing each wind turbine individually and with aggregation of a wind farm to be represented by a single large wind turbine model. Copyright © 2007 John Wiley & Sons, Ltd.

Power Fluctuations From Large Wind Farms

This paper deals with power fluctuations from wind farms. The time range in focus is between one minute and up to a couple of hours. In this time range, substantial power fluctuations have been observed during unstable weather conditions. A wind power fluctuation model is described, and measured time series from the first large offshore wind farm, Horns Rev in Denmark, are compared to simulated time series. The comparison between measured and simulated time series focuses on the ramping characteristics of the wind farm at different power levels and on the need for system generation reserves due to the fluctuations. The comparison shows a reasonable agreement between simulations and measurements, although there is still room for improvement of the simulation model.

Modeling of Wind Turbines Based on Doubly-Fed Induction Generators for Power System Stability Studies

This paper deals with modeling of the doubly-fed induction generator (DFIG) and the corresponding converter for stability studies. To enable efficient computation, a reduced-order DFIG model is developed that restricts the calculation to the fundamental frequency component. However, the model enhancement introduced in this paper allows the consideration of the alternating components of the rotor current as well, which is necessary for triggering the crowbar operation. Suitable models are presented for the rotor and grid side converters as well as the dc-link, taking into account all four possible operating modes. The proposed model for speed and pitch angle control can be used when wind and rotor speed variations are significant. Simulation results are presented for model verification purposes and also for demonstrating the dynamic behavior of a large offshore wind farm connected through a long undersea cable to the high voltage grid.

Modelling and measurements of power losses and turbulence intensity in wind turbine wakes at Middelgrunden offshore wind farm

AbstractUnderstanding of power losses and turbulence increase due to wind turbine wake interactions in large offshore wind farms is crucial to optimizing wind farm design. Power losses and turbulence increase due to wakes are quantified based on observations from Middelgrunden and state‐of‐the‐art models. Observed power losses due solely to wakes are approximately 10% on average. These are relatively high for a single line of wind turbines due in part to the close spacing of the wind farm. The wind farm model Wind Analysis and Application Program (WAsP) is shown to capture wake losses despite operating beyond its specifications for turbine spacing. The paper describes two methods of estimating turbulence intensity: one based on the mean and standard deviation (SD) of wind speed from the nacelle anemometer, the other from mean power output and its SD. Observations from the nacelle anemometer indicate turbulence intensity which is around 9% higher in absolute terms than those derived from the power measurements. For comparison, turbulence intensity is also derived from wind speed and SD from a meteorological mast at the same site prior to wind farm construction. Despite differences in the measurement height and period, overall agreement is better between the turbulence intensity derived from power measurements and the meteorological mast than with those derived from data from the nacelle anemometers. The turbulence in wind farm model indicates turbulence increase of the order 20% in absolute terms for flow directly along the row which is in good agreement with the observations. Copyright © 2007 John Wiley & Sons, Ltd.

Quantification of Condition Monitoring Benefit for Offshore Wind Turbines

Condition monitoring (CM) systems are increasingly installed in wind turbines with the goal of providing component-specific information to wind farm operators, theoretically increasing equipment availability via maintenance and operating actions based on this information. In the offshore case, economic benefits of CM systems are often assumed to be substantial, as compared with experience of onshore systems. Quantifying this economic benefit is non-trivial, especially considering the general lack of utility experience with large offshore wind farms. A quantitative measure of these benefits is therefore of value to utilities and operations and maintenance (O & M) groups involved in planning and operating future offshore wind farms. The probabilistic models presented in this paper employ a variety of methods including discrete-time Markov Chains, Monte Carlo methods and time series modelling. The flexibility and insight provided by this framework captures the necessary operational nuances of this complex problem, thus enabling evaluation of wind turbine CM offshore. The paper concludes with a study of baseline CM benefit, sensitivity to O & M costs and finally effectiveness of the CM system itself.

