In several countries, field tests for excitation system model verification have become mandatory. Therefore on-site transfer function identification of hydro units are compulsory. Using excitation system implemented Pseudo-Random Binary Sequence signal instead of a solid state spectrum analyser is a commercial solution. A correct transfer function of a generating unit is of high importance for modelling the power system, since it helps to understand behaviour, predict failures and improve stability and dynamic performance. Especially on hydro units these tests can verify tuning quality of excitation systems and power system stabilisers.