Some old and new thoughts on phase angle regulator protection

¿Smart Grid¿ can be defined as getting the power to the customer the smartest way possible with the least amount of losses. One of the ways to control the power flow over the transmission line is to insert a phase angle regulating transformer (PAR) in the line. PAR´s have long been dreaded by the protection engineer as a very complex device with many different currents that make it tedious at best to develop a complete protection scheme. The problem is compounded by currents changing with tap position on the load tap changer. This paper will review the theory and application of phase angle regulating transformers. Then review the classical protection given PAR´s. finally, suggesting new approaches to the protection problem made easier by the use of microprocessor based transformer differential relays.