Electrical collector system options for large offshore wind farms

The work presented is part of the EC FP6 DOWNVIND project activities, which focus on the requirements and implications of very large offshore wind farms. A comparative analysis of various design options for the electrical collector system of large offshore wind farms is presented, and the advantages and disadvantages in terms of their steady-state performance and economics are discussed. The case under consideration is that of a proposed 1-GW wind farm located off the northeast coast of Scotland. The impact on power losses and voltage level changes on the collector system busbars are investigated under various operating conditions. Contingency conditions of losing one of the cables to the hub end are also explored for collector system designs with redundant cables. Finally, the authors introduce an alternative design, based on conceptual ringed arrangements, and its advantages are illustrated and discussed.

Influence of Wind Direction on Intense Power Fluctuations in Large Offshore Windfarms in the North Sea

In certain weather conditions, intense power fluctuations have been observed in the Horns Rev offshore windfarm (160 MW) located in the North Sea. This Note presents the results showing a correlation between wind direction and power fluctuations in this large offshore windfarm.

Steady State Electrical Design, Power Performance and Economic Modeling of Offshore Wind Farms

A load flow model has been developed for the evaluation of thirteen different electrical architectures for large offshore wind farms. In a case study, these architectures have been evaluated for two wind farm sizes (100 and 500 MW) and two distances to shore (20 and 60 km). The case study has shown that systems C1 (string layout) and C2 (star layout), have the lowest contribution of the electrical system to the price per kWh (Partial Levelized Production Cost PLPC). C1 and C2 system prices are 19.7 and 24.9 MEuro (100 MW, 20 km), 36.9 and 42.1 MEuro (100 MW, 60 km), 91.7 and 109.5 MEuro (500 MW, 20 km) and 132.9, 150.7 MEuro (500 MW, 60 km). For comparison another case study with prolonged life-time and predicted price decrease of power electronic components is shown too.

Excessive Over-Voltage in Long Cables of Large Offshore Windfarms

Large offshore windfarms are connected to onshore transmission grids through long high-voltage cables. When a large offshore windfarm goes into isolated operation with its high-voltage cable, for example by an action of the on-shore main-circuit breaker, there is a risk of significant over-voltage in the cable and the windfarm transformer. This paper analyses such a situation for both an existing and a planned large offshore windfarm in Denmark.

Large penetration of wind and dispersed generation into Danish power grid

The present Danish power grid is characterised by large penetration of wind power and local combined heat and power (CHP) units. Today, there are two large offshore wind farms commissioned in the Danish power grid. Construction of two more offshore wind farms (430 MW) with grid connection to the Danish transmission power grid will be by the year 2009. The Danish power grid is undergoing a change from a classical power network with few centralised large power plants into a power network with a significant amount of wind power and dispersed generation. Presently, there must be at least three large power plant units in operation in Western Denmark to control voltage and frequency and maintain security of the power system operation. The technology change to wind power and dispersed generation implies that power supply and control from the large power plants – and, consequently, also ancillary services such as, e.g. voltage and frequency control capability – have to be reduced in favour of dispersed generation and wind. Such a change constitutes a challenge with respect to maintaining security of the power system operation in different operational situations, and creates a need for new solutions with voltage and frequency control and compliance with planned power balance between the Danish power system and the neighbouring areas. This article focuses mainly on issues with regard to short-term voltage stability of the power grid.

Loss evaluation of HVAC and HVDC transmission solutions for large offshore wind farms

This paper presents a comparison of transmission losses for different technical transmission solutions for large offshore wind farms. Three technical solutions are analyzed, i.e. HVAC, HVDC Line Commutated Converter (LCC) and HVDC Voltage Source Converter (VSC). The losses for each technology are calculated for wind farms with different ratings and various distances to shore. In addition, solutions with combinations of two and the three different transmission technologies are analyzed and compared. Based on this comparison, further analysis regarding the economical feasibility can be performed in order to determine the most economic solutions for the transmission system of an offshore wind farm.

Simulation Model of the Transmission Grid for a Large Offshore Windfarm, Used in Education and Research at the Technical University of Denmark

A small test software-model for the transmission system containing a number of central power plants, consumption centres, local wind turbines and a large offshore windfarm has been implemented in the simulation tool Powerfactory (DigSilent). This model demonstrates the interaction between electricity-producing wind turbines and a realistic transmission system, for instance short-term voltage stability and ride-through capability of the wind turbines. The model has been developed by the Danish Transmission System Operator Energinet.dk for use by the Centre for Electric Technology (CET) at Technical University of Denmark. It has been applied in the education of electric power engineers and in research projects. An example of its use is presented.

