#
Short-Circuit Analysis of DER-Based Microgrids in Connected and Islanded Modes of Operation^{ †}

^{*}

^{†}

## Abstract

**:**

## 1. Introduction

## 2. The Microgrid Concept

## 3. DER Models

- Synchronous machines directly connected to the grid;
- Induction machines directly connected to the grid;
- IBDERs;
- DFIMs.

#### 3.1. Synchronous Machines Directly Connected to the Grid

#### 3.2. Induction Machines Directly Connected to the Grid

#### 3.3. Inverter-Based DERs

#### 3.4. Doubly-Fed Induction Machines

## 4. Short-Circuit Calculation Procedure

- ${\widehat{Z}}_{k}^{+}$, ${\widehat{Z}}_{k}^{-}$, ${\widehat{Z}}_{k}^{o}$ represent sequence domain Thévenin impedances seen from bus k
- ${\widehat{U}}_{ka}^{}$ is the known pre-fault phase a voltage at bus k.

## 5. Results

## 6. Discussion

- From Table 3 and Table 4, the differences between fault currents for the 3LG fault, in grid-connected and islanded modes are huge. If we analyze the fault currents at the protective devices’ locations, it is obvious that their magnitudes differ widely, with the fault currents in grid-connected mode being up to 8.2 times higher than in the islanded mode.
- Based on the results from Table 3 and Table 4, it is obvious that standard “set and forget” relay protection methods cannot be used for microgrids aimed to operate in both, grid-connected and islanded modes. Rather, novel methods for adaptive relay protection shall emerge, that would monitor the grid conditions in real-time, and adapt the protective equipment settings based on the real-time data. This is one of the authors’ future research directions, derived from the results obtained during this research.
- Based on the results from Table 5, it can be concluded that the results for the grid-connected mode, obtained by in-house-developed software solution match well with the results from Typhoon HIL’s hardware-in-the-loop device. The differences are less than 2%. However, for the islanded mode, the differences in the obtained results are much higher and reach up to 15.36%. This is because the fault calculation procedure used for this research [10], is specially aimed for radial distribution networks, with one main power source (slack bus) and DERs that are much weaker than the main source. This works very-well for the grid-connected mode where the utility grid is the main source, but provides less accurate results for the islanded mode, where DERs share the entire load among themselves. Thus, a novel fault calculation procedure, that will work well and provide highly accurate results regardless of the operation mode, shall emerge. This is the second important future research direction that is derived from the results of the current research.
- Finally, the third direction for the future research will be focused on analyzing a short period immediately upon the fault occurrence, in which the fault currents of IBDERs are not limited. In addition, the influence of these currents on the protective equipment selection and planning process will be analyzed.

## 7. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**A real-life microgrid example [23].

**Figure 7.**Fault current contribution response of IBDER connected to bus 14 during 3LG short-circuit at bus 9 in time domain obtained with Typhoon HIL setup.

L-G | L-L | L-L-G | L-L-L-G | |
---|---|---|---|---|

${\widehat{J}}_{k}^{+}$ | $\frac{{\widehat{U}}_{ka}^{}}{{\widehat{Z}}_{k}^{+}+{\widehat{Z}}_{k}^{-}+{\widehat{Z}}_{k}^{o}}$ | $\frac{{\widehat{U}}_{ka}^{}}{{\widehat{Z}}_{k}^{+}+{\widehat{Z}}_{k}^{-}}$ | $\frac{\left({\widehat{Z}}_{k}^{-}+{\widehat{Z}}_{k}^{o}\right){\widehat{U}}_{ka}^{}}{{\widehat{Z}}_{k}^{+}{\widehat{Z}}_{k}^{-}+{\widehat{Z}}_{k}^{+}{\widehat{Z}}_{k}^{o}+{\widehat{Z}}_{k}^{+}{\widehat{Z}}_{k}^{o}}$ | $\frac{{\widehat{U}}_{ka}^{}}{{\widehat{Z}}_{k}^{+}}$ |

