Presently there is a great interest in underground cable transmission lines in extra high and high voltage level networks. The undergrounding of the transmission system can lead to the decrease of the resonance frequencies resulting in high overvoltages and possible power system component damages. It is therefore necessary to analyze possible resonance problems in the mixed power transmission system. In this work a frequency dependent model for transmission systems is implemented and the frequency scan results are presented. In order to gain a better understanding of the nature and extent of the resonance, a resonance mode analysis is also executed. The proposed procedure is tested on two examples: a ten-node power system and the Swiss extra high voltage (EHV) power transmission system. Power systems comprising solely overhead lines and different variations of mixed power system are examined.