Transient stability of a distribution subsystem during fault-initiated switching to islanded operation

This paper investigates the transient stability of a load-rich distribution subsystem during the switching process to islanded mode instigated by a permanent fault. When operating in islanded mode, the subsystem must maintain a generation-load power balance and use at least one distributed generator (DG) to regulate the system frequency and voltage. Therefore, switching control must be executed after the disconnection of the main grid and a strategy which includes a DG coordination method and a single-step load shedding scheme is proposed. Other factors also have a substantial impact on the system transient performance, including the type of subsystem load, the DG penetration level, the fault clearance time and the switching control delay. To perform the study, a distribution subsystem was simulated using PSCAD/EMTDC software, consisting of a mix of synchronous and inverter-based DGs and a combination of static loads and dynamic motor loads. Simulation results show the proposed switching control strategy can effectively ensure successful switching from grid-connected to islanded mode under different fault conditions and DG penetration levels.