Design considerations in power generation from offshore wind farms

The global efforts to reduce carbon emissions from power generation have favoured renewable energy resources such as wind and solar in recent years. The generation of power from the renewable energy resources has become attractive because of various incentives provided by government policies supporting green power. Among the various available renewable energy resources, the power generation from wind has seen tremendous growth in the last decade.

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Aug 24, 2017
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Author(s): R.C. Bansal, K. Musasa, Y. Mishra, K. Gajrani

Abstract

This article discusses various advantages of the upcoming offshore wind technology and associated considerations related to their construction. The conventional configuration of the offshore wind farm is based on the alternative current internal links. With the recent advances of improved commercialised converters, voltage source converters based high voltage direct current link for offshore wind farms is gaining popularity. The planning and construction phases of offshore wind farms, including related environmental issues, are discussed here.

Introduction

Power generation from fossil fuels has been prevalent in the past two centuries because of various reasons, such as higher energy density, relative abundance, cheaper cost and established technology. Coal-fired power plants currently fuel around 41% of global electricity and in some countries, such as South Africa, it can be as high as 93% [1]. However, it is estimated that if the fossil fuels were to be consumed at the present rate, they will not last longer and would be finished by 2050 [2]. Besides, electricity generation from coal is not a favourable choice owing to several environmental concerns. The increasing awareness of global warming and its impact on planet is favouring the use of renewable energy solutions, such as solar, hydro and wind. Among various renewable generation technologies, wind energy has shown tremendous growth in the last decade. Onshore wind technology has matured overtime and has enabled more than 318 GW of wind farms installed globally by 2013 [3].

The offshore wind technology, whereas, is relatively new, and is expected to grow in the coming years as the technology evolves and the cost of integration reduces. Recently, offshore wind energy has been recognised as one of the most promising choices and is categorised as the future key of power generation technology for renewable energy [4]. Almost 5 GW of offshore wind farms are installed globally by 2013, where most of them are in Europe [3–5].

The offshore wind technology is preferred over onshore because of several reasons as listed below:

  1. The wind resource offshore is greater than onshore, enabling more power generation from fewer turbines. In terms of wind speed, the offshore wind park operates potentially at higher wind speed compared to the one constructed onshore. On the averaged point of view, the wind speed on the sea is more than 25% of wind speed on land [5] at the same WTG height, which leads to a higher power production for the offshore system. Investment costs for offshore wind power are much higher than those for onshore installation [3]; although the cost is higher, the greater energy output of offshore turbines can compensate for the higher operational and construction costs. Furthermore, the economies of scale for offshore technology, which cannot be as easily obtained for onshore (physical size of the wind farm can be restricted onshore), makes the offshore wind power technology more attractive.
  2. One of the major obstacles of interconnecting onshore wind farms is the need of long transmission lines to transfer the power to the load centres [6]. As there are plenty of global cities located close to coastlines, especially in Europe and Asia-Pacific, preventing the need for long distance transmission lines.
  3. In a very densely populated coastal region with high property values, the development of offshore wind farm may be preferred over onshore ones. Although it may face public opposition, but carefully designed offshore wind project would anytime be preferred than its counterpart.

Despite several advantages, the integration of offshore wind farms faces few challenges, including high maintenance cost, type of technology such as high voltage alternating current/high voltage direct current (HVAC/HVDC), environmental approvals related to disturbance caused in marine ecosystem, community opposition owing to possible effect on coastal aesthetics and so on. The increase of turbine power capacity and reduction of maintenance costs are crucial for the future success of upcoming offshore wind farms. The size and weight of the offshore wind turbine (WT) components and foundation design are also important considerations and require due diligence during planning and execution phase.

This study discusses few challenges facing offshore wind farm technology and is organised as follows: Generators and Speed Control section describes generator and mode of speed control for offshore wind turbine. Various transmission systems and their voltage or current control mode, including the telecommunication system, are discussed in the Transmission, Control and Communication System section. Layout of Offshore Wind Farm describes the layouts for offshore wind farms. The planning and execution phase related to the construction of the offshore wind farm and related environmental concerns are discussed in Offshore Wind Farm Planning, Execution and Environmental Issues followed by a conclusion in the final section.

Generators and speed control

The turbine-generators used in the offshore wind farms may operate either at fixed speeds or variable speeds. Each of these two speed configurations has its own advantages and disadvantages. The fixed-speed wind farm based on squirrel-cage induction generator (SCIG) is more attractive because of its robust construction, low cost, low maintenance, long life, low power to weight ratio and ease of integration [7]. However, the reactive power demand of a large offshore wind farm based on SCIG may not be satisfied by the grid. Therefore, the deficit reactive power may lead to voltage control problems as well as increased energy losses within the electrical network. To meet the reactive power demand of the wind farm, well-designed reactive compensation is usually required. On the other hand, the main advantage of the variable speed is its ability to extract the maximum active power, which achieves the maximum aerodynamic efficiency over a wide range of wind speeds, thus it results in increased energy capture. It is also able to support the grid voltage through its reactive power controlling capability. However, the complexity of the variable speed WT makes it more costly. The use of doubly fed induction generators for offshore wind farm development and its coordinated control scheme is one of the most current research developments in the field [8].

