New approach for assessing the risk of harmonic amplification on the French network.

RTE is responsible for ensuring a sufficient margin with respect to the planning levels, taking into account the contribution of unloaded radial cable systems.

Detailed harmonic studies are performed to assess whether a new radial underground cable connection may cause harmonic voltage planning levels to be exceeded. The judicious selection of appropriate study cases is imperative to ensure the efficient and economic management of resources. Based on different offshore and onshore windfarm connection projects experience,

RTE carries out detailed harmonic studies for any radial underground or submarine cable connection project where the length is greater or equal to 15 km. While this criterion is straightforward to apply, it does not consider the specific characteristics of the connection area.

Furthermore, its application leads to many studies.

This paper presents a new approach developed by RTE to assess at a preliminary stage, the risk of harmonic amplification associated with the connection of radial underground cables to the French transmission grid. The objective is to determine the appropriate metric for ascertaining whether a detailed harmonic study is necessary for a given project.

The method involves the use of large-scale models of the French transmission network to compute harmonic amplification gains resulting from the virtual connection of radial underground cables at all 400 kV and 225 kV substations. The calculations are carried out by extracting the nodal admittance matrix from network models generated with EMT tools [4]. A comprehensive database is built, containing harmonic gain values at each connection point for various cable lengths and harmonic orders. By defining generic critical gain thresholds that may lead to planning level violations, this database enables early identification of projects that may pose harmonic amplification risks and require further analysis. The paper details the methodology, assumptions, and tools used to develop the harmonic gain database, as well as its application in assessing future projects.

Furthermore, given that critical gain thresholds are defined in relation to background harmonic levels, a dedicated section presents monitoring ambition for the French transmission grid. This involves the deployment of harmonic sensors at key locations across the French transmission grid.