This paper reports on the results of recent research and development efforts to study and assess various impacts of ferroresonance phenomenon on operating performance of power transformers and to investigate ways to mitigate the effects of such phenomenon. Three-phase core-type transformers are used in the study. The results of the technical investigation and the associated ferroresonance experimental work performed showed that ferroresonance can cause dangerous overvoltages and overcurrents in three-phase coretype transformers. Sensitivity analysis results are also presented to determine the impact of various design and operating parameters on the resulting overvoltages and overcurrents caused by ferroresonance.