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There is a number of phase shifting transformers (PST) installed in power systems that needs to be taken into account when assessing a transient stability. A conventional method of transient stability analysis a digital simulation can easily satisfy demanded model complexity while direct methods require energy functions that adequately represent a post-fault system. We present two ways how to enter effects of phase angle regulator (PAR) that is a member of PST into a power systems' energy function. Consequently, several new energy functions are presented. Two cases were considered: first uses advantages of a reduced system; and a second one a structure preserving framework. Two numerical examples validate the proposed energy functions by comparing critical clearing times obtained and by a step-by-step time-domain simulation.