Differential Protection Challenges in Auto Transformers with Tertiary Delta Windings - A Case Study

Autotransformers are vital components in power systems, ensuring the secure interconnection of Extra High Voltage (EHV) and High Voltage (HV) networks. They demand reliable protection systems due to their role in transferring substantial energy. This paper addresses the challenges of differential protection in autotransformers with tertiary delta windings, particularly when delta winding current is unmeasured. The paper discusses the principles of differential protection based on ampere-turn balance and the algorithms employed by modern numerical relays. It explores the impact of zero sequence compensation on protection sensitivity and phase identification and underscores the importance of measuring delta winding current. Two real-world cases illustrate the challenges faced in autotransformer differential protection, where the relay detected current in all three phases, making fault identification difficult. By calculating the differential currents, the paper provides insights into the relay's behavior in these scenarios, emphasizing the need for delta winding current measurement to enhance protection selectivity.