Harmonic distortion analysis involving submarine cables for offshore wind power plants

Abstract

With offshore wind power plants (OWPPs) getting larger and more common, the connection requirements at the point of common coupling (PCC) become more important. OWPPs often use converters that produce harmonics, and reactive components such as long high voltage AC (HVAC) submarine export cables further influence the harmonic content seen at the PCC. This thesis looks at voltage harmonics up to the 63rd order at the PCC between an OWPP and the main grid. An aggregated type IV OWPP with grid-following converters was modeled in PSCAD, and the harmonic content was analyzed in MATLAB using a fast Fourier transform (FFT). The simulated cases varied three factors: grid strength (a stronger and a weaker grid, relative to each other), HVAC export cable length (50 km and 100 km), and the order of voltage harmonic injected from the main grid (5th and 9th). Results are presented in the voltage and harmonic impedance domains. The impedance analysis focuses on the resistive component, since the reactive component remained inductive for all simulated cases. Grid strength was the dominant factor in the post-energization transient. Weaker grids produced a total harmonic distortion (THD) of 31-41 %, which is 2-4 times higher than the 9-19% seen for stronger grids in the same cases. A sensitivity check on later FFT windows showed that both grid configurations settle at a similar low THD around 2 %. The 2-4 times ratio is therefore specific to the post-energization transient, rather than the steady-state. The aggregated model is appropriate for studying the interaction between grid strength, cable parameters, and injected harmonics. Within the factors studied, grid strength had a much larger effect on THD than cable length, which means that grid reinforcement is more impactful than cable routing for limiting harmonic distortion at the PCC. The harmonic content was dominated by lower order harmonics, which is relevant for the design of harmonic filters and STATCOM compensation.