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Small-Signal Modelling and Stability Assessment of Phase-Locked Loops in Weak Grids

1
Department of Electrical Engineering, Faculty of Mathematical and Physical Sciences, University of Chile (DIE), Av. Tupper 2007, Santiago 8370451, Chile
2
PEMC Group, University of Nottingham, Nottingham NG7 2RD, UK
*
Author to whom correspondence should be addressed.
Energies 2019, 12(7), 1227; https://doi.org/10.3390/en12071227
Received: 24 February 2019 / Revised: 15 March 2019 / Accepted: 20 March 2019 / Published: 30 March 2019
(This article belongs to the Special Issue Grid-Connected Converters)
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Abstract

This paper proposes a low-complexity small signal model for a 3-leg converter connected to a balanced three-phase, three-wire weak grid and synchronised to this grid using a PLL implemented in a synchronous rotating d-q axis. A thorough analysis of the system stability as a function of the PLL bandwidth and the short circuit ratio (SCR) of the grid is performed based on a linearised model. By using the proposed model, an improved design process is proposed for the commonly used dq-PLL that accounts for the potential stability issues which may occur in weak grids. Using the proposed approach, it is possible to optimise the PLL design to find the fastest PLL that can operate stably considering the SCR of the grid. In addition, the proposed model is very simple, resulting in a straightforward design tool that could also be used for online stability monitoring. The method is validated through simulations and experimental results from a 5kW laboratory system. View Full-Text
Keywords: phase-locked loop (PLL) stability; small signal analysis; grid-connected converter; weak grid; PLL bandwidth analysis; short circuit ratio (SCR) phase-locked loop (PLL) stability; small signal analysis; grid-connected converter; weak grid; PLL bandwidth analysis; short circuit ratio (SCR)
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Burgos-Mellado, C.; Costabeber, A.; Sumner, M.; Cárdenas-Dobson, R.; Sáez, D. Small-Signal Modelling and Stability Assessment of Phase-Locked Loops in Weak Grids. Energies 2019, 12, 1227.

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