Abstract
This paper presents a dynamic simulation electric model of a case study involving a prospective scenario of a cluster of six offshore platforms, disconnected from the mainland grid, by a GW-scale wind farm and a green-hydrogen energy hub. With this concept, the overall goal is to reduce carbon emissions associated with oil and gas production on the platforms by replacing fossil electricity generation, generally produced by low-efficiency single-cycle gas turbines, with renewable generation. The purpose of the presented model is to investigate how the electric system should be designed and operated to ensure a stable and secure electricity supply to the platforms, where the balancing of active power fluctuations caused by varying wind speeds at the wind farm is one of the fundamental challenges that needs to be addressed. This dynamic model, which is simulated with DIgSILENT PowerFactory, is made publicly available for reproducibility purposes. The energy hub is fed by the massive wind farm and, in turn, feeds the cluster of platforms via a system of underwater high voltage cables. This hydrogen energy hub contains a grid-forming battery system, grid-following electrolysis units, and grid-following fuel cells. The six offshore platforms are modelled in the following way: one platform is based on a publicly available model, named LEOGO; and the other five are modelled as aggregated mixes of constant power and constant impedance loads. The battery system operates with a virtual synchronous machine scheme and provides inertial and primary power reserves. This scheme is equivalent to a traditional speed governor with frequency droop, which results in steady-state frequency deviations after power imbalances between production and consumption. To correct these steady-state errors, a centralized secondary frequency controller located at the hub’s power management system is employed. This controller measures the ac frequency at the energy hub’s main high voltage ac busbar and sends active power setpoints to the secondary frequency reserves, which are composed by the electrolyser and fuel cell energy storage systems. The power output from the wind farm is generated from its mean value encompassing wake losses and power spectral density characterizing correlated wind fluctuations between turbines arising from farm-scale turbulence. A surrogate model wrapping up data from state-of-the-art aerodynamic simulations is used for this purpose. The electric model of the cluster of platforms, green-hydrogen energy hub, and wind farm is simulated during various operating conditions. These conditions include normal operation at higher and lower wind speeds, partial losses of electric load or production, and weather fronts with abrupt changes in wind speed and/or direction. The results provide valuable insights which can be used for furthering the design and optimization of the hub concept, which includes, among others, sizing of batteries depending on maximum ramp rates of fuel cells and electrolysers, avoiding or mitigating instabilities induced by interactions among power electronic converters, and evaluating the need for reactive power compensation at the platforms.
