Abstract
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.
Highlights
V OLTAGE-SOURCE CONVERTERS (VSCs) are commonly used with renewable energy resources, flexible power transmission systems, and electrified transportation systems
It is revealed that when only one equilibrium point exists during the fault, the synchronous reference frame (SRF)-phase-locked loop (PLL) cannot stabilize voltage-source converters (VSCs) no matter how large the damping ratio is adopted
This article has analyzed the impact of the PLL on the transient stability of VSCs during grid faults
Summary
V OLTAGE-SOURCE CONVERTERS (VSCs) are commonly used with renewable energy resources, flexible power transmission systems, and electrified transportation systems. Considering VSC–grid interactions, a large-signal nonlinear model of the SRF-PLL is reported in [10], which reveals that the LOS will be inevitable if the VSC does not have equilibrium points during grid faults. To avoid modifying the injected current profile, the damping ratio of the SRF-PLL can be increased to enhance the transient stability of VSCs [12]–[15], [22]. It is revealed that when only one equilibrium point exists during the fault, the SRF-PLL cannot stabilize VSCs no matter how large the damping ratio is adopted This transient instability can be avoided if the SRFPLL is reduced as a first-order PLL by freezing the integral controller. Time-domain simulations and experimental tests validate the theoretical findings and the performance of the adaptive PLL
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