A new type of generator model named permanent-magnet-excited induction machine (PMIM) was tested for offshore WT [8], which includes both the advantages of the induction machine (low maintenance and stable operation) and the permanent magnet excited synchronous machine (high torque and good efficiency). Its generator is coupled directly to the turbine and the network. The intermediate gearbox and power converter are removed, thus the weight of the offshore WT and the operational losses are reduced. The PMIM operates at a constant speed, no tracking of the maximum wind speed by lack of pitch control leading to lower power coefficient. This disadvantage may be overcome by the use of HVDC transmission system, which links the offshore grid to the onshore AC network. The power converter station (rectifier at the offshore side) can be used to control the frequency of the offshore wind park, thus adjust the speed of the turbines. However, this machine is still on the prototype level and is not yet commercially available.

Transmission, control and communication system

Either of the HVAC or HVDC transmission systems can be adopted for wind farms. Transmitting power over large distances using HVAC requires reactive power compensation devices adding to the cost. Economic factor of HVDC compared to HVAC depend largely on cable length and cable losses [9,10]. Traditionally, HVAC has been preferred because of lower grid interconnection costs; recent advances in HVDC technology has gained widespread interest and opens other avenues for wind farm interconnection. The HVDC link has high power control capability with fast modulation ability and can transmit power over long distance with less voltage drop [11]. The HVDC system which incorporates the line-commutated control (LCC) circuit could be cheaper, compared to the one incorporated with the voltage source control (VSC) configuration [12]. The LCC-HVDC is highly susceptible to AC network disturbances, resulting in converter commutation failures which can temporarily shut down the complete wind farm. LCC uses thyristors, which are line commutating, whereas VSC uses insulated-gate bipolar transistors, which are self-commutating. The use of VSC circuit operating through the pulse width modulation (PWM) has several advantages over the LCC system, such as lower harmonic distortion of the AC-side voltage and fewer auxiliary filters (filters will require PWM but not with multilevel convertors). VSC-based HVDC system is able to independently control both active and reactive power exchanged with the AC grid. Therefore, they can help voltage regulation and are able to operate in weak or even dead AC networks. The major drawback of the VSC-PWM technology is the high switching losses that depend on the switching frequency of the semiconductor devices.

Harmonic compensation is another requirement for large wind farms because of long-length cables. Resonance may occur between the onshore and offshore grids because of the high capacitance of the cable that leads to a distortion in the shape of the voltage and can be mitigated with the application of appropriate filters [13]. As wind resource is uncertain and always variable, the series power flow controller and variable frequency transformer are used to reduce power fluctuations from the offshore wind farm [14]. Static synchronous compensators (STATCOM), series static synchronous compensator (SSSC) and unified power flow controller (UPFC) are used to improve the power system performance. The UPFC is used to mitigate the power fluctuation and also to ensure the stability of interconnected grid. The dynamic equivalent model of the static compensator was derived based on the space vector d–q representation [14], and it has been noted that UPFC has better damping characteristics than the system which operates with STATCOM. However, STATCOM provides a fast transient voltage control and has been widely selected for offshore WT. The SSSC is effectively employed for power flow enhancement in transmission line. Also, it was observed that UPFC combines the advantages of both STATCOM and SSSC [15].

The supervision of offshore wind farms can be monitored from the land through a communication line, which provides monitoring of the system and planning of system maintenance. The performance assessment of communication network based on the transmission control protocol/Internet protocol (TCP/IP) and the multi-protocol label switching were presented in [16,17]. These recorders provided real time recording with high degree of reliability. A test system for a 120 MW medium-scale offshore wind farm using the TCP/IP interface was analysed in [17]. The network congestion scenarios were simulated and network dynamics were assessed in terms of latency. The dynamic records from the wind park, such as small perturbations in grid parameters, fault condition and so on, can also be transferred to the onshore control centre. All the operational planning was coordinated from the control centre. The recording process is as presented in Fig 1.

Fig 1: Communication system diagram [17]

Layout of offshore wind farm

Recent studies have found the use of DC collector and DC transmission line as a promising solution [18]; rather than AC collector and AC transmission line, or AC collector and DC transmission line. The most commonly employed topology for the DC collector is the series-parallel (SP) connection as shown in Fig 2.

Fig 2: SP connection of WTs [18]

For the SP topology, when a sudden fault occurs at one of the WT unit in a series-branch, all the branches are affected. The envisaged solution is to bypass the faulted unit in order to maintain the branch active [18, which imposes an overvoltage to the fault-free WT units connected in the same series-branch. The situation worsens, if the overvoltage exceeds the rated value, leading to the disconnection of the whole series branch. This is resolved by providing auxiliary connection paths, using the matrix interconnected topology [18]. If one WT unit is faulted in the branch, the faulted branch will be disconnected and replaced by the auxiliary branch comprising of similar units as the one of the faulted-branch.

