Transmission Requirements for Off-shore and On-shore Wind Integration : A study of the merits of AC and HVDC transmission connection options

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Elliott, Douglas and Bell, Keith and Finney, Stephen (2013) Transmission Requirements for Off-shore and On-shore Wind Integration : A study of the merits of AC and HVDC transmission connection options. Electric Power Research Institute, USA.

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Abstract

This report presents a study of two transmission technologies, AC and HVDC, that could be employed to connect a distant wind farm to the Great Britain (GB) electricity transmission system. A review of the GB electricity transmission system industry codes makes it clear that particular consideration must be given in the design stage to the power factor and voltage control requirements at the interface between the wind farm and the transmission system. The ability of the transmission link and the wind turbines to ride through voltage dips within the GB transmission system is also important. A steady-state analysis was conducted that studied the reactive power production characteristics of a long high-voltage AC transmission cable. 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. At distances greater than 160km, it must also be supplemented at the cable mid-point. This allows the voltage profile along the cable to be regulated and ensures the availability of sufficient active power transfer capacity. Dynamic studies that investigate the response of both AC and HVDC transmission technologies to close-up and distant faults on the GB transmission system were also carried out. When either technology is used to connect a wind farm instead of a large synchronous generator to 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. In the particular scenario that was studied, AC transmission had a detrimental effect on the system stability, while HVDC had a beneficial effect. A study of the costs associated with using either transmission technology, both capital and operational, showed 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 longer. Costs are sensitive to reactive power compensation costs at the interface between wind farm and transmission system, as well as to the transmission capacity and distance. The energy that is lost within the transmission link was determined. The cost of the lost Renewable Obligation Certificates (ROCs) associated with these losses would have a big impact on operational costs. Under the current UK offshore transmission owner (OFTO) regime, these costs would fall to the GB transmission system operator, and the wind farm operator would not loose ROCs. However, in the case of ‘generator self-build’ of the connection before sale to an OFTO, the wind farm developer might be more strongly incentivized to choose the connection option that has the lowest capital cost rather than one that has the lowest lifetime cost.

ORCID iDs

Elliott, Douglas, Bell, Keith ORCID logoORCID: https://orcid.org/0000-0001-9612-7345 and Finney, Stephen ORCID logoORCID: https://orcid.org/0000-0001-5039-3533;
CORE (COnnecting REpositories)