Delayed current zeros due to out-of-phase synchronizing

Out-of-phase synchronizing occasionally occurs in power stations. The resulting fault current may exhibit delayed current zeros. These delayed current zeros have totally different causes and are quite dissimilar in comparison with the well-known delayed current zeros associated with generator terminal faults. The rapid movement of the rotor from initial out-of-phase angle delta(0) to delta = 0 results in a very small a.c. component and a dominant d.c. component when delta = 0 is reached. The various parameters influencing the extent of the delayed current zeros have been analyzed and are discussed. The mast influential parameter is the initial out-of-phase angle delta(0). Other important parameters are inertia constant of the turbo set and initial deviation from synchronous speed. The influence of incorrect generator modelling on the result has also been demonstrated. The effect of circuit-breaker are voltages on delayed current zeros has been examined and a comparison made between generator voltage and HV-side circuit-breakers. From this comparison follows that while circuit-breakers on both sides of the step-up transformer are necessary from a protection point of view, the generator circuit-breaker should preferably be used for synchronizing operations.