TRANSIENT SIMULATION AND ANALYSIS OF A 3-PHASE 5-LIMB STEP-UP TRANSFORMER FOLLOWING AN OUT-OF-PHASE SYNCHRONIZATION

This paper presents the theoretical and the experimental analysis of the electromagnetic transient following the out-of-phase synchronization of a three-phase five-limb step-up transformer; this means an abnormal condition where the angle between phasors representing the generated voltages and those representing the power network voltages at the instant of closure of the connecting circuit breaker is not near zero, as normal, but may be as much as 180-degrees (phase opposition). When this happens, the peak values of the transient currents in the windings of the transformer might be sensibly higher than those of the failure currents estimated in a conventional way; in addition they correspond to unbalanced magnetomotive forces (MMF) in the primary and secondary windings of each phase of the machine. The currents and fluxes during the transient are computed by a non-conventional circuital non-linear model of the transformer simulated by the ElectroMagnetic Transient Program (EMTP). The results of an experimental validation made on a specially built 100 kVA three-phase five-limb transformer are also reported.