With the rise in global energy demand and growing environmental concerns, renewable energy development has become a key trend in the energy field. The capacity of distributed photovoltaic - storage systems is on the upswing, with numerous power electronic devices being integrated into the grid. During short - circuits, the outgoing line current’s amplitude is limited and its phase angle is regulated. Thus, traditional power - frequency relay protection methods are no longer viable. Also, current protection methods often fail when new energy output is low. This paper proposes a longitudinal protection method for grid - connected photovoltaic - storage systems using the improved S - transform. First, the S - transform is applied to the currents at both ends to obtain frequency - domain fault features. Then, the high - frequency transient energy distribution at both ends is calculated. Faults inside and outside the zone are distinguished based on the difference in this energy. A PSCAD - based simulation model is built and experimentally verified. Results show that the improved S - transform - based protection can accurately detect faults.