Summary This paper proposes an improved transient negative-sequence calculation formula for a large-capacity nuclear power generator system under unbalanced network conditions. Because of the disadvantages of traditional transient negative-sequence ratings, I2*2 t is concluded to be inadequate for rating the generator imbalance capability. In fact, many factors, such as the possible fault asymmetry, the duration of the fault and non-periodic components, should also be considered when rating a generator. According to the inherent relation between rotor heating and generator imbalance capability, the non-periodic component is added to the improved transient negative-sequence formula. In this paper, an AP1000 nuclear turbo-generator was examined as a model to systematically analyse the electromagnetic properties and generator transient negative-sequence capability affected by different types of typical short circuits. A comprehensive comparison based on accident examples, experimental results and extensive simulations shows that the improved transient negative-sequence formula not only significantly enhances the assessment accuracy of large-capacity generator imbalance capability but also provides a theoretical reference for the related research of large-capacity generator systems. Copyright © 2015 John Wiley & Sons, Ltd.