Transmission capacity management of subsea cables for the grid connection of offshore wind farms

摘要

TenneT is responsible for connecting offshore wind farms (OWF) in the German North Sea to the onshore transmission grid. Wind farms with a low amount of installed power which are near to the shore are connected by HVAC connections. For larger OWFs and greater distances from the coast HVDC links are used. For the subsea cables special permit requirements do exist when crossing the nature reserve Wadden Sea. The most important restriction is the 2K-criterion. It specifies that the temperature of the sea ground must not be heated up by more than 2 K compared to the sea ground which is not influenced by the cable [1]. Hereby the fluctuating power infeed of the wind farms connected to the cable, as well as in case of AC cables the capacitive charging current, etc. have to be taken into account. Hence, the emitted thermal loss energy and therefore the transmission capacity are restricted below the technical limit of the cables. As an important damage reduction concept against forced outages of grid connection system (GCS) and as interim solutions during periods of system unavailability TenneT is planning to provide transmission reserves by using parallel cables. Therefore a method was developed to guarantee that the 2K-criterion is fulfilled and simultaneously as much energy is transmitted to the shore as possible. As only wind farms are connected to the cables, the load of the system is fluctuating due to the available wind. Further the thermal system (cable and surrounding soil) has a very large time constant which leads to the problem that conventional controllers (e.g. PI-controller) cannot be used for dynamic power rating of the HVDC link. Instead of this TenneT developed a predictive feed forward approach based on an optimization algorithm using an evolutionary strategy [5, 6]. With this optimization method it is possible to transmit more energy by using time periods with low wind conditions. Therefore the outage costs are minimized.