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Emphasis in this paper is on the fault ride-through operation impact on the wind turbines structural loads. Grid faults are typically simulated in power system simulation tools using simplified drive train mechanical model, approach which doesn't allow a thorough investigation of structural loads. This paper presents a two-steps simulation procedure, where two complimentary tools are combined, i.e. power system simulation tool and advanced aeroelastic simulation code. The goal is to investigate the whole integrated wind turbine design and to provide insight both into the structural as well as the electrical design of the wind turbine response during grid faults. The two-step simulation procedure is assessed by means of a simulation example. The effect of a grid fault on the structural part of a typical fixed speed wind turbine, equipped with an induction generator, is assessed. Emphasis in this paper is on the fault ride-through operation impact on the wind turbines structural loads. Grid faults are typically simulated in power system simulation tools using simplified drive train mechanical model, approach which doesn't allow a thorough investigation of structural loads. This paper presents a two-steps simulation procedure, where two complimentary tools are combined, i.e. power system simulation tool and advanced aeroelastic simulation code. The goal is to investigate the whole integrated wind turbine design and to provide insight both into the structural as well as the electrical design of the wind turbine response during grid faults. The two-step simulation procedure is assessed by means of a simulation example. The effect of a grid fault on the structural part of a typical fixed speed wind turbine, equipped with an induction generator, is assessed.

This paper introduces the power quality issues of wind power installations in a historic perspective, as the development from a few small wind turbines connected directly to the low voltage grid, to the present system with high penetration on the medium voltage distribution grids and two large offshore wind farms connected at transmission level. In this perspective, the power quality issues are divided into local issues particularly related to the voltage quality in the distribution systems and global issues related to the power system control and stability. Power quality characteristics of wind turbines and wind farms are described according to national and international standards, and measurements from wind farms are presented.

The objective of this work is to illustrate the impact of the grid faults on the wind turbine structural loads. Grid faults are typically simulated in detailed power system simulation tools, which by applying simplified mechanical models, are not able to provide a throughout insight on the structural loads caused by sudden disturbances on the grid. On the other hand, structural loads of the wind turbine are typically assessed in advanced aerolastic computer codes, which by applying simplified electrical models do not provide detailed electrical insight. This paper presents a simulation strategy, where the focus is on how to access a proper combination of two complimentary simulations tools, such as the advanced aeroelastic computer code HAWC2 and the detailed power system simulation tool DIgSILENT, in order to provide a whole overview of both the structural and the electrical behaviour of the wind turbine during grid faults. The effect of a grid fault on the wind turbine flexible structure is assessed for a typical fixed speed wind turbine, equipped with an induction generator.

In this paper, it is analysed the coherence between wind speeds located in a horizontal plane corresponding to hub height of wind turbines in a large wind farm. The coherence is calculated through real data from Nysted Offshore Wind Farm. Concretely, the wind speed measured in the 72 Wind Turbines and in 2 of the meteorological masts during 9 months. The results are analysed in the scale of power fluctuations in large offshore wind farms. This analysis shows the needing of a new spectral coherence model. The work presented in this paper has been done in the research Project ”Power Fluctuations from large offshore wind farms” financed by the Danish Transmission System Operator Energinet.dk as PSO 2004 project number 6506. A. Vigueras-Rodr´ıguez is supported by the Spanish Ministerio de Educaci´on y Ciencia through the grant program “Becas FPU” and from the national research project “ENE2006-15422-C02-02.

This paper provides an overview of two technologies for connecting offshore wind power plants (offshore WPPs, OWPPs) to high-voltage direct current (HVDC) networks: voltage source converters (VSCs) and diode rectifiers (DRs). Current grid code requirements for the connection of such power plants are also addressed, and their implications when using such technologies are discussed.

Variability of large scale wind power generation is dependent on several factors: characteristics of installed wind power plants, size of the area where the plants are installed, geographic dispersion within that area and its weather regime(s). Variability can be described by ramps in power generation, i.e. changes from time period to time period. Given enough data points, it can be described with a probability density function. This approach focuses on two dimensions of variability: duration of the ramp and probability distribution. This paper proposes an index based on these two dimensions to enable comparisons and characterizations of variability under different conditions. The index is tested with real, large scale wind power generation data from several countries. Considerations while forming an index are discussed, as well as the main results regarding what the drivers of variability experienced for different data.

