Ancillary services analysis of an offshore wind farm cluster-technical integration steps of a simulation tool

In this publication, the authors present methodology and example results for the analysis of ancillary services of an offshore wind farm cluster and its electrical power system. Thereby the operation tool Wind Cluster Management System (WCMS) is used as simulation tool to evaluate certain planning scenarios. Emphasis is made on two topics: 1) the integration of high voltage direct current (HVDC) technology to the WCMS, 2) the ancillary service analysis. As examples, voltage source converter based HVDC (VSC-HVDC) and the provision of reserve respectively balancing power are discussed in detail. The analyzed study case considers the Kriegers Flak area while the associated power system connects wind farms to Sweden, Denmark and Germany.EC/FP7/ENERGY-2011-1/ 28279

Offshore Wind Farm-Grid Integration: A Review on Infrastructure, Challenges, and Grid Solutions

Recently, the penetration of renewable energy sources (RESs) into electrical power systems is witnessing a large attention due to their inexhaustibility, environmental benefits, storage capabilities, lower maintenance and stronger economy, etc. Among these RESs, offshore wind power plants (OWPP) are ones of the most widespread power plants that have emerged with regard to being competitive with other energy technologies. However, the application of power electronic converters (PECs), offshore transmission lines and large substation transformers result in considerable power quality (PQ) issues in grid connected OWPP. Moreover, due to the installation of filters for each OWPP, some other challenges such as voltage and frequency stability arise. In this regard, various customs power devices along with integration control methodologies have been implemented to deal with stated issues. Furthermore, for a smooth and reliable operation of the system, each country established various grid codes. Although various mitigation schemes and related standards for OWPP are documented separately, a comprehensive review covering these aspects has not yet addressed in the literature. The objective of this study is to compare and relate prior as well as latest developments on PQ and stability challenges and their solutions. Low voltage ride through (LVRT) schemes and associated grid codes prevalent for the interconnection of OWPP based power grid have been deliberated. In addition, various PQ issues and mitigation options such as FACTS based filters, DFIG based adaptive and conventional control algorithms, ESS based methods and LVRT requirements have been summarized and compared. Finally, recommendations and future trends for PQ improvement are highlighted at the end

Impedance analysis of harmonic resonance in HVDC connected Wind Power Plants

During the last years the development of HVDC connected offshore wind power plants increased. As the first wind farms of this type were commissioned, an unexpected phenomenon occurred. Electrical harmonic resonance in offshore AC grid led to outages of the HVDC transmission system. The thesis introduces the phenomenon and compare different methods of its analysis. The study focuses on harmonic frequencies identification excited through the resonance phenomena between the elements within WPP's inner AC network. The analysis includes observations from three tested topology cases by different methods: frequency sweep and harmonic resonance modal analysis. The comparison is performed for diverse converter models: voltage source based, current source based and nonlinear impedance model obtained by harmonic linearization method. The results of the analysis are verified by the outcome attained in DIgSILENT Power Factory software. The study also includes the stability analysis based on Nyquist criterion and interpreted in Bode diagrams. Furthermore, the result of investigation exposes the clues for possible subsequent implementation of harmonic filters as well as for beneficial control of converters. Feasible measures for resonance mitigation from literature are described and proposed

A review on DC collection grids for offshore wind farms with HVDC transmission system

Abstract: Traditionally, the internal network composition of offshore wind farms consists of alternating current (AC) collection grid; all outputs of wind energy conversion units (WECUs) on a wind farm are aggregated to an AC bus. Each WECU includes: a wind-turbine plus mechanical parts, a generator including electronic controller, and a huge 50-or 60-Hz power transformer. For a DC collection grid, all outputs of WECUs are aggregated to a DC bus; consequently, the transformer in each WECU is replaced by a power converter or rectifier. The converter is more compact and smaller in size compared to the transformer. Thus reducing the size and weight of the WECUs, and also simplifying the wind farm structure. Actually, the use of offshore AC collection grids instead of offshore DC collection grids is mainly motivated by the availability of control and protection devices. However, efficient solutions to control and protect DC grids including HVDC transmission systems have already been addressed. Presently, there are no operational wind farms with DC collection grids, only theoretical and small-scale prototypes are being investigated worldwide. Therefore, a suitable configuration of the DC collection grid, which has been practically verified, is not available yet. This paper discussed some of the main components required for a DC collection grid including: the wind-turbine-generator models, the control and protection methods, the offshore platform structure, and the DC-grid feeder configurations. The key component of a DC collection grid is the power converter; therefore, the paper also reviews some topologies of power converter suitable for DC grid applications

DC Grids : Motivation, Feasibility and Outstanding Issues : Status Report for the European Commission Deliverable : D5.4

Wind energy is already a mainstay of clean power generation in Europe, with over 100GW of capacity installed so far, and another 120GW anticipated by 2020 according to various analysts. Much of this capacity is expected to be installed offshore, as it is a windier and the source is steadier compared to onshore wind energy. Hence, offshore wind has been envisaged as making a critical contribution to Europe’s demand for electrical energy and to minimising the carbon emissions associated with meeting that demand

A comparison of AC and HVDC options for the connection of offshore wind generation in Great Britain

This paper presents a comparison of two forms of cable connection of a distant offshore wind farm to a transmission system: AC and HVDC. The requirements of relevant industry standards in Great Britain (GB) that drive a connection design and, hence, its cost are highlighted along with an analysis of the ways in which AC cable connections might be made to comply while facilitating export of active power. Dynamic studies investigating responses to grid-side short circuit faults show that, in the particular scenarios studied, an AC connection of a wind farm in the place of a large synchronous generator is marginally detrimental while an HVDC connection is beneficial. A comparison of costs shows that the cross-over distance at which HVDC is cheaper than AC for wind farms of different sizes occurs at longer distances than have hitherto commonly been assumed, and AC connections benefit from reactive compensation not only at the point of common coupling and wind farm end but also at the connection mid-point

Loss allocation in a distribution system with distributed generation units

In Denmark, a large part of the electricity is produced by wind turbines and combined heat and power plants (CHPs). Most of them are connected to the network through distribution systems. This paper presents a new algorithm for allocation of the losses in a distribution system with distributed generation. The algorithm is based on a reduced impedance matrix of the network and current injections from loads and production units. With the algorithm, the effect of the covariance between production and consumption can be evaluated. To verify the theoretical results, a model of the distribution system in Brønderslev in Northern Jutland, including measurement data, has been studied