  The transient stability of traditional power systems is concerned with the
ability of generators to stay synchronized with the positive-sequence voltage
of the network, whether for symmetrical or asymmetrical faults. In contrast,
both positive- and negative-sequence synchronizations should be of concern for
inverter-based generation (IBG) under asymmetrical faults. This is because the
latest grid codes stipulate that IBG should inject dual-sequence current when
riding through asymmetrical faults. Currently, much less is known about the
synchronization stability during asymmetrical faults. This significantly
differs from the positive-sequence synchronization alone because the coupled
dual-sequence synchronization is involved. This paper aims to fill this gap.
Considering the sequence coupling under asymmetrical faults, the dual-sequence
synchronization model of IBG is developed. Based on the model, the conditions
that steady-state equilibrium points should follow are identified. The
conditions throw light on the possible types of synchronization instability,
including the positive-sequence dominated instability and the negative-sequence
dominated one. For different types of instability, the dominant factors are
analyzed quantitatively, which are reflected by the limit on the current
injection amplitude. Exceeding the limit will lead to the loss of both
positive- and negative-sequence synchronizations. The model and the analysis
are verified by simulations and hardware-in-the-loop experiments.
