Analysis of the influence of different cable modelling for DC series offshore wind farm

In this paper, a dc series offshore wind farm with an onshore modular multilevel converter (MMC) is presented with consideration of different cable models. Normally, the dc cable is represented either by a small capacitance or by pi-sections, which implies that the frequency-dependent effects of the HVDC cable are neglected. One should be careful when using these cable models for high frequency simulations, especially for dc series offshore wind farm, whose topology forms a resonance circuit and consequently, accurate model of cable should be adopted to represent the transient characteristic of the HVDC transmission system in a proper way. This paper shows that the two most common lumped parameter types of DC cable model: small capacitance and pi-section model are unable to provide equivalent damping effect as the Wideband cable model or universal line model (ULM). The Wideband model is suitable for Electromagnet Transients (EMT) analysis while its distributed parameters make it impossible to be transferred into a state-space representation. This paper first introduces the topology of a DC series offshore wind farm. Then the resonance characteristic of the wind farm topology is analysed based on different cable models. Finally the offshore wind farm is simulated with these three types of cable models and controlled by modular multilevel converter (MMC).