This paper has found that the zero sequence voltage conveyed from the primary and secondary side of the transformer to generator terminal is so different when a single-phase-to-earth occurs on the primary side of the transformer for a non-neutral-grounding generator. Based on the results of theoretical calculation and confirmation from the field operation， with the arc-suppression-soil-earth system， the generator’s first harmonic zero sequence over voltage protection will be not good for trip， if it must， then should be changed to trip from single， the grounding of arc-suppression reactor under compensation should be changed to the distribution transformer with high-impedance grounding.