Offshore Wind Energy Generation

Control, Protection, and Integration to Electrical Systems

Olimpo Anaya-Lara, David Campos-Gaona, Edgar Moreno-Goytia, Grain Adam

• Publisher: Wiley
• Year: 2014
• ISBN:
  9781118701713
• Edition: 1
• Topic: Tecnologia e ingegneria
• Subtopic: Risorse energetiche

The offshore wind sector's trend towards larger turbines, bigger wind farm projects and greater distance to shore has a critical impact on grid connection requirements for offshore wind power plants. This important reference sets out the fundamentals and latest innovations in electrical systems and control strategies deployed in offshore electricity grids for wind power integration.

Includes:

• All current and emerging technologies for offshore wind integration and trends in energy storage systems, fault limiters, superconducting cables and gas-insulated transformers
• Protection of offshore wind farms illustrating numerous system integration and protection challenges through case studies
• Modelling of doubly-fed induction generators (DFIG) and full-converter wind turbines structures together with an explanation of the smart grid concept in the context of wind farms
• Comprehensive material on power electronic equipment employed in wind turbines with emphasis on enabling technologies (HVDC, STATCOM) to facilitate the connection and compensation of large-scale onshore and offshore wind farms
• Worked examples and case studies to help understand the dynamic interaction between HVDC links and offshore wind generation
• Concise description of the voltage source converter topologies, control and operation for offshore wind farm applications
• Companion website containing simulation models of the cases discussed throughout

Equipping electrical engineers for the engineering challenges in utility-scale offshore wind farms, this is an essential resource for power system and connection code designers and pratitioners dealing with integation of wind generation and the modelling and control of wind turbines. It will also provide high-level support to academic researchers and advanced students in power and renewable energy as well as technical and research staff in transmission and distribution system operators and in wind turbine and electrical equipment manufacturers.

1
Offshore Wind Energy Systems

1.1 Background

With construction restrictions inhibiting the deployment of wind turbines onshore, offshore installations are more attractive (e.g. in the UK) (The Crown State, 2011). By mid-2012, offshore wind power installed globally was 4620 MW, representing about 2% of the total installed wind power capacity. Over 90% of the offshore wind turbines currently installed across the globe are situated in the North, Baltic and Irish Seas, along with the English Channel. Most of the rest is in two demonstration projects off China's coast. According to the more ambitious projections, a total of 80 GW of offshore wind power could be installed worldwide by 2020, with three quarters of this in Europe (GWEC, 2013).

All current offshore wind installations are relatively close to shore, using well-known onshore wind turbine technology. However, new offshore wind sites located far from shore have been identified, with clusters of wind farms appearing at favourable locations for wind power extraction, like in the UK Dogger Bank and German Bight (Figure 1.1) (European Union, 2011). The depths of the waters at these sites are in excess of 30 m.

Figure 1.1 Europe's offshore wind farms in operation, construction and planning (Source: www.4coffshore.com/offshorewind).

1.2 Typical Subsystems

The typical subsystems in an offshore wind farm are shown in Figure 1.2. At first glance, it comprises the same elements of an onshore wind farm. However, the environment in which a turbine operates allows a distinction to be made. Considering that the nature of the sea state will act to prohibit accessibility of wind turbines for repair, there is a greater need for offshore wind turbines to be reliable and not require regular repair. This requirement means that the designs and controllers of offshore wind turbines differ from those seen with onshore wind turbines. This is to ensure that performance is maximised whilst minimising cost (German Energy Agency, 2010).

Figure 1.2 Subsystems of an offshore wind farm installation (Anaya-Lara et al., 2013).

In the offshore environment, loads are induced by wind, waves, sea currents, and in some cases, floating ice (Figure 1.3), introducing new and difficult challenges for offshore wind turbine design and analysis. Accurate estimation and proper combination of these loads are essential to the turbine and associated controllers design process. Offshore wind turbines have different foundations to onshore wind turbines. The foundations are subjected to hydrodynamic loads. This hydrodynamic loading will inevitably exhibit some form of coupling to the aerodynamic loading seen by the rotor, nacelle and tower. This is an additional problem that must be considered when designing offshore wind turbines. Ideally, the total system composed of rotor/nacelle, tower, substructure and foundation should be analysed using an integrated model (Nielsen, 2006). Development of novel wind turbine concepts optimised for operation in rough offshore conditions along with better O&M strategies is crucial. In addition, turbine control philosophy must be consistent and address the turbine as a whole dynamic element, bearing in mind trade-offs in terms of mechanical performance and power output efficiency (Anaya-Lara et al., 2013).

Figure 1.3 Impacts on a bottom-fixed wind turbine (Fischer, 2006).

At the wind farm level, the array layout and electrical collectors must be designed on a site-specific basis to achieve a good balance between electrical losses and wake effects. For power system studies, it is typical to represent the wind farm by an aggregated machine model (and controller). However, more detailed wind farm representations are required to take full advantage of control capabilities, exploring further coordinated turbine control and operation to achieve a better array design. Full exploitation of the great potential offered by offshore wind farms will require the development of reliable and cost-effective offshore grids for collection of power, and its transmission and connection to the onshore network whilst complying with the grid codes. It is anticipated that power electronic equipment (e.g. HVDC and FACTs), and their enhanced control features, will be fundamental in addressing these objectives.

1.3 Wind Turbine Technology

1.3.1 Basics

Wind turbines produce electricity by using the power of the wind to drive an electrical generator (Fox et al., 2007; Anaya-Lara et al., 2009). Wind passes over the blades generating lift and exerting a turning force. The rotating blades turn a shaft that goes into a gearbox, which increases the rotational speed to that which is appropriate for the generator. The generator uses magnetic fields to convert the rotational energy into electrical energy. The power output goes to a transformer, which steps up the generator terminal voltage to the appropriate voltage level for the power collection system.

A wind turbine extracts kinetic energy from the swept area of the blades (Figure 1.4).

Figure 1.4 Horizontal-axis wind turbine.

The power in the airflow is given by (Burton et al., 2001; Manwell et al., 2002):

(1.1)

where ρ is the air density, A is the swept area of the rotor in m², and

is the upwind free wind speed in m/s. The power transferred to the wind turbine rotor is reduced by the power coefficient, C_p:

(1.2)

A maximum value of C_p is defined by the Betz limit, which states that a turbine can never extract more than 59.3% of the power from an air stream. In practice, wind turbine rotors have maximum C_p values in the range 25–45%. It is also conventional to define a tip-speed ratio, λ, as

(1.3)

where ω is the rotational speed of the rotor and R is the radius to ti...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• About the Authors
• Acronyms and Symbols
• Chapter 1: Offshore Wind Energy Systems
• Chapter 2: DFIG Wind Turbine
• Chapter 3: Fully-Rated Converter Wind Turbine (FRC-WT)
• Chapter 4: Offshore Wind Farm Electrical Systems
• Chapter 5: Grid Integration of Offshore Wind Farms – Case Studies
• Chapter 6: Offshore Wind Farm Protection
• Chapter 7: Emerging Technologies for Offshore Wind Integration
• A: Voltage Source Converter Topologies
• B: Worked-out Examples
• Index
• End User License Agreement