A novel methodology for life estimation and performance evaluation of offshore wind farms high voltage AC export cables is presented. The method applies Dynamic Temperature Prediction (DTP) analysis using a Thermo-Electrical Equivalent model (TEE). Furthermore, it is suggested how the cable lifetime might be inferred based on the accumulated ageing effects. Afterwards, a sensitivity analysis of the seabed temperature variations is performed. Finally, a holistic procedure for calculating more accurately the electrical power losses of the cable is presented. Results show that an important increase of the total installed power, or cross-section reduction, can be achieved compared to traditional sizing methods.

This paper deal with the dynamics of an autonomous no storage wind-diesel system, comprising a diesel generator and a controlled wind system with a hypo/hyper synchronous cascade. The objective is to maximize the wind energy penetration rate, by an optimization control system, respecting the quality standard concerning the frequency deviation in the AC local grid. Also, the influence of the diesel drive train on the system’s dynamics performances is discussed.

The purpose of this work is to illustrate the impact of a grid fault on the mechanical loads of a wind turbine. Grid faults generate transients in the generator electromagnetic torque, which are propagated in the wind turbine, stressing its mechanical components. Grid faults are normally simulated in power system simulation tools applying simplified mechanical models of the drive train. This paper presents simulations of the wind turbine load response to grid faults with an advanced aeroelastic computer code (HAWC2). The core of this code is an advanced model for the flexible structure of the wind turbines, taking the flexibility of the tower, blades and other components of the wind turbines into account. The effect of a grid fault on the wind turbine flexible structure is assessed for a typical fixed speed wind turbine, equipped with an induction generator.

Recently proposed as a viable alternative for connecting offshore wind farms (OWFs) to HVDC networks, diode rectifiers have prompted increasing interest from both industry and academia. However, before technical connection requirements for such solutions can be determined, more studies are needed to assess their capabilities to contribute to the secure operation of the networks connected to them.This study assesses the capability of an OWF to provide frequency support to an onshore AC network, when connected through a HVDC link having a diode-rectifier-based (DR-based) offshore terminal and a voltage-source-converter-based (VSC-based) onshore terminal. The primary focus is fast frequency response (FFR), which contributes to the stabilisation of the onshore AC network during the first stage of large frequency excursions by decreasing the (magnitude of the) rate of change of frequency. The kinetic energy stored in the rotating masses of the wind turbines (WTs) is considered as the main source of additional power/energy for such response during onshore underfrequency events. The WTs areoverloaded (i.e. forced to overproduce) to extract some of that stored kinetic energy when providing FFRduring onshore underfrequency events. A semi-aggregated OWF representation is considered in order to examine the dynamics of each grid-forming wind turbine within a string when providing FFR, while achieving reasonable simulation times.Simulation results corroborate that the new connection concept (and corresponding changes in WT control) does not have a significant impact on the capability of OWFs to provide FFR to onshore AC networks. That is OWFs connected to HVDC via DR-based offshore terminals can indeed provide FFR, by means of OWF-level active power controls similar to those developed for VSC-HVDC-connected OWFs, while its grid-forming WTs share the reactive power consumption/production and keep the offshore frequency and voltage within their normal operating ranges.

The power-transfer capability curve is widely used by Offshore Wind Farms (OWFs) planner when designing their grid connection. An improved method for calculating power-transfer capability curves of OWFs cables is presented in this paper. What differentiates this method compared to the traditional approach, is the consideration of the high power variability and low capacity factors of OWFs, instead of assuming continuous nominal conditions. The method is based on an iterative approach, aiming to determine the maximum total installed power of an OWF, that a cable can support in function of its total length (effective length from the Offshore Substation, OSS, to the Onshore Connection Point, OCP); in order to do so, operational constraints such as: voltage swing limit, Surge Impedance Limit (SIL), and thermal limit are taken into account. By means of this strategy, is possible to estimate more accurately and realistically the power limits and binding constraints, hence exploiting the cables’ capacities under particular installation and operating conditions. The translation from rated conditions towards dynamic behaviours, permits the inclusion of more realistic states of the system, for instance, accounting not only for the wind speeds fluctuations, but also the variation of boundary temperatures (seabed), and other thermal parameters which have strong influence over buried cables’ thermal performance. The transmission cables are modelled considering a uniform distribution of their electrical parameters, inductance and capacitance, by means of the attenuation constant and characteristic impedance. Likewise, A Thermo-Electrical Equivalent Model (TEE) is applied for the thermal analysis given its good solution quality-computation time balance. The proposed methodology is applied to a 800 mm2 cable, with the results showing an estimated increase of OWF total installed power of 110% for a total length of 120 km, when compared to the traditional method.

