Offshore Substation Research Articles

Nearshore and offshore wind development: Costs and competitive advantage exemplified by nearshore wind in Denmark

Nearshore wind development has been seen as the cost-reducing option that could shrink the cost gap between onshore and offshore development. The cost advantage is linked to more shallow water and shorter connection to shore even avoiding an offshore substation. Public tendering for offshore wind in Denmark has opened up for near-shore wind turbine farms as an alternative for lowering the cost of new offshore wind development. Whether these proposed near-shore locations will manage to significantly lower costs is not clear. The tenders have resulted in bids that are at comparable levels for the nearshore and the further offshore wind farms. We compare the cost drivers and possible cost differentials with preferences for locating wind farms further away from the coast. The primary cost driver is water depth, and in the Danish case water depth is increasing slowly or is not even correlated with the distance from shore. Therefore the willingness to pay for moving turbines away from the coast may be sufficiently high to balance the increased cost. The actual comparison of costs and willingness to pay must be carried out for the specific case with cost characteristics and willingness to pay by the affected population.

A New Magnetic-Linked Converter for Grid Integration of Offshore Wind Turbines Through MVDC Transmission

A medium-voltage ac network is commonly used to interconnect wind turbines to the offshore substation, which can create significant power losses, as the distance between wind turbine and offshore substation can vary from 5 to 9 km. The copper-conductor-based high-voltage dc (HVDC) or high-voltage ac (HVAC) transmission lines are generally used to transmit the electrical power from the offshore substations to the onshore grids. With the recent advancement of the high-temperature superconducting (HTS) cables, it is now possible to replace the traditional HVAC/HVDC transmission lines with a medium-voltage dc (MVDC) cables. A new MVDC converter is proposed in this paper to convert the generator low ac output voltage to MVDC directly. The proposed new technology not only eliminates the requirement of low-frequency heavy and large step-up transformers and line filters, but can also eliminate the voltage conversion stages to allow the entire power transmission networks to use MVDC HTS cables. A compact, lightweight, and efficient high-frequency common magnetic link is used in the proposed converter. The magnetic link can inherently solve the isolation problem. In this paper, a scale-down prototype converter is developed in the laboratory to validate the feasibility of the proposed MVDC converter.

Detailed modelling and simulations of an all‐DC PMSG‐based offshore wind farm

A detailed modelling method and corresponding simulations for an all-DC offshore wind farm equipped with a permanent magnet synchronous generator (PMSG) are elaborately discussed and conducted. This all-DC wind farm is designed in series-connected topology with an offshore DC substation and a voltage-sourced converter−high-voltage direct current (VSC-HVDC) transmission system. Some major components including wind turbine, synchronous generator, VSC, and isolated full bridge converter are all modelled based on the complete electrical models and engineering parameter settings. Containing the start process of the system, the simulations under various wind speed conditions and the specific faults analysis under different situations are carried out in the MATLAB/SimulinkTM environment. Simulation results reveal the comprehensive operating characteristics of this all-DC offshore wind farm and its system response to faults.

Electrical Cable Optimization in Offshore Wind Farms—A Review

A state-of-the-art review of the optimization of electrical cables in offshore wind farms (OWFs) is presented in this paper. One of the main contributions of this paper is to propose a general classification of this problem, framed in the general context of the OWFs design and optimization (OWiFDO). The classification encompasses two complementary aspects. First, the optimum sizing of electrical cables, with the three main approaches used nowadays, static-rated sizing, dynamic load cycle profile, and dynamic full time series, is conceptually analyzed and compared. The latest techniques and advances are described, along with the presentation of potential research areas not thoroughly addressed today, such as dynamic cable rating, and cable’s lifetime estimation under time-varying conditions. Second, the network optimization of large OWFs is thoroughly presented, dividing the problem with a bottom–top approach: cable layout of the collection system, wind turbines (WTs) allocation to offshore substations (OSSs), number and location of OSSs, and interconnection between OSSs and onshore connection points (OCPs). A comparison among different methods is performed, taking into consideration the main engineering constraints. Global optimization, specifically, binary programming (BIP) or mixed-integer linear programming (MILP), is envisaged as the best way to tackle this topic. The full combinatorial problem is found to be better addressed following a top–bottom approach, combining exact formulations with high-level heuristics, or holistically with evolutionary algorithms.