The optimal layout of an offshore wind farm may lead to a considerable saving of the operation, maintenance and cost of the components. The study conducted in [19] concentrates on optimising the offshore wind farm configuration, including the transmission line to shore. The cost and reliability are dependent on a particular configuration. The optimal layout of a wind farm obviously relays on the published standards topologies, such as [19] the one for system collector: radial, single-sided ring, double-sided ring, star and multi-ring; and the one for transmission system: HVAC and HVDC. The adopted optimal combinations can be restricted to a standard configuration. It is essential to optimise the offshore wind farm design in order to fully benefit from the potential cost savings.

In the layouts of offshore wind farms, there are also three groups of electrical system configurations [15]. First, the AC system comprising the WT generator, the AC/AC transformer and the HVAC transmission line; secondly, the AC/DC system that includes the WT generator, the AC/DC converter and the HVDC transmission line; and thirdly, the DC system comprising the WT generator, the DC/DC converter and the HVDC transmission line. The conventional offshore wind farm is constructed using the traditional AC system, which includes a very bulky 50/60 Hz transformer. An alternative solution is to use a DC distribution system, which includes a DC/DC converter. These converters consist of a rectifier, a coupling transformer and an inverter. The advantage of this concept is the reduced size and weight of the 50/60 Hz transformers. Research is being undertaken on the design of the internal DC cables [20] as the collector requires an internal DC link cable with better efficiency.

Offshore wind farm planning, execution and environmental issues

Successful installation of offshore wind farm requires several factors to be considered that includes obtaining permission from a regulatory authority, project planning, preparation, health & safety, and environmental issues, installation methods, transportation, execution, operation & maintenance (O&M) strategies and so on [21]. Offshore wind farm project planning is a complex issue that requires consideration of types of foundation and design, which further depends upon WT size, metoclean and seabed conditions.

An offshore WT as compared to onshore WT requires four additional factors; that is, water depth, wave load, seabed conditions and turbine induced frequencies to be considered for designing foundation. The four most commonly used WT foundations are: (i) Monopile, a steel tube of 4–8 m diameter used for a water depth of 25 m; (ii) Gravity base, usually made of concrete of 15–25 m in diameter and has weight of 1500–4500 tonnes; (iii) Tripod, as the name indicates is a three-leg steel foundation and commonly used for water depth of 15–40 m. Tripod is expensive and takes a long time to install compared to monopile; (iv) Jacket, which is normally square foot, has low weight and generally used for large water depths [22].

Metoclean conditions require the size and direction of waves, needing specialised knowledge of hydrology. The knowledge of seabed conditions (hard/soft subsurface) is critical to determine the depth of the stable foundation. Furthermore, the bending moments and forces from WT get influenced by metoclean conditions which in turn, affect the foundation.

A permit is required from a regulatory authority before a wind farm can be installed and this process varies among different countries. For example, the federal government maintains regulatory authority to grant permission for installing wind farms in USA and the Crown Estate maintains regulatory authority in UK. Permit to install offshore wind farms are always accompanied by health, safety and environmental conditions, to ensure the safety of the people involved in the project and environment. It must be ensured that relevant standards (e.g. ISO 18000 for health and safety and ISO 14000 for environment) are followed for offshore wind farm projects.

As building offshore wind farms requires construction works in marine environments, proper approval from marine authorities is also required. In UK, Food and Environment Protection Act (FEPA) [23] regulates the offshore wind farm construction to protect the marine environment and to minimise the interference with other legitimate uses of the sea.

Although the interference with other legitimate uses of the sea such as access to marine life and fishing lanes can be minimised during construction and operation phase, there are a few disturbances caused to marine ecosystem during construction phase. The increased level of turbidity, the smothering from re-suspended sediments, the traffic and the noise created during construction phase scare the marine life and may force them to relocate temporarily. Although the impact on the social and economical matter such as fishing and transport maritime is very low, the disturbance caused to the marine ecosystem services can be one of the challenges facing future offshore wind farms. Quantifying these effects and reporting them on regular basis would be required by the wind farm owners and regulators certainly play a very important role.

Offshore wind industry is labelled as green industry, and the industry would like to ensure that the marine environment will not be damaged due to commissioning and decommissioning of offshore wind farms. It has to be ensured that there is proper waste management, no oil spills and components are fabricated in an environmentally-friendly manner.

Conclusion

The wind energy technology is identified recently as an effective means producing clean energy. With the emphasis to maximise the output energy from the wind generator plant, or to maximise the efficiency, the energy produced from the offshore wind farms has a better output–input energy ratio than the one installed on land. In this paper, some of the technological prospects of offshore wind power are briefly reviewed, such as the layout of the wind farm, the planning, the impact and also the issues related to transmission systems. It has been noticed that recently much attention is directed on the implementation of internal DC configuration or DC collector. The HVDC system has many advantages compared to the HVAC system. The effect of interconnecting a HVDC transmission line from the offshore wind park to the onshore AC grid and also the performance analysis of DC cables will be part of future prospects.

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Go to the profile of Ramesh Bansal

Ramesh Bansal

Professor and group head (power), University of Pretoria

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