During these last two decades, transmission system operators (TSOs) have been facing profound changes such as penetration of renewables, DC networks, redefinition of European market mechanisms leading to major evolutions in the way to operate the electrical transmission grids. The goal of carbon neutrality implies that most of the challenges for TSOs are still to come. All those changes imply drastic changes in the way to perform grid analysis and have appropriate computation modules to perform efficient security assessment. Legacy tools are evolving slowly with low information about used approximations. RTE, the French TSO, believes that Open Source is the perfect answer to build power system tools in an efficient, flexible, transparent, and collaborative way. RTE has significantly invested in these Open Source tools and would like to share his experience with demo of tools currently in operation based on PowSyBl framework.