As the UK Government sets its sights on increasing the use of renewable sources, the requirement for offshore wind generation is going to dramatically rise. Not only is the wind farm itself important, but the design of the network and quality of the power are crucial to ensure the energy can be harvested and distributed to its maximum capacity.  Here Dr Norman Macleod, technical marketing director for Alstom Grid, looks at these requirements

There are many policies and agreements in place that will no doubt have an impact on how we generate and distribute energy in the future. The government’s target to cut emissions in half by 2025 will play a large role in the increase of renewable technologies and solutions, making them an essential part of the future energy landscape.  Although this is currently up for debate and the target may be relaxed, it is still demonstrative of the move towards a greener society.

Other contributing factors, which mean the UK will have to increasingly turn to renewable energy sources, include the EU 2020 renewable energy target. This has been put in place to ensure that a fifth of Europe’s energy should come from renewable sources by 2020. 
In addition, an extension to the Kyoto agreement is also currently being considered, which will add to the number of factors driving the UK to increasingly turn to renewable energy sources.

Wind turbines will no doubt play an important role in delivering these ambitious targets. In fact, it is predicted wind generated energy is set to increase from 158 GW as it was in 2009, to 410 GW in 2014. A large proportion of this renewable energy will come from offshore wind generation, as opportunities to site turbines on shore are limited.

A challenge currently being addressed by the industry is to prepare European Grids for the influx of renewable energy. Alstom Grid is involved in an EU funded project with 25 other partners, including leading academic establishments and system operators. Known as the Twenties Transmitting Wind project, together these organisations will address this need for change and make decisions on key issues such as the capabilities that network operators will need to implement.

The Twenties project is aiming to have conclusions by early 2014, and will use real-life, large-scale demonstrations to assess the critical technology required to improve the pan-European transmission.

Alongside collaborative projects such as this, new technologies and solutions are being explored to support the move towards renewables and ensure reliability and performance of the European energy networks.

For offshore wind farms that are categorised as remote – more than 50 km from the coastline – direct current often provides the best solution, delivering both technical and economical advantages. It is for this reason Alstom Grid has developed a specific offshore wind solution, the HVDC MaxSine technology, which combines HVDC systems and Voltage Source Converters (VSC).  The solution utilises chain link, or multi-level technology to deliver the best results.

Within this HVDC MaxSine technology, each link in the convertor consists of a DC capacitor and an IGBT switch.  By using the correct switching sequence, the VSC can generate a voltage waveform at a controlled magnitude and phase angle. This allows the HVDC MaxSine convertor to control the power flow from the wind farm to the onshore network, as well as controlling the reactive power flow on both sides of the HVDC system.

So what are the benefits of this technology? The first is the real and reactive power can be carefully controlled to ensure the network is kept at the desired level.  In addition, the requirement for harmonic filtering is removed, which makes the implementation of the system very straightforward, as no additional technology is needed to ensure power quality is at its best.

Other benefits include the small footprint of the equipment, which is well suited for offshore platforms where space is a premium.  Standard power transformers can be used, and the HVDC MaxSine has the capability to operate down to zero MW power flow.

There are many other aspects of network design and functionality that are also being considered across the industry. There is currently a lack of a DC breaker that can operate at an appropriate voltage level, and this is commonly identified as a hindrance to a reliable network.  Without this, the complete protection plan design is incomplete. As there are no multi-terminal DC grids with wind farm connections in operation yet, no standards for this technology are available.

As part of the Twenties programme, DC breaker specifications are being explored with a  view to delivering the best possible protection and fault management for offshore wind farm networks. Fault event characterisation and development of a DC breaker prototype are the current priorities for the partnership.

At present, a benchmark model is being worked with to establish DC breaker specifications and requirements.  The factors that have been considered include the frequency range of interest (between 300 and 2000 Hz) and how these change as the inductance and capacitance are varied.

While Alstom Grid is working on the network aspect of offshore wind farms, the power division of the company is developing turbine equipment specifically for offshore locations. These turbines require larger diameter rotors and longer blade lengths than their onshore counterparts.  As part of the company’s commitment to a greener future, Alstom is currently working with developers, taking an active role in ensuring solutions are available for round 3 of the Crown Estate project. Through this project, Alstom will be collaborating with Belwind, Belgian wind farm developer, to demonstrate its next generation of 6 MW offshore turbines as part of a 40 MW site in Belgium.

Power quality is also a vital consideration for wind farm operators and network designers.  While some technologies – such as the HVDC MaxSine – do not require additional harmonic filtering, some applications will require this as an additional service.

Frequency range, voltage unbalance and voltage range can all distort the harmonics of the network and will result in unusable power being generated, as well as having detrimental effects on the equipment. This is in turn may result in costly downtime and an increase in operating costs – both very undesirable outcomes.
There are a number of factors that need to be taken into consideration in order to design the right reactive power compensation and harmonic filtering. These are grid code requirements, network dynamics at the interconnection point and wind farm characteristics. It may be appropriate to carry out feasibility studies, load surveys and system modelling before committing to a particular technology.

As we move into the future, it will also become increasingly important for wind farms to consider additional methods to maximise production, availability and reliability. These solutions are already in place and widely used in industrial and manufacturing settings, including asset management, predictive maintenance and corrective maintenance.

With the UK and Europe set to increase its reliance on renewable technologies, in particular wind power, now is the right time for invested parties to be working towards creating an energy network that can be depended upon.  Power quality is clearly of high importance, in equal measure with developing the best solutions for safe and reliable energy. Alstom Grid is proud to be part of a pioneering industry – one which presents a range of challenges, but a world of possibilities.