Offshore Wind Farm Integration Research Articles

Using High Voltage DC Transmission and Multi-Terminal DC Networks for Offshore Wind Farm Integration

Large offshore wind farms have been planned and some are currently under construction in Europe. Multi-terminal HVDC transmission technology using voltage source converters has been proposed for integrating large offshore wind farms into transmission grids. This article introduces some of the main concepts and challenges associated with the design and development of large offshore DC systems. Different converter topologies and offshore DC grid configurations are discussed. Droop based DC voltage control and power dispatch strategies are described and strategies for safe ride through onshore AC grid fault are explored. System behaviour and protection requirement during faults on the DC grid are highlighted.

Off-shore wind farm development: Present status and challenges

Abstract Offshore wind farm (OWF) is an emerging technology in the wind energy conversion system. These wind resources are abundant, stronger, and are more consistent in terms of their availability than land-based wind resources. As a matter of fact significantly higher energy production is achieved due to larger wind turbine ratings and stronger wind profiles. This paper highlights the present scenario and challenges in development of offshore wind power. The challenges and opportunities that exist in the development stages of an offshore wind farm project, from exploration to erection and installation of wind turbines, construction of platforms and laying of sea cables, up to maintenance and de-commissioning, involving important technical aspects are addressed. An application of high voltage direct current (HVDC) transmission for integration of large scale offshore wind farm with onshore grid is attractive as compared to high voltage alternating current (HVAC) transmission system. To make the offshore wind farm feasible, reliable and secure, the different aspects in its planning, design and operation are also reviewed in this paper.

Research on hybrid multi-terminal high-voltage DC technology for offshore wind farm integration

Multi-terminal high-voltage DC (MTDC) technology is a promising way to transmit large amounts of offshore wind power to the main grids. This paper proposes a hybrid MTDC scheme to integrate several offshore wind farms into the onshore power grids at different locations. A hybrid four-terminal HVDC system comprising two onshore line commutated converters (LCCs) and two voltage source converters (VSCs) connecting an offshore wind farm is constructed in PSCAD/EMTDC. A coordination control scheme based on the VSCs’ AC voltage control and the LCCs’ DC voltage droop control is designed to ensure smooth system operation and proper power sharing between onshore AC grids. The operational characteristics of the system are analyzed. In addition, a black start-up method without any auxiliary power supply for the VSCs is proposed. The transmission scheme is tested through simulations under various conditions, including start-up, wind speed variation, and the disconnection of one VSC or of one LCC.

A framework for data integration of offshore wind farms

Operation and maintenance play an important role in maximizing the yield and minimizing the downtime of wind turbines, especially offshore wind farms where access can be difficult due to harsh weather conditions for long periods. It contributes up to 25–30% to the cost of energy generation. Improved operation and maintenance (O&M) practices are likely to reduce the cost of wind energy and increase safety. In order to optimize the O&M, the importance of data exchange and knowledge sharing within the offshore wind industry must be realized. With more data available, it is possible to make better decisions, and thereby improve the recovery rates and reduce the operational costs. This article describes the development of a framework for data integration to optimize the remote operations of offshore wind farms.

Integration of Offshore Wind Farm Using a Hybrid HVDC Transmission Composed by the PWM Current-Source Converter and Line-Commutated Converter

This paper investigates the feasibility of the application of a hybrid HVDC transmission system for the grid integration of offshore wind farms. The proposed hybrid HVDC consists of a pulse width modulated current source converter (PWM-CSC) and a line-commutated converter (LCC). The PWM-CSC is connected to the offshore wind farm and the LCC connects the onshore grid. The hybrid topology takes advantages from self-commutated converters as well as LCCs. On the one hand, LCC-based HVdc is the most mature technology with the lowest power losses and lowest cost. On the other hand, PWM-CSC has the same features that a voltage source converter for offshore applications, i.e., the ability to operate without an external commutation voltage, reactive power control capability, and a relative small footprint. Moreover, both the PWM-CSC and the LCC are current source converters and hence the coupling can be effortlessly done. The control design for the entire system is presented and verified using numerical simulations. Simulations are performed using PSCAD/EMTDC under different conditions including changes in the wind speed and ac and dc faults.

