Analysis of the effectiveness of grid codes for offshore wind farms connected to onshore grid via VSC-based HVDC

The definition of adequate Grid Codes is essential to ensure an efficient and secure operation of offshore wind farms and their integration into the interconnected electricity system. The German Ordinance on System Services by Wind Energy Plants (SDLWindV) as legal base for the corresponding Grid Codes demands wind turbine generators (WTG) and the whole farm to provide a wide range of reactive power supply. Furthermore fault-ride-through (FRT) capabilities are requested, i.e. the ability to keep connected in case of faults and to adjust active and reactive power supply as response to grid voltage and frequency deviations within a short-time frame. While these requirements have their justification in supporting AC-onshore-grids, it has to be analysed whether the potential of the WTGs can be exploited in offshore wind farms coupled by Voltage Source Converter-based High Voltage Direct Current links (VSC HVDC). Subsequently it is to discuss if a simplification of the Grid Codes in accordance with the secure and efficient integration of a VSC-based HVDC connected wind farm is possible. For the studies carried out in this paper, an offshore wind farm model is presented, which consists of 40 single wind turbines with FRT-capable doubly-fed induction generators (DFIG) and a VSC-based HVDC link to the onshore grid. The VSC-based HVDC is equipped with a control strategy to handle offshore and onshore faults. The offshore converter works as a reference machine while the onshore converter operates with a Vdc/Vac control scheme, designed for an appropriate integration into the interconnected electricity system. Various cases with different control strategies, such as Q(U) or fixed Q control, were investigated to study the steady-state offshore grid operation and the corresponding fault behaviour. Results indicate the possibility to operate the offshore grid in steady-state on a low loss level while maintaining good voltage quality with a distinctly tighter P-Q-range of the WTG than required from the Grid Codes with limited impact on the dynamic behaviour. It can be demonstrated that voltage drops during a three phase (symmetrical) fault as well as the voltage recovering time are mostly independent from the pre-fault reactive power supply. The latter is dominated by the offshore converter and the requested WTG reactive current support during faults. The results imply that initiating a discussion about a modification of the ordinance with a downsized reactive power range for VSC-HVDC connected wind farms is entitled and necessary