Offshore pile load tests in Chalk to support the design of jacket foundations

Iberdrola developed the 350 MW Wikinger offshore windfarm (OWF) in the German Baltic Sea where ground conditions are mainly dominated by deposits of the Upper Quaternary series and post-glacial deposits overlying Cretaceous Upper Chalk (low-density, grade A1/A2) of Maastrichtian age and at some locations with Paleogene Limestone (high-density to very high density, grade A1/A2). Current pile design methods for open tubular steel piles in Chalk highlighted significant design uncertainties for open tubular piles that would lead to unnecessarily conservative pile dimensions of 70 m embedded length or longer for the 70 four-legged jacket wind turbine support structures and the six-legged offshore substation. For mainly axially loaded piles, German legislation requires the execution of dynamic pile load tests on production piles at a minimum of 10% of wind turbine generator locations immediately following pile installation. This practice offers no opportunity for pile design optimisation. If positive, the pile tests can confirm design assumptions, but risk costly delays in OWF commissioning if the dynamically ‘measured’ capacities fall below design requirements. After careful consideration of costs and benefits, and with the clear advantages of eliminating ambiguity of test results, it was decided that offshore tests at the Wikinger OWF offered the best return on investment regarding foundation design optimisation, assurance and certification. To address these concerns, in advance of final design, offshore dynamic and fully autonomous static pile load tests were commissioned. These were completed 10 weeks after pile driving on 50%-downscaled piles with penetrations of up to 31 m in water depths of around 40 m. This paper provides an overview of the test results and how they were applied in detailed design and validated during construction through dynamic pile tests. Lessons learned are shared and conclusions are drawn regarding on-site offshore pile test results and their transfer into the foundation design and planning process.

Optimization of Electric System for Offshore Wind Farm Based on Lightweight Substation

The electrical system of offshore wind farms is the key to transmit the power from wind turbines to onshore power grid. With the increase of the offshore wind farms capacity and the distance from shore, the traditional centralized offshore substations is facing the challenges of capacity and construction. To solve the above problems, we propose the lightweight substation and an improved k-medoids clustering method to find lightweight substation locations, in addition, we establish the life cycle cost model of electrical system. The example of a real offshore wind farm is taken as an optimization object. The final result shows that the lightweight substation can decrease the economic costs of the offshore wind farm electrical system.

Next generation polymeric high voltage direct current cables—A quantum leap needed?

The attention given to high voltage direct current (HVDC) cables appears to be at an all-time high. In the July/August 2017 issue of IEEE Electrical Insulation Magazine, the focus was on different aspects of the design and testing of HVDC cable systems. There are several reasons for this, one of the most important probably being the worldwide attention on harvesting renewable energy at a scale never seen before. The prices of building large offshore windfarms in Europe as reflected in a recent normalized cost of electricity have unexpectedly dropped in the last year, as illustrated in Figure 1, which will further accelerate the development of HVDC cables to link offshore and onshore substations. The winning bid in 2016 for the Borssele II, 700-MW windfarm in the Netherlands was as low as 54.5€ ($61)/MWh [1].

Overview of offshore wind farm configurations

Offshore wind energy has been attracting great attention. Compared with onshore wind power systems, offshore wind power applications present significantly greater economic challenges mainly due to the required bulky and costly offshore substation. To lower the cost of offshore wind power systems, various configurations are proposed in both industry and academia. The present work investigates existing offshore wind farm configurations.

