1 Department of Electrical Engineering, Technical University of Denmark2 Center for Electric Power and Energy, Department of Electrical Engineering, Technical University of Denmark3 DONG Energy A/S
The need for knowledge about interactions between components in a power system is directly related to the complexity of the system. This has been reflected in the creation of several international research groups investigating the topic. Focus has often been put on electromagnetic transients, harmonics and interaction between components and the power system and especially on the interaction and resonance between the transformer,-cables and the circuit breaker. These phenomena are also important in offshore wind farms, where the specific and often complex structure of the collection grid can cause resonances between cables and transformers, which might lead to potentially harmful overvoltages / currents. This paper shows how the topology of the collection grid of offshore wind farms influences the occurrence of electrical resonances in medium- and high-frequency region. Broad band models were applied for the main components involved, such as cables, transformers and capacitors. Cables were modelled as a travelling wave type, taking into account the frequency dependency of all cables’ parameters. Regarding the transformer, a representation has to be chosen including leakage inductances, winding resistances and winding capacitances. Considering the frequency range of the present investigations, up to about 1 MHz, a lumped representation of the transformer characteristics was deemed sufficient. Breakers and capacitors are modelled as ideal components. The chosen wind farm layout includes both simple radials and branched structures, the latter generating additional points of discontinuity, resulting in additional resonance frequencies. The analyses include admittance frequency sweeps and time domain simulations. Admittance frequency sweeps are based on the principle of driving point admittances at selected locations inside the wind farm. Time domain simulations of energized radials were performed (both in case of turbines being connected and not), in order to investigate the shape and magnitude of voltage at the locations. Considerable overvoltages are shown to occur in certain frequency ranges. The results show the influence of specific parameters being varied depending on the farm layout. In particular, cable lengths and transformer broad band characteristics turned out to have significant impact on the results.