${\widehat{J}}_{k}^{-}$ | $\frac{{\widehat{U}}_{ka}^{}}{{\widehat{Z}}_{k}^{+}+{\widehat{Z}}_{k}^{-}+{\widehat{Z}}_{k}^{o}}$ | $-\frac{{\widehat{U}}_{ka}^{}}{{\widehat{Z}}_{k}^{+}+{\widehat{Z}}_{k}^{-}}$ | $\frac{-{\widehat{Z}}_{k}^{o}{\widehat{U}}_{ka}^{}}{{\widehat{Z}}_{k}^{+}{\widehat{Z}}_{k}^{-}+{\widehat{Z}}_{k}^{+}{\widehat{Z}}_{k}^{o}+{\widehat{Z}}_{k}^{+}{\widehat{Z}}_{k}^{o}}$ | 0 |

${\widehat{J}}_{k}^{0}$ | $\frac{{\widehat{U}}_{ka}^{}}{{\widehat{Z}}_{k}^{+}+{\widehat{Z}}_{k}^{-}+{\widehat{Z}}_{k}^{o}}$ | 0 | $\frac{-{\widehat{Z}}_{k}^{-}{\widehat{U}}_{ka}^{}}{{\widehat{Z}}_{k}^{+}{\widehat{Z}}_{k}^{-}+{\widehat{Z}}_{k}^{+}{\widehat{Z}}_{k}^{o}+{\widehat{Z}}_{k}^{+}{\widehat{Z}}_{k}^{o}}$ | 0 |

