Abstract
The effect of frequency variation on system stability becomes crucial when a voltage source converter (VSC) is connected to a weak grid. However, previous studies lack enough mechanism cognitions of this effect, especially on the stability issues in DC voltage control (DVC) timescale (around 100 ms). Hence, this paper presented a thorough analysis of the effect mechanism of frequency variation on the weak grid-connected VSC system stability in a DVC timescale. Firstly, based on instantaneous power theory, a novel method in which the active/reactive powers are calculated with the time-varying frequency of voltage vectors was proposed. This method could intuitively reflect the effect of frequency variation on the active/reactive powers and could also help reduce the system order to a certain extent. Then, a small-signal model was established based on the motion equation concept, to depict the effect of frequency variation on the weak grid-connected VSC system dynamics. Furthermore, an analytical method was utilized to quantify the effect of frequency variation on the system’s small-signal stability. The quantitative analysis considered the interactions between the DC voltage control, the terminal voltage control, phase-locked loop, and the power network. Finally, case studies were conducted, and simulation results supported the analytical analyses.
Highlights
In many parts of the world, including China, wind farms are often located far away from load centers [1]
A novel power calculation method in which the active/reactive powers are calculated with the time-varying frequency of voltage vectors was proposed
Comparisons of simulation responses and eigenvalue analysis validated that the established model could hold the main behaviors of concern, even under weak grid conditions
Summary
In many parts of the world, including China, wind farms are often located far away from load centers [1]. To analyze the dynamics of power systems with large scale VSCs in DVC timescale, [14] proposes a small-signal model of VSC based on the motion equation concept In this model, the self-characteristics of VSC are depicted by the relationship between the unbalanced active/reactive powers and the phase/amplitude dynamics of the VSC’s internal voltage Drawing lessons from conventional SG-dominated power systems, the reactance of the power network under the nominal frequency was utilized to calculate the active/reactive powers by references [14,15,16] Owing to this nominal-frequency-based power calculation method, the existing motion-equation-concept models cannot depict the effect of frequency variation on the weak grid-connected VSC system dynamics. Description of Grid-Connected VSC System and Motion Equation Concept in DVC Timescale
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