As a flow control device, a phase-shifting transformer (PST) regulates power flow distribution by adjusting the phase angle between the voltage and current at the ends of a transmission line. However, due to the PST’s inherent phase-shifting angle and equivalent impedance, its integration into a line inevitably impacts the existing distance protection, potentially leading to maloperation or the failure to operate of the protection. While some mitigation strategies have been proposed, they have not fully resolved this issue. Addressing the problem of distance protection maloperation or failure to operate caused by PST integration, this paper, from an engineering perspective and based on extensive simulation experiments, investigates the adaptability boundaries of existing distance protection schemes to PST integration.