Performance Indices for Assessing the Power Quality of Islanded Operation of Distributed Generators


Currently, most utilities require the immediate disconnection of distributed generators in case of unintentional islanding, due to safety and technical reasons. However, there is a trend for allowing the islanded operation of distributed resources under specific conditions, so that the reliability of the energy supply to the consumers can be improved. In this case, the power quality inside the energized island must be assured by the distributed generators’ operators, which means that the transient and steady-state voltage and frequency of the isolated systems must be within acceptable limits. In this context, in this paper we proposed four indices for assessing the technical performance of the islanded operation of distributed generators. The indices are based on power quality issues, and they were proposed by considering the unplanned islanding of a synchronous distributed generator, its transition from the grid-connected mode to the islanded mode and the steady-state operation after a successful islanding. Results have shown the indices application to different operating scenarios, including a load shedding scheme, and they proved to be an interesting tool to technically evaluate the islanded operation of distributed generators, under a power quality perspective.

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The authors acknowledge the National Council for Scientific and Technological Development (CNPq), under processes 306528/2015-0 and 474443/2013-2, for funding this research.

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Correspondence to Rogério Lúcio Lima.

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Appendix 1 Restriction time calculation, \(t_{REST}\)

Appendix 1 Restriction time calculation, \(t_{REST}\)

The restriction time (\(t_{REST}\)) is the only index associated with DG protection and was proposed by Trindade et al. (2010). The restriction time is calculated by several simulations of electromagnetic transients in which the time of changing the post-islanding control mode is gradually increased. For each change in the instant of the DG control mode conversion (PQ to f-V), it is checked whether the frequency relay disconnects the DG. If the relay operates, the change time used in the previous simulation is determined as the restriction time for the current active power imbalance level. Otherwise, the DG control conversion time is increased and new simulation is performed to check if frequency relay is operated or not.

This process is repeated until the restriction time for the current level of active power imbalance is obtained. It is noteworthy that the process developed in Trindade et al. (2010) to determine the restriction time was the same applied in this study.

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Lima, R.L., Vieira, J.C.M. Performance Indices for Assessing the Power Quality of Islanded Operation of Distributed Generators. J Control Autom Electr Syst (2021).

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  • Distributed generation
  • Intentional islanding
  • Power distribution
  • Power quality
  • Time domain simulation