Dynamic loadability of cables in grids connected to offshore wind farms

Abstract

Abstract:

As grids become more and more saturated due to addition of more renewable energy sources in the system, the power demand keeps on increasing. This puts a pressure on the electrical utilities especially the power cables transferring power from one voltage level to another. Therefore, prior investigation into the effect of dynamic loading of such power cables is required before implementing the system in the physical grid.
One such saturated system is the implementation of offshore wind farm cables and the utilities connected to it.
The objective of this study was to check the effect of the dynamic nature of power flow through the connected cable system. In order to get a quantified result the degradation of the cable is measured in terms of the lifetime consumption of the cables under different loading schemes.
The offshore grid connection from Ijmuiden ver area in the North sea operated by Tennet has been identified as a case study for explaining the results. The offshore platforms are connected to the converter station with the help of 3-core 66kV HVAC cables. The connection from the converter station to the onshore grid is with the help of 2 525kV 1-core HVDC cables. The setup has been modelled with the help of two separate models for both the AC and DC cable sross-sections respectively. The models were developed in MATLAB (analytical) and Comsol Multiphysics
5.0 (numerical) to check the effect of dynamic loading of the cables with the help of different current vs. time profiles. The temperature variation with time was checked for the two cable systems.The parameters of soil thermal resistivity and the burial depth were varied from (0.2[W/mK],1200[mm]) to (0.36[W/mK],1500[mm]).
For the AC cable, it was observed that the lifetime consumption increases on changing the soil thermal conductivity and burial depth from 0.36[W/mK],1500[mm] to 0.2[W/mK],1200[mm]. The percentage lifetime consumption is dependent on the time duration for which the cable has been overloaded with a current value greater than the steady state value of 310[A] corresponding to 90◦C. The worst case change between the two cases was observed as going from 0.4439% to 0.5295% per year.Thus, showing that prior investigation is needed before loading the cable with a certain current profile under a given scenario.
For the AC case, on overloading the cable with a current value about 1.2 times the normal value, the maximum percentage lifetime consumption observed was 0.6208%. For the DC cable, the lifetime consumption was measured in terms of the time to failure of the insulation. This parameter is a function of the radial thickness of the insulation from the conductor screen to the insulation. The time to failure increases as one moves away from the conductor screen. Greater insulation deterioration was found to happen with the burial depth and soil thermal conductivity being (1200[mm],0.2[W/mK]) as compared to the case (1500[mm],0.36[W/mK]).A reversal in the
trend of time to failure was observed when the cable was overloaded with a current value of 1.2 times the normal current. The worst case mean value of time to failure observed was 39.68[y] for the normal case.