Online parametric estimation of grid impedance under unbalanced grid conditions

Nabil Mohammed*, Mihai Ciobotaru, Graham Town

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

22 Citations (Scopus)
48 Downloads (Pure)


Whereas power-electronics-based power systems are expected to enable more integration of renewable energy sources, they could pose crucial challenges including stability issues due to the Thévenin impedance seen by inverters. Such problems could be mitigated by enabling the inverters to estimate the grid impedance by including a grid impedance estimation technique into their control loop. However, one aspect which has been overlooked thus far is that the accuracy of such grid impedance estimation techniques is significantly affected by various grid conditions. For instance, the unbalance in three-phase power systems causes unwanted oscillations at double the fundamental frequency in the inverters control loops. Therefore, this paper proposes a simple and reliable online estimation of the grid impedance under unbalanced conditions. The technique is based on wide-band impedance estimation incorporated into the control loop of the positive sequence of a three-phase grid-connected inverter equipped with a positive- and negative-sequence control (PNSC) strategy. Additionally, complex curve fitting is utilized to obtain parametric models of the grid impedance. To demonstrate the efficacy of the proposed grid impedance estimation technique, extensive case studies are performed. These include: (1) unbalanced operations of both resistive-inductive (RL) and resistive-inductive-capacitive (RLC) models of the grid, (2) background harmonics, and (3) asymmetrical impedances of the network.

Original languageEnglish
Article number4752
Pages (from-to)1-21
Number of pages21
Issue number24
Publication statusPublished - 13 Dec 2019

Bibliographical note

Copyright the Author(s) 2019. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.


  • impedance estimation
  • signal injection
  • PRBS
  • unbalanced voltage
  • sequences controller
  • grid-connected inverters


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