137 research outputs found
Design-Oriented Transient Stability Analysis of Grid-Connected Converters with Power Synchronization Control
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 at the unstable equilibrium point of the
post-fault operation. This fixed CCA facilitates the design of power system
protection. Moreover, it is also found that the PSC-VSC can still
re-synchronize 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
Design-Oriented Transient Stability Analysis of PLL-Synchronized Voltage-Source Converters
Differing from synchronous generators, there are lack of physical laws
governing the synchronization dynamics of voltage-source converters (VSCs). The
widely used phase-locked loop (PLL) plays a critical role in maintaining the
synchronism of current-controlled VSCs, whose dynamics are highly affected by
the power exchange between VSCs and the grid. This paper presents a
design-oriented analysis on the transient stability of PLL-synchronized VSCs,
i.e., the synchronization stability of VSCs under large disturbances, by
employing the phase portrait approach. Insights into the stabilizing effects of
the first- and second-order PLLs are provided with the quantitative analysis.
It is revealed that simply increasing the damping ratio of the second-order PLL
may fail to stabilize VSCs during severe grid faults, while the first-order PLL
can always guarantee the transient stability of VSCs when equilibrium operation
points exist. An adaptive PLL that switches between the second-order and the
first-order PLL during the fault-occurring/-clearing transient is proposed for
preserving both the transient stability and the phase tracking accuracy.
Time-domain simulations and experimental tests, considering both the grid fault
and the fault recovery, are performed, and the obtained results validate the
theoretical findings
Control of Grid-Forming VSCs: A Perspective of Adaptive Fast/Slow Internal Voltage Source
Grid-forming (GFM) capability requirements are increasingly imposed on
grid-connected voltage-source converters (VSCs). Under large grid disturbances,
GFM-VSCs need to remain stable while providing GFM services. Yet, such
objectives, as pointed out in this paper, inherently lead to conflicting
requirements on the dynamics of internal voltage source (IVS) of GFM-VSCs,
i.e., the fast IVS dynamics is needed to avoid the loss of synchronism with the
grid, whereas the slow IVS dynamics is preferred for maintaining GFM
capability. To tackle this challenge, an adaptive fast/slow IVS control is
proposed, which switches GFM-VSC between fast and slow IVS dynamics based on
system needs. The proposed method enhances the transient stability of GFM-VSC,
whilst maximizing its capability of providing GFM service. Further, the
approach is robust to different grid strengths and different types of grid
disturbances. The experimental results verify the theoretical findings and the
effectiveness of the proposed control method
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