Optimizing wind farm cable routing considering power losses

Wind energy is the fastest growing source of renewable energy, but as wind farms are getting larger and more remotely located, installation and infrastructure costs are rising. It is estimated that the expenses for electrical infrastructures account for 15–30% of the overall initial costs, hence it is important to optimize their design. This paper focuses on offshore inter-array cable routing optimization. The routing should connect all turbines to one (or more) offshore substation(s) while respecting cable capacities, no-cross restrictions, connection-limits at the substation, and obstacles at the site. The objective is to minimize both the capital that must be spent immediately in cable and installation costs, and the future reduced revenues due to power losses. We present a Mixed-Integer Linear Programming approach to optimize the routing using both exact and math-heuristic methods. In the power losses computation, wind scenarios are handled efficiently as part of the preprocessing, resulting in a model of only slightly larger size. A library of real-life instances is introduced and made publicly available for benchmarking. Computational results on this testbed show the viability of our methods, proving that savings in the order of millions of Euro can be achieved.

Hierarchical Optimization for the Double-Sided Ring Structure of the Collector System Planning of Large Offshore Wind Farms

High reliability double-sided ring collector systems have been applied in practice for long distance and off the coast large-capacity offshore wind farms. To reduce investment and improve reliability, it is important to optimize and evaluate the double-sided ring collector systems. In this paper, a capital cost optimization model is developed to solve the mutual coupling effects among the system structure, power flow, and short circuit current in the collector system constraints. Based on the characteristics of the offshore electrical collector systems, the optimization model is divided into the offshore substation layer, the wind turbine layer, and the submarine cable layer. The fuzzy clustering algorithm, single parent genetic algorithm, and multiple traveling salesman solution technique are integrated and applied to solve the hierarchical optimization model. Finally, the proposed model and the optimization algorithms are tested in a real, large-scale offshore wind farm. The simulation results are compared with the radial collector systems from an economics and reliability point of view. The simulation results show that the double-sided ring design will achieve more profit with higher reliability in the long run. The results can also provide a benchmark for the collector system design of large-scale offshore wind farms.

Optimal Design for Offshore Wind Farm considering Inner Grid Layout and Offshore Substation Location

This paper introduces an efficient methodology to design optimally cable layout of inner grid as well as location of offshore substation. The methodology is composed of two optimizers: one is inner grid layout optimizer, which consists of several algorithms such as the k-clustering-based genetic algorithm, the minimum spanning tree, and the cable selection; the other one is offshore substation location optimizer based on the pattern search method. While performing iteratively two optimizers, alternatives for cable layout and candidates for location of offshore substation are generated and evaluated in the economic and reliability aspects, respectively. Optimal layout of offshore wind farm is determined so as to minimize the total cost which is a sum of construction, power loss, maintenance, and reliability costs.

Optimization and Reliability Evaluation of an Offshore Wind Farm Architecture

This paper presents an original approach aiming to obtain the optimum configuration of an offshore wind farm (OWF). Thanks to cost models, we take into account the costs off all the parts of the electrical network. The optimization platform, based on a genetic algorithm, also allows us to evaluate the reliability of an OWF. This approach is used to compare the topologies obtained in different cases on a real OWF, the “Banc de Guerande.” The selected design of the OWF is compared to the results obtained using different methods: cost optimization, or both cost and reliability optimization. The optimization results show that the ring topologies give interesting results if the total cost including the expected energy not supplied during the lifetime of the OWF are taken into account. The number of offshore substations and their positions are also considered in the optimization: we show that the introduction of more substations in order to obtain better performances is possible at a reasonable cost. The presented optimization tool can help to design such farms taking into account several constraints.