The development of offshore wind farms depends on many technical and economic parameters, which requires an integrated planning approach. Some parameters can be controlled by the wind farm developer, and some are determined by the regulatory authorities. Others can be outside control of both. One aspect of optimizing wind farm development is overplanting, which represents the capacity optimization between installed generation capacity and transmission capacity, which allows for a minor share of energy curtailment to increase the overall value of the farm. Quantifying this value also depends on the regulatory framework, which is analyzed here by comparing United Kingdom and Danish conditions. Using a discounted cash flow model, we find that UK conditions are favorable to overplanting from the developer perspective, where the benefit of using the transmission cable more efficiently supports overplanting. In the Danish case, the private‐economic incentive to overplant is minor due to the constraint of linking subsidies to a certain energy output. On the other hand, when the cost of turbines declines relative to transmission system costs, overplanting tends to become more attractive. This is mostly the case when installing wind farms further offshore compared to projects closer to the coast, which has been exerted more and more exerted in the recent years. In a socioeconomic perspective, overplanting is also a viable method to optimize the wind farm technically and economically, if the cost of the additional turbines is less than the value of the additional generation that can be fed into the grid by the transmission line. This article is categorized under: Wind Power > Economics and Policy Wind Power > Systems and Infrastructure

Offshore HVDC grids are envisaged to render more reliable and economical solutions for transferring more offshore wind energy to onshore power systems. From the reliability point of view, the most promising operational/control strategy of HVDC grids is the dc voltage droop control. This control provides autonomous power-sharing among converters and promotes the frequency support to onshore power systems from other ends of HVDC grids without using fast communication links. The simultaneous use of frequency and dc voltage droops in an HVDC converter can transform frequency deviations into dc voltage changes. Other converters with dc voltage droop can react to the changes by regulating their power flow. Therefore, autonomous frequency support can be established through an HVDC grid. Although this method is reliable, fast, and economical, it gives rise to interactions between different droops, which can result in a reduction in the HVDC power that system operators can expect during frequency events. The present study investigates the effectiveness and challenges of the droop-based (communication less) control for providing frequency support in offshore HVDC grids. The frequency support is described mathematically, and a static transfer function between onshore frequency and HVDC power is derived as a function of the power system and control parameters. The function can be used to compensate for the adverse effect of frequency and dc voltage droop interactions; however, it is challenging in the case where an HVDC grid has more than one onshore ac systems since the transfer function of one ac system will depend on the parameters of other ac systems. A signal-flow model is developed for offshore HVDC grids to quantify the effects of onshore frequency deviations on different electrical parameters of the grid. Using this model the transfer function mentioned before can be obtained more easily. A point-to point and a four-terminal HVDC connections are simulated and communication less control is numerically compared with communication-based control.

In the framework of the FP7 project EERA DTOC: Design Tool for Offshore wind farm Cluster, a new software supporting the planning of offshore wind farms was developed, based on state-of-the-art approaches from large scale wind potential to economic benchmarking. The model portfolio includes WAsP, FUGA, WRF, Net-Op, LCoE model, CorWind, FarmFlow, EeFarm and grid code compliance calculations. The development is done by members from European Energy Research Alliance (EERA) and guided by several industrial partners. A commercial spin-off from the project is the tool ‘Wind & Economy’. The software has been compared and validated to a wide extent. Around 10 wake models have been compared to SCADA data from the Horns Rev 1 offshore wind farm in the North Sea, and the Lillgrund and Rødsand-2 wind farms in the Baltic Sea. The Rødsand-2 wind farm is located nearby the Nysted-1 wind farm, thus an investigation of the wake influence between dual operation twin farms was possible. Furthermore both micro- and mesoscale wake models have been compared to satellite-based wind farm wake data in the North Sea. Regarding the planning of the electrical grid, both inter-array and long-distance cables were modelled by the software and several tests were performed. The calculations include the smoothing effect on produced energy between wind farms located in different regional wind zones and the short time scales relevant for assessing balancing power. The grid code compliance was tested for several cases and the results are useful for wind farm planning of the grid and necessary components and controls.

This study presents models and tools for the assessment of the impact that providing frequency support has on doubly-fed generator (DFIG) wind turbine structural loads and drive train. The focus is on primary frequency support, aiming at quantifying the impact on wind turbines acting as frequency containment reserve and providing inertial response. The sensitivity of wind turbine load indicators—load duration- distribution and maximum load values—to inertial response control actions and different torsional models of drive train is investigated. The analysis is done by co-simulations of an aeroelastic code and electrical models. In this simulation framework, the impact that power system conditions can have on wind turbines, and vice versa the support that wind turbines can offer to the power system can be investigated.

