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The power synchronization control (PSC) has been increasingly used with voltage-source converters (VSCs) connected to the weak ac grid. This paper presents an in-depth analysis on the transient stability of the PSC-VSC by means of the phase portrait. It is revealed that the PSC-VSC will maintain synchronization with the grid as long as there are equilibrium points after the transient disturbance. In contrast, during grid faults without any equilibrium points, the critical-clearing angle (CCA) for the PSC-VSC is identified, which is found equal to the power angle of the unstable equilibrium point of the postfault operation. This fixed CCA facilitates the design of power system protection. Moreover, it is also found that the PSC-VSC can still resynchronize with the grid after around one cycle of oscillation, even if the fault-clearing angle is beyond the CCA. This feature reduces the risk of system collapse caused by the delayed fault clearance. These findings are corroborated by simulations and experimental tests.
- Phase portrait
- power synchronization control (PSC)
- transient stability
- weak grid