A Novel Multiterminal VSC-HVdc Transmission Topology for Offshore Wind Farms

Multiterminal voltage source converter (VSC) topologies of high voltage direct current (HVdc) transmission systems (M-HVdc) for the integrating large offshore wind farms are scrutinized. A novel two wind farm with one onshore substation (2WF-1SS) topology is proposed. M-HVdc system standards are defined along with control schemes. HVdc switch gear and HVdc-link topologies are reviewed, considering the need of HVdc circuit breakers, dc cables, and extra offshore substations. Numerous M-HVdc-link topologies are analyzed and compared based on certain techno-economic criteria: length, number and ratings of HVdc circuits, cost, flexibility, stability, number of HVdc circuit breakers, and offshore substations. Operation of various topologies with respect to the designed control system is analyzed and compared through PSCAD/EMTDC simulations considering two different tests: permanent VSC disconnection and a dc line-to-line fault. Results show that 2WF-1SS is a promising topology for future multiterminal HVdc grids.

A Medium-Frequency Transformer-Based Wind Energy Conversion System Used for Current-Source Converter-Based Offshore Wind Farm

Offshore wind farms with series-interconnected structures are promising configurations because bulky and costly offshore substations can be eliminated. In this paper, a medium-frequency transformer (MFT)-based wind energy conversion system is proposed for such wind farms based on current source converters. The presented configuration consists of a medium-voltage permanent magnet synchronous generator that is connected to a low-cost passive rectifier, an MFT-based cascaded converter, and an onshore current source inverter. Apart from fulfilling traditional control objectives (maximum power point tracking, dc-link current control, and reactive power regulation), this study endeavors to ensure evenly distributed power and voltage sharing among the constituent modules given the cascaded structure of the MFT-based converter. In addition, this paper thoroughly discusses the characteristic of decoupling between the voltage/power balancing of the modular converter and the other control objectives. Finally, both simulation and experimental results are provided to reflect the performance of the proposed system.

Combined optimization for offshore wind turbine micro siting

In order to minimize the wake loss, wind turbines (WT) should be separated with large intervening spaces. However, this will incur an increase in the capital expenditure on electrical systems and even in the operation and maintenance costs. In order to realize a cost-effective wind farm, an integrated optimization method in which the positions of the WTs and offshore substations (OS) and the cable connection configuration are optimized simultaneously is proposed in this paper. Since the optimization variables are both continuous and discrete, the mixed integer particle swarm optimization (MIPSO) algorithm is adopted to minimize the levelized production cost (LPC) of the wind farm. Simulation results are given for validating the proposed approach and comparison is made with results obtained using other methods. It is found that the proposed method can reduce the levelized production cost (LPC) by 5.00% and increase the energy yields by 3.82% compared with the Norwegian centre for offshore wind energy (NORCOWE) reference wind farm layout. This is better than the traditional method which only achieves a 1.45% LPC reduction although it increases the energy yields by 3.95%.

Overall Optimization for Offshore Wind Farm Electrical System

AbstractBased on particle swarm optimization (PSO), an optimization platform for offshore wind farm electrical system (OWFES) is proposed in this paper, where the main components of an offshore wind farm and key technical constraints are considered as input parameters. The offshore wind farm electrical system is optimized in accordance with initial investment by considering three aspects: the number and siting of offshore substations (OS), the cable connection layout of both collection system (CS) and transmission system (TS) as well as the selection of electrical components in terms of voltage level and capacity. Because hundreds of optimization variables, continuous or discrete, are involved in the problem, a mix integer PSO (MIPSO) is required to obtain the solution. The fuzzy C‐means clustering (FCM) algorithm is used to partition the wind farm into several sub regions. The collection system layout in each sub region as well as the connection scheme between offshore substations are optimized by an adaptive PSO‐minimum spanning tree algorithm (APSO‐MST) which has been proposed in a previous work. The simulation results show that the proposed optimization platform can find an optimized layout that save 3.01% total cost compared with the industrial layout, and can be a useful tool for OWFES design and evaluation. Copyright © 2016 John Wiley & Sons, Ltd.