Coordinated Control for Wind Turbine and VSC-HVDC Transmission to Enhance FRT Capability

The integration of large-scale offshore wind farms into electricity networks without causing stability hazards is one of the future challenges for interconnecting large offshore wind farms into national grids. This paper presents enhanced control strategies for offshore wind farm arrays interconnected via an HVDC link. To improve fault ride- through capabilities of the HVDC link and wind farms, a frequency controller is proposed and the third harmonic injection technique is applied as a means to improve the reliability of the offshore arrays. This new controller can easily coordinate the power flow from different sources (i.e. large-scale wind farms and conventional power stations). The power production from the offshore wind farms will depend upon the type of wind turbines installed. Thus, the performance of wind farms based on DFIG and FRC wint turbines is compared aiming to give a better understanding and to identify areas where control improvements can be introduced to optimise offshore wind power transmission.

Operation and Power Flow Control of Multi-Terminal DC Networks for Grid Integration of Offshore Wind Farms Using Genetic Algorithms

For achieving the European renewable electricity targets, a significant contribution is foreseen to come from offshore wind energy. Considering the large scale of the future planned offshore wind farms and the increasing distances to shore, grid integration through a transnational DC network is desirable for several reasons. This article investigates a nine-node DC grid connecting three northern European countries — namely UK, The Netherlands and Germany. The power-flow control inside the multi-terminal DC grid based on voltage-source converters is achieved through a novel method, called distributed voltage control (DVC). In this method, an optimal power flow (OPF) is solved in order to minimize the transmission losses in the network. The main contribution of the paper is the utilization of a genetic algorithm (GA) to solve the OPF problem while maintaining an N-1 security constraint. After describing main DC network component models, several case studies illustrate the dynamic behavior of the proposed control method.

A novel concept of offshore wind-power collection and transmission system based on cascaded converter topology

SUMMARY This paper presents a new concept for the grid integration of large-scale offshore wind farms. In the offshore wind farm, the wind turbines are integrated into the offshore DC collector grid by a cascaded converter. The high-voltage level for the high-voltage direct current (HVDC) transmission purpose is built by series connections of low voltage modules, without the use of an offshore step-up substation. The output power of each module is controlled by the switching of the related half-bridge, and a modified straightforward voltage balancing strategy is proposed to balance the storage capacitor voltages in the turbines. Supplementary balancing strategies are also presented to assist in balancing the capacitor voltages, when the differences in the wind velocities flowing to the turbines become larger. Because of the stable capacitor voltages, self-commutated voltage source converter (VSC) is adopted as the onshore inverter station connecting the HVDC link and the onshore AC grid. The VSC station not only regulates the DC-bus voltage of the HVDC link, but also provides reactive power compensation to the onshore grid. The time-domain simulation results show that the proposed offshore wind-power collection and transmission system provides a new option for integrating large-scale offshore wind farms. Copyright © 2012 John Wiley & Sons, Ltd.

Offshore wind farm with centralised power conversion and DC interconnection

The study of a hypothetical large offshore wind farm based on centralised power conversion and interconnected to the grid using a multiterminal parallel high voltage direct current (HVDC) link is presented. The 300 MW wind farm consists of 60 squirrel-cage based 5 MW generators connected to a common DC bus using ten voltage source converters (VSCs). The transmission system converters provide variable speed generator control, and therefore individual converters are not required for each wind generator, implying savings in wind farm costs. The paper studies the technical and economical benefits of the proposed topology, as well as the selection of the main components. A detailed analysis of the control circuits for both generator and grid facing converters, with respect to primary control functions, is also given. PSCAD/EMTDC simulation of the proposed concept is presented for realistic wind signals. The results confirm operation at an average optimum coefficient of performance at each respective generator group, as well as satisfactory stability even for severe wind speed changes. The proposed concept reduces the costs associated with DC interconnection and may simplify integration of large offshore wind farms at substantial distances.

The Effect of Large Offshore and Onshore Wind Farms on the Frequency of a National Power Supply System; Simulation Modelled on Germany

The paper describes the simulated effect of large windfarms on the frequency of the interconnected grid system within which they are operating. The study is modelled on Germany. The features of the well-tested and widely used power system simulation package Power System Dynamics (PSD) were expanded so that windfarms with all commonly used wind turbine generators and their control structures can be simulated. In particular, the windfarm interaction with the power grid can be studied. Additionally, the effects of topology and wind conditions at the location of the wind turbines on the output power have been incorporated into the simulation. The result reveals that the stochastic nature of wind power at significant capacity may cause noticeable frequency fluctuation on the whole grid. Without countermeasures, the fluctuating power may use a disproportionately large component of the primary control power reserve of the grid system. The paper describes options for the integration of large offshore and onshore windfarms in the task of primary frequency control.