Fault Ride Trough requirements by grid codes: transient analysis and solutions for 1-10 MW range ORCs

Up to recently, it was rather usual that any grid disturbance outside certain limits (like a short grid fault causing a deep voltage sag) implied a small size (hundreds of kW or megawatts range) electricity production unit, as an ORC is, to be disconnected from the grid and stop. The increasing share of renewable plants, compared to the overall generated power in a grid, forced the utilities to take into account the problem that a sudden shut-off of many generators could even cause a major grid black-out, possibility that would be negligible if the contribution to the overall power of the disconnected generator/s was minor. For this reason, more recently many International grid codes ask the generators connected to the grid to remain connected when a voltage sag, within a specified voltage-time profile, occurs. The paper lists and analyzes the grid codes requirements of some European countries and shows the results of a detailed study carried out on a 770 kW ORC unit, considering the transient behaviour of the system during a grid fault from an electrical point of view and taking into account the dynamic response of the ORC unit as a whole. The analysis is carried out considering to drive either an asynchronous generator or a synchronous one. Furthermore, some considerations are added to explain which solutions should be adopted in the design and selection of the electric generator driven by the turbine as well as of the main auxiliaries of the ORC (for example the frequency converter powering the feed pump) and other ancillary systems to avoid a shutdown of the unit during a short-circuit grid fault.     «

Up to recently, it was rather usual that any grid disturbance outside certain limits (like a short grid fault causing a deep voltage sag) implied a small size (hundreds of kW or megawatts range) electricity production unit, as an ORC is, to be disconnected from the grid and stop. The increasing share of renewable plants, compared to the overall generated power in a grid, forced the utilities to take into account the problem that a sudden shut-off of many generators could even cause a major grid...     »