Abstract
Recent studies show that the loss of stability for a voltage-source converter (VSC) in weak-grid connection is largely related to its synchronization unit, i.e., the phase-locked loop (PLL). This paper studies the synchronization stability of a system comprised by two VSCs in parallel connection to a weak grid. A reduced transfer function based small-signal model, which can allow for the interactions between PLL and converter outer power controls, is first proposed. Then, an improved net damping criterion is used to analyze the damping and stability characters of such system under various operating conditions and different controller configurations. Compared to the conventional net damping criterion, the used criterion has wider applicability in terms of stability judgment. Case studies show that the studied system tends to be unstable at weak-grid or heavy-loading conditions. The instability can be in the form of oscillations or monotonic divergence, in which, the latter is more likely to occur for the converters without grid voltage regulation capabilities. Besides, the net damping-based sensitivity studies can provide guidance on control tuning or design for stability enhancement. Detailed model-based time domain simulations are conducted to verify the analysis results.
Highlights
With the rapid development of renewable generation, large numbers of power electronics converters penetrate the power systems for energy conversion and transmission
The used voltage-source converter (VSC) module is much simpler than the detailed one in [3] for synchronization stability studies owing to the neglection of fast inner current controls, but more accurate compared to the models given in [9,14] for counting the interactions between phase-locked loop (PLL) and outer power controls
An improved net damping criterion is used between PLL and the outer power controls
Summary
With the rapid development of renewable generation, large numbers of power electronics converters penetrate the power systems for energy conversion and transmission. Literature [6,7] both find that the interactions between active power controls and PLL can lead to instability at weak-grid conditions in wind generation and high-voltage direct transmission (HVDC) systems. These results indicate that the dynamics of VSC outer power controls need to be modeled for synchronization stability studies. The used VSC module is much simpler than the detailed one in [3] for synchronization stability studies owing to the neglection of fast inner current controls, but more accurate compared to the models given in [9,14] for counting the interactions between PLL and outer power controls.
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