Using airborne and satellite SAR for wake mapping offshore

AbstractOffshore wind energy is progressing rapidly around Europe. One of the latest initiatives is the installation of multiple wind farms in clusters to share cables and maintenance costs and to fully exploit premium wind resource sites. For siting of multiple nearby wind farms, the wind turbine wake effect must be considered. Synthetic aperture radar (SAR) is an imaging remote sensing technique which offers a unique opportunity to describe spatial variations of wind speed offshore. For the first time an airborne SAR instrument was used for data acquisition over a large offshore wind farm. The aim was to identify the turbine wake effect from SAR‐derived wind speed maps as a downstream region of reduced wind speed. The aircraft SAR campaign was conducted on 12 October 2003 over the wind farm at Horns Rev in the North Sea. Nearly simultaneous measurements were acquired over the area by the SAR on board the ERS‐2 satellite. In addition, meteorological data were collected. Both aircraft and satellite SAR‐derived wind speed maps showed significant velocity deficits downstream of the wind farm. Wind speed maps retrieved from aircraft SAR suggested deficits of up to 20% downstream of the last turbine, whereas satellite SAR‐derived maps showed deficits of the order of 10%. The difference originated partly from the two different reference methods used for normalization of measured wind speeds. The detected region of reduced wind speed had the same width as the wind turbine array, indicating a low degree of horizontal wake dispersion. The downstream wake extent was approximately 10 km, which corresponds well with results from previous studies and with wake model predictions. Copyright © 2006 John Wiley & Sons, Ltd.

Quantifying offshore wind resources from satellite wind maps: study area the North Sea

AbstractOffshore wind resources are quantified from satellite synthetic aperture radar (SAR) and satellite scatterometer observations at local and regional scale respectively at the Horns Rev site in Denmark. The method for wind resource estimation from satellite observations interfaces with the wind atlas analysis and application program (WAsP). An estimate of the wind resource at the new project site at Horns Rev is given based on satellite SAR observations. The comparison of offshore satellite scatterometer winds, global model data and in situ data shows good agreement. Furthermore, the wake effect of the Horns Rev wind farm is quantified from satellite SAR images and compared with state‐of‐the‐art wake model results with good agreement. It is a unique method using satellite observations to quantify the spatial extent of the wake behind large offshore wind farms. Copyright © 2006 John Wiley & Sons, Ltd.

Coordinated Control of an HVDC Link and Doubly Fed Induction Generators in a Large Offshore Wind Farm

Doubly fed induction generators (DFIGs) are an economic variable-speed solution for large wind turbines while high-voltage dc (HVdc) transmission is being considered for the grid connection of some offshore wind farms. This paper analyzes the need for coordinating the control of the DFIGs and the HVdc link so that the two topologies can work together, giving system designers and operators a choice that may be useful in some applications. It is desired that individual generators be controlled for power tracking in a way similar to that used when they are connected directly to an ac grid, although a grid voltage reference for the DFIG control is no longer available as an independent source in this case. The study shows that machine control should explicitly maintain the flux level, which then allows the HVdc link to regulate the local system frequency and, indirectly, voltage amplitude. Interactions between DFIGs and the HVdc link are investigated and simulations performed to verify the proposed control strategy.

Analytical modelling of wind speed deficit in large offshore wind farms

AbstractThe proposed model for the wind speed deficit in wind farms is analytical and encompasses both small wind farms and wind farms extending over large areas. As is often the need for offshore wind farms, the model handles a regular array geometry with straight rows of wind turbines and equidistant spacing between units in each row and equidistant spacing between rows. Firstly, the case with the flow direction being parallel to rows in a rectangular geometry is considered by defining three flow regimes. Secondly, when the flow is not in line with the main rows, solutions are suggested for the patterns of wind turbine units corresponding to each wind direction. The presentation is an outline of a model complex that will be adjusted and calibrated with measurements in the near future. Copyright © 2006 John Wiley & Sons, Ltd.

Connecting renewables: The challenge of integrating large offshore wind farms

Considerable changes in world energy markets, and the need to integrate renewable energy sources into the network, is presenting the industry with new challenges. Phill Cartwright of AREVA T&D considers the issues arising from the connection of wind farms to the grid.