Abstract
To prepare for the future high penetration level of renewable energy sources, the power grid’s technical boundaries/constraints for the correct operation of powerelectronics interfaced devices need to be further examined and defined. This paper investigates the challenge of integrating Voltage Source Converters (VSC) into low inertia power grids, where the system frequency can vary rapidly due to the low kinetic energy buffer available, which used to be provided by the rotational inertia of synchronous generators. The impact of rate of change of frequency (ROCOF) on the PLL dynamics and its subsequent influence on the VSC power stage output is explained. The Bonaire island network is presented as case study. The performance of the VSC is analyzed under a fast ROCOF event, which is triggered by a short circuit fault. A down-scaled experiment is used to validate the Bonaire island network simulation results. It shows that the phase angle error measured by the synchronous-reference frame phase-locked loop (SRF-PLL) is proportional to the slope of the ROCOF and inversely proportional to its controller integral gain constant.
Highlights
A weak grid is characterized as an AC power system with a low short-circuit ratio (SCR) and/or inadequate mechanical inertia (IEEE standard 1204-1997 [1])
To study the impact of a fast rate of change of frequency (ROCOF) on the phase locked loop (PLL) dynamics and the voltage-sourced converter (VSC) power stage output, the ideal voltage source is triggered by a frequency ramp-down event (ROCOF = 10 Hz/s), where the mechanical inertia coupling is not considered
The emulated frequency ramp-down event is shown by the green curve in Figure 8b, where system frequency starts to decline at 2 s and it settles at 46 Hz in 0.4 s (ROCOF = 10 Hz/s)
Summary
A weak grid is characterized as an AC power system with a low short-circuit ratio (SCR) and/or inadequate mechanical inertia (IEEE standard 1204-1997 [1]). For a large inter-connected power system, the total kinetic energy buffer provided by all the synchronous generators in the system is large In this case, local disturbances (e.g., generator trip, load rejection, short circuit fault etc.) cause only mild frequency variation thanks to the total system mechanical inertia. Inspired by this event and expected future challenges associated with battery storage, grid frequency support in a power grid with high penetration of renewable energy sources, this paper investigates the impact of fast rate of change of frequency (ROCOF) on the grid-connected VSC phase locked loop (PLL) dynamics.
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