Abstract: A robust ground-fault protection system is disclosed which can possess a reduced sensitivity to system noise. In a ground-fault protection system for an electrical machine such as a generator, an injection signal with an injection frequency fi is applied to the electrical machine in order to generate a periodic bias voltage on a conductor of the electrical machine, and a response signal thereto is evaluated. The injection frequency can be adapted such that a flexible approach respective of a most recent value of a system quantity of the electrical machine, can provide increased stability and reliability of the ground-fault protection system.