Offshore wind power has a significant development potential, especially in North Europe. The geographical concentration of offshore wind power leads to increased variability and in the case of critical weather conditions it may lead to sudden and considerable loss of production. In this context, the chances of losing several GW of wind power due to critical weather conditions in a very short time period could potentially jeopardize the whole system's reliability and stability. Forecasting such events is not trivial and the results so far are not encouraging. When assessing the impact of the variability for the 2020 Danish power system, one can see that in the worst case, up to 1500 MW of power can be lost in 30 minutes. We present results showing how this issue is partially solved by the new High Wind Storm Controller presented by Siemens in the TWENTIES project.

Recently, offshore renewable technologies have shown a rapid growth with the advancement in technology together with reduction of capital investment per unit size. This movement is further catalyzed by various government policy decisions and new set targets on renewable energy (RE) production. Due to higher wind speed, limited land availability and flexibility of transporting very large structures (with the increased size of a single wind turbine) offshore wind power plants provide commendable benefits over onshore wind plants. At present, only medium voltage ac is used in the offshore wind collection systems. However, the use of medium voltage direct current (MVdc) at the collection level is becoming more attractive due to advantages like lower systems losses, zero reactive power compensation and flexibility of power flow control compared to traditional ac collection systems. This paper intends to review opportunities and key challenges associated with the emerging MVdc technology for offshore wind collection systems. Attention is given to the DC wind turbine concept and selecting a suitable MVdc collection system topology with appropriate control modes.

Using probabilistic forecast information in control algorithms can improve the performance of wind farms during periods of extreme winds. This work presents a wind farm supervisor control concept that uses probabilistic forecast information to ride-through a storm with softer ramps of power. Wind speed forecasts are generated with a statistical approach (i.e. time series models). The supervisor control is based on a set of logical rules that consider point forecasts and predictive densities to ramp-down the power of the wind farm before the storm hits. The potential of this supervisor control is illustrated with data from the Horns Rev 1 wind farm, located in the North Sea. To conclude, an overview of ongoing and future research in the Radar@Sea experiment is given. This experiment aims at improving offshore wind power predictability and controllability through the increased use of meteorological information, and particularly weather radar images.

Restrained generation from wind and solar PV can provide a frequency response that is fast in comparison to conventional generation. However, the reduced inertia in a dispatch with little conventional generation can cause very fast initial drop in the frequency and it may be difficult to mitigate that with wind and solar PV alone. In the paper, we outline a method to compare frequency control strategies in such a system, present typical response characteristics of different options, and simulate some example cases for frequency containment in the Iberian power system. The results are demonstrative and additional analysis is required to draw more robust conclusions and to conclude on the costs and benefits of using wind and PV for frequency response.

Currently, more wind power is expected to enter into the electrical network all over the world. Therefore the power system becomes more vulnerable and dependent on the wind energy production. The European countries with already grid connection requirements for wind power have revised them in the last two years while other countries have started to address these requirements. In all cases the grid codes reflect the penetration of the wind power into the electrical network both at distribution and transmission levels or a future development is prepared with these demands. Issues as power controllability, power quality, fault ride-through and grid support capability during network disturbances are addressed through these demands. The present paper provides a world wide overview of the existing grid codes for wind power.

Emphasis in this paper is on the fault ride-through operation impact on the wind turbines structural loads. Grid faults are typically simulated in power system simulation tools using simplified drive train mechanical model, approach which doesn't allow a thorough investigation of structural loads. This paper presents a two-steps simulation procedure, where two complimentary tools are combined, i.e. power system simulation tool and advanced aeroelastic simulation code. The goal is to investigate the whole integrated wind turbine design and to provide insight both into the structural as well as the electrical design of the wind turbine response during grid faults. The two-step simulation procedure is assessed by means of a simulation example. The effect of a grid fault on the structural part of a typical fixed speed wind turbine, equipped with an induction generator, is assessed.

There are several methods for resonance identification in power systems each of which has its own advantages and disadvantages. In this paper, three methods for resonance identification are discussed; namely: Impedance Scan (ImS), Harmonic Resonance Modal Analysis (HRMA), and the method based on Determinant of the Transfer Function (DTF). Going to the mathematical concept behind each method, and giving the physical meaning of these concepts in power systems, the applicability of using these methods in power systems is investigated. In this way, it is concluded that ImS is suitable for graphical representation of the resonances, HRMA is good if more insight about the resonances is needed, while DTF, which has the advantage of representing the behavior of the system with a single number, can have some limitations that need to be taken into account when used in resonance studies.