Delayed current zeros due to out-of-phase synchronizing [of turbogenerators]

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 δ₀ to δ=0 results in a very small AC component and a dominant DC component when δ=0 is reached. The various parameters influencing the extent of the delayed current zeros have been analyzed and are discussed. The most influential parameter is the initial out-of-phase angle δ₀ . Other important parameters are inertia constant of the turbogenerator 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 arc voltages on delayed current zeros has been examined and a comparison made between generator voltage and HV-side circuit-breakers. From this comparison, it 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