EDF SEI operates over 20 isolated micro grids, notably in French overseas territories and off the coast of Brittany. So far, those micro grids have mostly been powered by diesel engines ; renewable energy sources and storage systems are progressively installed to reduce fuel consumption and emissions.

Due to the diversity of installations, there is a growing need to harmonize neutral earthing systems in medium-voltage (MV) distribution networks in order to simplify equipment procurement and ensure consistent protection practices, particularly for inland municipalities in French Guiana.

Recently, abnormal overvoltages were observed in the MV network of Saint Martin. A dedicated R&D study identified the neutral earthing scheme as the root cause of excessive potential rise. As a mitigation measure, a modification of the neutral earthing system was implemented at the end of 2025, pending a long-term solution. This event motivated a more general study aimed at systematically assessing the technical advantages and limitations of various neutral earthing regimes applied to isolated microgrids, with the objective of identifying a technically optimal solution adaptable to the wide range of microgrids operated by EDF SEI. The study evaluates different neutral earthing configurations based on three main criteria:

Personnel safety assessed through touch voltage,

Equipment protection assessed through healthy phase overvoltage,

Residual current and voltage protections sensitivity.

A dedicated calculation code was developed in MATLAB to compute earth-fault currents and touch voltages under various operating conditions. The results were validated against detailed simulations performed using EMTP‑RV. This calculation tool enables efficient evaluation of personnel safety and protection performance for a wide range of configurations, including generator power, neutral earthing impedance, MV cable length, and fault resistance.

The results show that solidly earthed systems generally lead to the highest touch voltages, making them unfavorable from a personnel safety perspective, despite their good fault detection capability. In addition, the analysis highlights resonance phenomena between generator inductances and underground cable capacitances, which may cause overvoltages exceeding line-to-line voltage on healthy phases, increasing the risk of double earth faults. This behavior is particularly pronounced in isolated neutral systems, where no resistive damping is present.

The study concludes that impedance-earthed neutral systems offer the best compromise for isolated microgrids by limiting overvoltage risks, preserving personnel safety, and ensuring adequate earth-fault protection sensitivity.