Sympathetic inrush current phenomenon with loaded transformers

• Sympathetic inrush current equations in a state-space form are derived.
• Physical background of the phenomenon is described.
• Specifics concerning loading of parallel transformers are provided.
• Measurements of WAMS-captured sympathetic event are presented.

The magnetization of a transformer is usually associated with the well-known phenomenon of inrush current. However, inrush current does not only affect the transformer being switched on. Instead, it has a significant impact also on all parallel connected transformers, which most certainly includes measurement transformers. This is known as a sympathetic inrush phenomenon. While the transformer being switched on might be a subject to a sudden high saturation level, the parallel transformers are gradually drawn to saturation as well, but of opposite polarity. The sympathetic inrush is expected in situations where the ohmic part represents a significant portion of the total system impedance. In contrast to other available literature on this subject, in this paper a modal approach to solving equivalent circuit by expressing differential equations in the state-equation form is selected. From the derivation of eigenvalues and eigenvectors, the phenomenon can be systematically investigated. A special attention will be given to circumstances, when the already operational transformer is fully loaded as within substations two or more transformers often operate in parallel, among which at least one of them is usually loaded. Finally, simulation results are compared to captured Wide Area Monitoring System (WAMS) measurements of the phenomenon and reasons for discrepancies are discussed.