Loss of neutral in distribution substation transformers: analysis of an actual event and experimental studies on a reduced-scale distribution network

This paper shows an actual lost neutral event that has occurred in a 69 kV/13.8 kV power transformer at the Equatorial Energia’s Santarém Substation using oscillography records, and its impacts on distribution network operation. Due the need to evaluate the system’s behaviour in scenarios of loss of neutral conductor, this paper presents a reduced-scale distribution system to emulate different events under this condition. In this reduced-scale network, there is the presence of a protection relay, which is used in real-world HV/MV distribution substations. An analysis of oscillography records captured from neutral lost events emulated in the reduced-scale network is presented. Studies considering the loss of the neutral conductor in power transformers contribute to the development of more effective detection and protection solutions. By analysing the real event presented in this paper and the various studies conducted using the reduced-scale network, it becomes evident that the type of grounding significantly changes the system’s response to disturbances and faults. Therefore, even in solidly grounded systems, it is important to include voltage-based protection elements to safeguard the assets of distribution companies against impacts resulting from lost neutral events in distribution substation transformers.