This thesis presents the development of a frequency-dependent model for metal oxide surge arresters (MOSAs) to better understand their behaviour under temporary overvoltages with a high harmonic content. This topic has gained importance due to the increasing use of long AC cables in connecting offshore wind farms to the transmission grid, which introduces a high capacitance to the grid and raises the risk of harmonic resonances. These resonances can lead to sustained temporary overvoltages, making accurate arrester modelling essential for selecting appropriate surge arresters to withstand these conditions.

Surge arresters are critical components for protecting high-voltage equipment from overvoltage events. The main objective of this work was to investigate the response of the arrester under both AC and DC voltages at a variety of frequencies, with particular focus on the low-current region of the characteristic voltage-current (V-I) curve.

To achieve this, High-voltage AC and DC tests were performed on a surge arrester block. The measured data were used to extract the resistive and capacitive components of the current, as well as to calculate the power losses. These results formed the basis for developing a four-branch nonlinear model. Curve-fitting techniques and various algorithms in MATLAB simulations were used to determine the model parameters, which were then validated against the experimental data. The model demonstrated good accuracy for peak voltages up to 9 kV and showed clear frequency-dependent behaviour, particularly in the 50 to 500 Hz range.

Overall, the final model aligned well with the RMS values of total current as well as the average power losses, with only minor deviations from the measured results. This work contributes to improving surge arrester modelling and lays the groundwork for further developments in frequency-dependent modelling within simulation tools such as ATP-EMTP.