Generator modeling for transformer transfer voltage study

This paper is focused on hydraulic generator modeling in a transient simulation for a transformer transfer voltage due to a lightning surge on a transmission line from the viewpoint of the insulation design of a hydraulic generator station. The transfer voltage of a generator step-up transformer is measured. Three generator models are investigated in comparison with the measured result. A conventional surge impedance model gives a satisfactory result of a calculated peak voltage, but an oscillating frequency disagrees with the measured one. A sub-transient reactance model composed of inductance elements and capacitance elements representing a generator subtransient reactance and an armature winding capacitance can produce a transfer voltage oscillation. However, it cannot simulate the crest value and the oscillating frequency of the measured voltage. A new model proposed in the paper is based on the subtransient reactance model. The inductance is modified by taking into account the frequency characteristic of a transfer voltage. And resistance elements are added in parallel with reactance elements to represent the damping of the voltage oscillation. The accuracy of the proposed model is confirmed to be satisfactory in both transfer voltage crest value and oscillating frequency in comparison with the measured results.