The integration of renewable energy systems (RES) and the increase of non-lineal loads (NLL) at high voltage level can impact the behaviour of the relay protection coordination. RES and NLL produce harmonics distortion on voltage and current signals respectively. Also, the harmonic flow can influence the response of electric equipment such as current transformer (CT) and capacitor voltage transformer (CVT). The harmonic analysis and frequency response attributes in power lines are complex and depend on line transposition. Therefore, legacy relay settings can be negatively affected on a power system with harmonic flow. This study aims to determine the frequency response of a ring-type transmission system that is connected to inverter-based resources and non-linear loads. The frequency scan results showed resonance at the fifth harmonic causing a significant increase of the fifth current and voltage harmonics. These results indicate that transmission lines frequency resonances must be identified to have an appropriate consideration before to define the relay settings.