Phase | A | B | C |
---|---|---|---|

I1 [A]/angle[°] | 84.09/−0.33 | 84.09/−120.33 | 84.09/119.67 |

I2 [A]/angle[°] | 14.14/−0.31 | 14.14/−120.31 | 14.14/119.69 |

I3 [A]/angle[°] | 41.89/−0.39 | 41.89/−120.39 | 41.89/119.61 |

I4 [A]/angle[°] | 14.14/−0.31 | 14.14/−120.31 | 14.14/119.69 |

I5 [A]/angle[°] | 0.10/−4.50 | 0.10/−124.50 | 0.10/115.50 |

I6 [A]/angle[°] | 0.10/−4.50 | 0.10/−124.50 | 0.10/115.50 |

I7 [A]/angle[°] | 27.94/−0.42 | 27.94/−120.42 | 27.94/119.58 |

I8 [A]/angle[°] | 0.10/−4.50 | 0.10/−124.50 | 0.10/115.50 |

I9 [A]/angle[°] | 0.10/−4.50 | 0.10/−124.50 | 0.10/115.50 |

I10 [A]/angle[°] | 13.83/179.78 | 13.83/59.78 | 13.83/−60.22 |

I11 [A]/angle[°] | 13.83/179.78 | 13.83/59.78 | 13.83/−60.22 |

I12 [A]/angle[°] | 13.97/−0.44 | 13.97/−120.44 | 13.97/119.56 |

I13 [A]/angle[°] | 13.83/179.78 | 13.83/59.78 | 13.83/−60.22 |

I14 [A]/angle[°] | 13.83/179.78 | 13.83/59.78 | 13.83/−60.22 |

**Table 3.**The Fault Calculation Results for 3LG Fault at the Bus 9 in grid-connected mode of operation.

Phase | A | B | C |
---|---|---|---|

I1 [A]/angle[°] | 3527.39/−53.53 | 3527.39/−173.53 | 3527.39/66.47 |

I2 [A]/angle[°] | 33.43/−109.14 | 33.43/130.86 | 33.43/10.86 |

I3 [A]/angle[°] | 17.76/−4.84 | 17.76/−124.84 | 17.76/115.16 |

I4 [A]/angle[°] | 3492.92/−53.36 | 3492.92/−173.37 | 3492.92/66.64 |

I5 [A]/angle[°] | 17.69/−118.40 | 17.69/121.60 | 17.69/1.60 |

I6 [A]/angle[°] | 17.69/−118.40 | 17.69/121.60 | 17.69/1.60 |

I7 [A]/angle[°] | 11.84/−4.88 | 11.84/−124.88 | 11.84/115.12 |

I8 [A]/angle[°] | 21.02/−86.41 | 21.02/153.59 | 21.02/33.59 |

I9 [A]/angle[°] | 3473.37/−53.21 | 3473.37/−173.21 | 3473.37/66.79 |

I10 [A]/angle[°] | 20.80/−133.38 | 20.80/106.62 | 20.80/−13.38 |

I11 [A]/angle[°] | 20.80/−133.38 | 20.80/106.62 | 20.80/−13.38 |

I12 [A]/angle[°] | 5.92/−4.90 | 5.92/−124.90 | 5.92/115.10 |

I13 [A]/angle[°] | 20.81/−94.43 | 20.81/145.57 | 20.81/25.57 |

I14 [A]/angle[°] | 20.82/−90.04 | 20.82/149.96 | 20.82/29.96 |

Phase | A | B | C |
---|---|---|---|

I1 [A]/angle[°] | 487.81/−76.25 | 487.81/163.75 | 487.81/43.76 |

I2 [A]/angle[°] | 42.57/−87.51 | 42.57/152.49 | 42.57/32.49 |

I3 [A]/angle[°] | 2.21/−26.68 | 2.21/−146.68 | 2.21/93.33 |

I4 [A]/angle[°] | 444.19/−75.46 | 444.19/164.54 | 444.19/44.54 |

I5 [A]/angle[°] | 21.12/−88.35 | 21.12/151.64 | 21.12/31.65 |

I6 [A]/angle[°] | 21.12/−88.35 | 21.12/151.64 | 21.12/31.65 |

I7 [A]/angle[°] | 1.48/−26.74 | 1.48/−146.74 | 1.48/93.26 |

I8 [A]/angle[°] | 20.97/−89.17 | 20.97/150.83 | 20.97/30.83 |

I9 [A]/angle[°] | 423.61/−74.83 | 423.61/165.17 | 423.61/45.18 |

I10 [A]/angle[°] | 20.82/−90.02 | 20.82/149.98 | 20.82/29.98 |

I11 [A]/angle[°] | 20.82/−90.02 | 20.82/149.98 | 20.82/29.98 |

I12 [A]/angle[°] | 0.74/−26.75 | 0.74/−146.75 | 0.74/93.25 |

I13 [A]/angle[°] | 20.82/−90.02 | 20.82/149.98 | 20.82/29.98 |

I14 [A]/angle[°] | 20.82/−90.02 | 20.82/149.98 | 20.82/29.98 |

Fault type | 3LG | ||
---|---|---|---|

Operation mode | Grid-connected | Islanded | |

I1 [A] | Fortran | 3527.39 | 487.81 |

Typhoon HIL | 3470.69 | 447.00 | |

I5 [A] | Fortran | 17.69 | 21.12 |

Typhoon HIL | 17.47 | 20.27 | |

I6 [A] | Fortran | 17.69 | 21.12 |

Typhoon HIL | 17.47 | 20.27 | |

I8 [A] | Fortran | 21.02 | 20.97 |

Typhoon HIL | 20.13 | 20.70 | |

I9 [A] | Fortran | 3473.37 | 423.61 |

Typhoon HIL | 3500.65 | 500.48 | |

I14 [A] | Fortran | 20.82 | 20.82 |

Typhoon HIL | 21.30 | 20.89 |

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**MDPI and ACS Style**

Simic, N.; Strezoski, L.; Dumnic, B.
Short-Circuit Analysis of DER-Based Microgrids in Connected and Islanded Modes of Operation. *Energies* **2021**, *14*, 6372.
https://doi.org/10.3390/en14196372

**AMA Style**

Simic N, Strezoski L, Dumnic B.
Short-Circuit Analysis of DER-Based Microgrids in Connected and Islanded Modes of Operation. *Energies*. 2021; 14(19):6372.
https://doi.org/10.3390/en14196372

**Chicago/Turabian Style**

Simic, Nikola, Luka Strezoski, and Boris Dumnic.
2021. "Short-Circuit Analysis of DER-Based Microgrids in Connected and Islanded Modes of Operation" *Energies* 14, no. 19: 6372.
https://doi.org/10.3390/en14196372