The Interaction Between Multiple Timescales of the Grid-Tied Voltage Source Converter

The voltage source converter's (VSC) control system and its interactions with the network are important factors that cause the instability of the grid-tied VSC. The control of VSC has a multiple timescale feature, which can be divided into DC-link voltage control (DVC) timescale and alternating current control (ACC) timescale. Existing research focuses on stability in a single timescale and rarely deals with the interaction between multiple timescales. Therefore, this paper uses the modal analysis to investigate the quantitative indicators and influencing factors of the interaction between different timescales of the grid-tied VSC. At the same time, the applicable scope of the models in DVC and ACC timescales is clarified, and the analysis results are preliminarily explained from a physical point of view. First, this paper briefly introduces the topology and control scheme of the grid-tied VSC and illustrates its multiple timescale dynamic characteristics. Next, the small-signal models of the grid-tied VSC in multiple timescales, DVC timescales, and ACC timescales are constructed. The accuracy of the above models is verified by the electromagnetic transient simulation and eigenvalue analysis. The participation factors for state variables and modes in different timescales are used based on the modal analysis method to measure the relative strength of the interaction between different timescales. The interaction variation between different timescales with the bandwidth of the phase-locked loop (PLL) and the short circuit ratio (SCR) is analyzed. By comparing the eigenvalue damping ratios and oscillation frequencies of the grid-tied VSC models in multiple timescales, DVC timescale, and ACC timescale, this paper shows that the interaction between multiple timescales has an impact on the system stability analysis results. Then, the deviations of eigenvalue damping ratios and oscillation frequencies of the DVC or ACC timescale models are analyzed when the PLL bandwidth and SCR vary. The applicable scope of the above models in a single timescale is clarified. Moreover, the theoretical analysis results are verified by electromagnetic transient simulation. Finally, the effects of the interaction between multiple timescales are initially explained from the physical level. The main conclusions of this paper are as follows. (1) The participation factors for state variables and modes in different timescales are defined as the different-scale participation factors, and different-scale participation factors can be used as a quantitative index to measure the interaction between multiple timescales. (2) When the PLL bandwidth increases or SCR decreases, the different-scale participation factors increase, which enhances the interaction between multiple timescales. (3) The influence of the interaction between multiple timescales on the analysis results of system stability is studied, and the applicable scope of the models in DVC and ACC timescales is proposed. The increase of PLL bandwidth or the decrease of SCR will enhance the interaction between multiple timescales, thus increasing the deviation of the eigenvalue analysis results of the models in a single timescale. (4) From the perspective of the parts in the DVC timescale, the inner control loops are equivalent to the damping links. Therefore, the eigenvalue analysis results of the model in the DVC timescale are lower in damping ratio and higher in oscillation frequency than those in the multiple timescale model. From the perspective of the parts in the ACC timescale, the outer control loops are equivalent to the excitation, so the eigenvalue oscillation frequency obtained by the model in the ACC timescale is much lower. © 2023 Chinese Machine Press. All rights reserved.