This paper shows influence of overvoltages caused by circuit breaker's poles lagging during switching operation of unloaded power transformer on electrical apparatus in transformer bay. In order to perform analysis of these overvoltages on personal computer it is essential to create appropriate power transformer model. Power transformer model for analysis of unloaded transformer switching must comprise adequately modelled power transformer's magnetising curve. Therefore simple method for power transformer's magnetising curve representation is presented based on results of open circuit test on power transformer that is available from transformer testing protocol. In reality this is optimisation problem that shows how much is curve calculated by computer simulation close to the one obtained by measurement. To solve this optimisation problem computer program was created by MatLab software based on evolutionary algorithm. Its results were used to model power transformer's magnetizing curve in EMTP/RV software and simulation results of one of circuit breaker's poles lagging during switching operation of modelled unloaded power transformer are presented. Simulation results showed that circuit breaker’s poles lagging during switching operation of unloaded power transformer could cause serial resonance or feroresonance induced overvoltages and then steep front of the vave overvoltages could occur depending on moment of circuit breaker’s contacts closure. Finally, influence of overvoltages caused by circuit breaker's poles lagging during switching operation of unloaded power transformer on electrical apparatus (current transformers and surge arresters) in transformer bay is shown on two cases encountered in Elektroprenos's BiH substations.nsformer, modelling, magnetizing curve, evolutionary algorithm, current transformer, surge arrester.