Innovative power flow regulating tap-changer control installed on multiple phase-shifting transformers

This paper describes the development and installation of a power flow regulating tap-changer control that has been installed on multiple phase-shifting transformers (PSTs) on the AEP system. These transformers are also known as phase angle regulating (PAR) transformers because they control the phase angle across a networked transmission system branch circuit to regulate the real power flow in that branch of the transmission grid. PSTs regulate the phase angle by inserting a quadrature voltage into each phase by action of a tap-changer mechanism. AEP installed the PSTs to optimize power flow on existing transmission assets until planned transmission system upgrades are complete. Historically, most PST installations have been manually controlled because the regulated quantity, power flow, is only indirectly controlled by the position of the tap changer. The actual change in power for each tap step is a function of system conditions so it is easily possible that an automatic control will hunt (control action overshoots the regulation dead band which causes the tap position to continually change). AEP desired a control that would provide automatic regulation to an operator set point. This requirement led to the development of a control with an adaptive bandwidth feature that learns system conditions to prevent the possibility of hunting. In addition to local and remote, automatic and manual control, the control includes automatic features such as one-button "take off bypass and put in service" and "put on bypass and remove from service" automatic switching sequences to ensure proper switching sequences and operator safety. The first of five units was energized in the summer of 2006 and the most recent was energized in the summer of 2007. All installations are operating satisfactorily.