Controllable series-shunt transformers regulate power flow by injecting adjustable voltages

facilitating the transition of distribution networks toward flexible bidirectional power supply and thereby significantly improving system flexibility and reliability. However

their integration changes the equivalent electrical parameters of the network

causing variations in the dynamic characteristics and distribution of short-circuit currents. Therefore

the applicability of existing protection principles and setting criteria needs to be further analyzed and verified. To address this issue

an analytical model of the controllable series-shunt transformer under normal operating conditions is established

and fault-current expressions under three-phase and phase-to-phase short-circuit conditions are derived. The influencing mechanisms of device parameters and operating states on fault-current magnitude and transient processes are analyzed. On this basis

combined with the setting principles of typical multi-level protection in distribution networks

the impacts of short-circuit current variations after device integration on existing protection settings and coordination are investigated. A sectionalizing protection improvement method based on the half-cycle integral ratio before and after current-limiting operation

as well as a setting verification scheme for branch protection

is then proposed. Simulation results provide a theoretical basis for fault analysis and multi-level protection adaptability studies in distribution networks with controllable series-shunt transformers.