High-voltage Direct Current to be a Key Player in the Creation of Smarter Grids
a report by Peter Jones
UK Engineering Manager Grid Systems, ABB
Today’s electricity networks rely mainly on large, centralised generating plants such as fossil fuel or nuclear facilities. This has enabled transmission and distribution network operators to develop successful control strategies that utilise the controllable nature of these plants in matching the more inelastic and uncontrollable demand from consumers. However, the increasing penetration of inherently intermittent renewable energy sources such as wind and solar power demands a change in this strategy. There is now a need for smarter grids that can respond rapidly, reliably and economically to large and unexpected fluctuations in supply. High-voltage direct current (HVDC) transmission technology – in particular, systems that use the latest voltage source converter (VSC) technology – allows rapid and precise control of voltages and power flows. It is reliable and economical, and can be used to flexibly enhance existing alternating current (AC) grids. VSC-based HVDC is also the first choice for transmitting power from large offshore wind farms to AC grids (see Figure 1).
Advantages of High-voltage Direct Current HVDC can contribute towards the creation of smarter grids in a number of ways, including the following.
•
Flexibility – it is well suited to provide fast responses to both operational changes and customer needs.
• Accessibility – it is accessible for all power sources, including renewable and local power generation.
• •
Reliability – it ensures both quality of supply and resilience against the uncertainties inherent in the production of renewable energy.
Economy – it provides efficient operation and bulk energy management, and the flexibility to adapt to new regulations.
In technical terms, HVDC technology supports:
• load flow control; • reactive power support; • voltage control; • power oscillations control; • flicker compensation;
Peter Jones is Engineering Manager, Grid Systems for ABB in the UK. He has been with ABB for 10 years, having joined as Network Solutions Manager, bringing 20 years of electricity industry experience from Scottish Power. He is involved in the engineering design of UK grid code compliant renewables transmission connections, UK transmission system reinforcements, high-voltage direct current (HVDC) interconnectors and UK smart grid technologies. Mr. Jones is a chartered
engineer and has a degree in electrical engineering, an MBA in technology management and is a Fellow of the Institution of Engineering and Technology (FIET).
• voltage quality; •
handling of asymmetrical loads; and • handling of volatile loads.
High-voltage Direct Current – The Smart Transmission Toolkit
For well over a century, high-voltage AC (HVAC) was seen as the natural choice for electrical power transmission. However, the capacitance per unit length makes AC cables impractical for transmitting large amounts of power over distances greater than 50–70km: a significant amount of reactive power is generated, and low-frequency resonances may result in instability. While classic HVDC technology has been commercially available since the mid 1950s, it has mainly been used for point-to-point, high-capacity bulk power transmission links over long distances or for the interconnection of asynchronous grids. Its active components are high-power thyristors. A typical application is China’s 800kV Xiangjiaba–Shanghai link, which provides the capacity to transmit 6,400MW over a distance in excess of 2,000km. The link has an overall energy efficiency of 93%, yet its land use is less than 40% of that needed by conventional technology. At more than 99.5%, availability is also high.
Over the past 13 years ABB has pioneered a new generation of HVDC based on VSC technology – HVDC Light – that uses series-connected power transistors rather than thyristor valves. It is ideal for integrating dispersed, renewable generation, especially wind power, into existing AC grids. It is also used for smart transmission and smart grids due to its great flexibility and adaptability.
The world’s first HVDC link to connect an offshore wind farm with an AC grid is the BorWin1 project. Based on HVDC Light technology, this 200km link connects the BARD Offshore 1 wind farm located off Germany’s North Sea coast to the HVAC grid on the German mainland. This link transmits 400MW at a DC voltage of ±150kV and was ready for service in late 2009 (see Figure 2).
When complete, BARD Offshore 1 will consist of 80 wind generators, each with a capacity of 5MW. These will feed their power into a 36kV AC cable system. This voltage will then be transformed to 155kV AC before reaching the HVDC Light converter station, located on a dedicated platform (see Figure 3). Here the AC is converted to ±150kV DC and fed into two 125km sea cables, which then continue into two 75km land cables, transmitting 400MW power to the land-based converter station at Diele in Germany.
High-voltage Direct Current Light Technology HVDC Light uses insulated-gate bipolar transistors (IGBTs) connected in series to reach the desired voltage level. This technology is used for power transmission, reactive power compensation and for harmonics and flicker compensation.
38 © TOUCH BRIEFINGS 2011
Wind
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