Current differential protection for transformer in multi-phase supply network, involves using derived currents to form difference currents for isolated network, and corrected currents for earthed network

摘要

The method involves acquiring first and second secondary currents (IL1os,..) proportional to currents in phase conductors (L1-L3;La-Lc) detected by current converters on first and second sides of the transformer (T), forming phase-related difference currents and monitoring their magnitudes. For a transformer with an odd number of switch groups the secondary currents of the relevant group are converted into derived secondary currents before forming difference currents. The derived currents are used to form the difference currents for an isolated network on the second side of the transformer and are used after correction with an earth current for an earthed network. AN Independent claim is also included for the following: (a) a current differential protection arrangement for a transformer.