Evaluation of grid forming controls and effects of pll during black start in microgrids
File(s)
Author(s)
Liu, Zhaoru
Type
Thesis
Abstract
With the increasing penetration of renewable energy sources, the power system becomes more vulnerable to power outages. Grid-forming (GFM) inverters are gradually deployed in inverter-based resources (IBR) for essential grid support and restoration in high renewable penetration scenarios. This thesis provides an in-depth analysis of different types of GFM control, including droop control, power synchronisation control (PSC), virtual synchronous generator (VSG), virtual synchronous machine (VSM) and matching control, etc. The operating principles, applications, and effectiveness of GFM controls are summarised under weak grid and black start conditions. Three GFM controls (PSC, VSG, droop control) are compared and analysed, highlighting their power sharing efficiency and response under line impedance mismatch. Furthermore, this thesis also investigates the dynamic behaviour and stability of various GFM and GFL controls in weak grids and determines the Short Circuit Ratio (SCR) threshold for stability. These comparisons and analyses offer insights for selecting and designing GFM control in weak grids.
Finally, the effective black-start strategies in microgrids are simulated, focusing on the performance comparison of GFM inverters both with and without PLLs, and examines how variations in PLL bandwidths influence the black-start process utilizing GFM inverters. This research reveals that GFMs with PLL have longer recovery times and greater voltage and frequency deviations. Higher PLL bandwidths can shorten overall black-start duration but increase system sensitivity, possibly leading to instability. This highlights the need for carefully designed PLLs in GFMs for weak grids and black-start situations, balancing quick recovery with stability. The study emphasises the significance of choosing suitable GFM controls that effectively manage response speed, system stability, and complexity under various grid conditions.
Finally, the effective black-start strategies in microgrids are simulated, focusing on the performance comparison of GFM inverters both with and without PLLs, and examines how variations in PLL bandwidths influence the black-start process utilizing GFM inverters. This research reveals that GFMs with PLL have longer recovery times and greater voltage and frequency deviations. Higher PLL bandwidths can shorten overall black-start duration but increase system sensitivity, possibly leading to instability. This highlights the need for carefully designed PLLs in GFMs for weak grids and black-start situations, balancing quick recovery with stability. The study emphasises the significance of choosing suitable GFM controls that effectively manage response speed, system stability, and complexity under various grid conditions.
Version
Open Access
Date Issued
2024-04-13
Date Awarded
01/08/2024
License URL
Advisor
Pal, Bikash
Publisher Department
Electrical and Electronic Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)