Life cycle assessment and net energy analysis of offshore wind power systems

This study attempted to evaluate the environmental impact and energy benefit of offshore wind power systems using life cycle assessment (LCA) and net energy analysis. The environmental impact of offshore wind power systems is based primarily on ferrous metal, which is used to install the foundations, towers, and nacelles. The impact categories with the greatest relevance were fossil fuels and respiratory inorganics. This study assumed that the life cycle of an offshore wind power system has four stages (production, installation, operation and maintenance, and end-of-life). Two scenarios were examined in this study. The major difference between the scenarios was that Scenario 2 included an offshore substation. The overall environmental impact in Scenario 2 was higher than that in Scenario 1 by approximately 10%. The net energy analysis in this study included the evaluations of cumulative energy demand (CED), energy return on investment (EROI), and energy payback time (EPT). For Scenarios 1 and 2, CED was 0.192 and 0.216 MJ/kWh, EROI was 18.7 and 16.7, and EPT was 12.8 and 14.4 months, respectively. Moreover, when the recycling of waste materials was considered, each scenario produced a 25% lower environmental impact, 30% lower energy requirement, and 4 months lower EPT.

Decision variables for floating offshore wind farms based on life-cycle cost: The case study of Galicia (North-West of Spain)

Despite the fact that offshore wind energy will tend towards the floating technologies their economic impact has not been practically evaluated yet. In this context, the aim of this article is to characterize the possible economic alternatives which can be considered in the life-cycle of floating offshore wind farms based on their life-cycle costs. Several groups of variables have been considered for determining these alternatives: wind turbines, floating platforms, mooring system, anchoring process, kind of seabed, electric system, installation, shipyard and maintenance. The method proposed has been applied to the case of the North-West coast of Spain (Galicia region). For this case test three different platforms have been analyzed: Tensioned Leg, Spar and Semisubmersible platforms. Results, which are presented by using maps of the area, conclude that offshore substation, string electric configuration and “tree fall” decommissioning method are the best options for minimizing the costs. In addition, the installation method will be conditioned by the type of floating platform and mooring material, whereas the disposition and anchor decisions are dependent on the geographic location. Results show how the methodology proposed is useful to characterize economically the life-cycle of a floating offshore wind farm.

Optimisation for offshore wind farm cable connection layout using adaptive particle swarm optimisation minimum spanning tree method

The wind farm layout optimisation problem is similar to the classic mathematical problem of finding the minimum spanning tree (MST) of a weighted undirected graph. Due to the cable current-carrying capacity limitation, the cable sectional area should be carefully selected to meet the system operational requirement and this constraint should be considered during the MST formulation process. Hence, traditional MST algorithm cannot ensure a minimal cable investment layout. In this study, a new method to optimise the offshore wind farm cable connection layout is presented. The algorithm is formulated based on the concept of MST and further improved by adaptive particle swarm optimisation algorithm. Since the location of the offshore substation (OS) has a significant impact on both the layout formulation and total cost of cables, the optimised location of OS is expected to be found together with the optimised cable connection layout. The proposed method is compared with the MST and dynamic MST methods and simulation results show the effectiveness of the proposed method.

Offshore substation workers' exposure to harmful factors - Actions minimizing risk of hazards

The current development of electric power industry in Poland, especially in the field of renewable energy sources, including wind power, brings about the need to introduce legislation on new work environment. The development of occupational safety and health (OSH) regulations that must be met by new workplaces, such as offshore substations becomes necessary in view of the construction of modern offshore wind power plants - offshore wind farms. Staying on offshore substation is associated with an increased exposure to harmful health factors: physical, chemical, biological and psychophysical. The main sources of health risks on offshore substations are: temperature, electromagnetic field, noise from operating wind turbines, direct and alternating current, chemicals, Legionella bacteria and social isolation of people. The aim of this article is to draw attention to the problem of offshore substation workers' exposure to harmful factors and to present methods of preventing and reducing the risk-related adverse health effects. In this paper, there are identified and described risks occurring on offshore substations (fire, explosion, lightning, accidents at work). Some examples of the means and the methods for reducing the negative impact of exposure on the human health are presented and discussed. The article also highlights the need to develop appropriate laws and health and safety regulations concerning the new working environment at the offshore substations. The review of researches and international standards shows that some of them can be introduced into the Polish labor market.