Transmission Requirements for Off-shore and On-shore Wind Integration

Elliott, Douglas and Bell, Keith and Finney, Stephen (2014) Transmission Requirements for Off-shore and On-shore Wind Integration. Electric Power Research Institute, USA.

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Abstract

a distant wind farm to the GB electricity transmission system in the UK. The two technologies are AC transmission and HVDC transmission with a particular focus on use of cables and the connection of offshore wind farms. A review of the GB electricity transmission system industry codes highlights that particular consideration must be given to the power factor and voltage control requirements at the interface between the wind farm transmission link when such a link is designed. Also the ability of the transmission link and the wind turbines to ride through voltage dips within the GB transmission system is important. A steady state analysis is conducted which studies the reactive power production characteristics of a long high voltage AC transmission cable. This highlights that in order to transmit energy by this means over distances greater than 80km the transmission cable must be supplemented with reactive compensation at the either end of the cable; and for distances greater than 160km also at the cable mid-point. This allows the voltage profile along the cable to be regulated and also ensures sufficient active power transfer capacity is available. Dynamic studies which investigate the response of both AC and HVDC transmission technologies to close-up and distant faults on the GB transmission system are carried out. These highlight in both technology cases that the performance requirements stipulated within the industry codes can be met. It is also demonstrated that when either technology is used to connect a wind farm in the place of a large synchronous generator in the GB system on the exporting side of a stability constrained transmission boundary, each causes slightly different angular displacement responses from the remaining synchronous generators in the system, the effect of AC transmission in the particular scenario studied being detrimental to the system stability, while the effect of HVDC is beneficial. A study of the costs associated with using either transmission technology, both capital and operational, show that there is a cross-over distance where the equivalent annual costs of HVDC transmission to connect a wind farm become lower than those of AC transmission for a similarly sized wind farm. For a 1000MW wind farm, this occurs at approximately 160km, whereas for smaller wind farms the cross-over point is found to be at a larger distance. Within this analysis it is shown that the costs of each technology are sensitive to reactive power compensation costs at the interface between the wind farm transmission link and the GB transmission system, as well as to the transmission capacity and distance. The energy that is lost within the transmission link is determined and it is found that the inclusion of the cost of lost Renewable Obligation Certificates (ROCs) that could be associated with the lost energy would have a big impact on the transmission link operational costs. Discussion of how the offshore transmission owner (OFTO) regime in the UK affects who accounts for the transmission losses highlights that the cost involved fall to the GB transmission system operator, and therefore the wind farm operator would not loose ROCs as a result of transmission losses. However, in the case of ‘generator self-build’ of the connection before sale to an OFTO, the wind farm developer might be more strongly incentivised to choose the connection option that has the lowest capital cost rather than one that has the lowest lifetime cost.

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