Abstract
Fault currents inside a grid-connected AC microgrid are significantly varied because fault current contributions of the main grid and DG units are different from each other due to various fault locations, fault types, and high penetration of inverter-based distributed generators (IBDGs) and rotating-based distributed generators (RBDGs). A traditional fault-analysis method cannot be sufficiently applicable for AC microgrids with the presence of both rotating-based distributed generators and inverter-based distributed generators. From the above viewpoint, this paper proposes a simplified and automated fault-current calculation approach for grid-connected AC microgrids to quickly and accurately calculate fault-current contributions from IBDGs and RBDGs as well as the grid fault-current contribution to any faulted microgrid sections. The simplified and automated fault-current calculation approach is mainly focused on grid-connected and small-sized low-voltage AC microgrids with the support of communication system. Under the grid-connected microgrid operation mode, fault-tripping current-thresholds of adaptive overcurrent relays are properly adjusted thanks to the proposed fault analysis method. Relying on fault-current distribution-coefficients of IBDGs, RBDGs, and the utility grid, the setting values of adaptive overcurrent relays in a low-voltage AC microgrid are effectively self-adjusted according to various microgrid configurations and the operation status of DG units during the grid-connected mode.
Appendices
System parameters of a radial 380V AC microgrid configuration.
| Components | Specifications1 | |
|---|---|---|
| Grid source | 500kVA, impedance – 4 %, 11.4kV | |
| Transformer | 500kVA, impedance – 4 %, 11.4/0.38kV | |
| Line i-j | 0.0722Ω/km, length – 1km | |
| DG-i | IBDG-i | 100kVA, impedance – 10 %, 380V, a maximum load current – 150A The minimum fault current, Ifault_DGi_min = 180A The maximum fault current, Ifault_DGi_max = 300A |
| RBDG-i | 100kVA, impedance – 10 %, 380V, a maximum load current – 150A | |
| DG-j | IBDG-j | 150kVA, impedance – 12 %, 380V, a maximum load current – 230A The minimum fault current, Ifault_DGj_min = 276A The maximum fault current, Ifault_DGj_max = 460A |
| RBDG-j | 150kVA, impedance – 12 %, 380V, a maximum load current – 230A | |
Note: A three-phase fault is assumed to occur in the microgrid as seen in Figure 4; thus only positive-sequence impedances are given in the table.
System parameters of a looped 380V AC microgrid configuration.
| Components | Specifications1 | |
|---|---|---|
| Grid power source | 500kVA, impedance – 4 %, 11.4kV | |
| Transformer | 500kVA, impedance – 4 %, 11.4/0.38kV | |
| Line i-j | 0.0722Ω/km, length – 1km | |
| Line i-k | 0.0722Ω/km, length – 1km | |
| Line k-m | 0.0722Ω/km, length – 1km | |
| Line m-j | 0.0722Ω/km, length – 1km | |
| DG-i | IBDG-i | 100kVA, impedance – 10 %, 380V, maximum load current – 150A The minimum fault current, Ifault_DGi_min = 180A The maximum fault current, Ifault_DGi_max = 300A |
| RBDG-i | 100kVA, impedance – 10 %, 380V, maximum load current – 150A | |
| DG-j | IBDG-j | 150kVA, impedance – 12 %, 380V, maximum load current – 230A The minimum fault current, Ifault_DGj_min = 276A The maximum fault current, Ifault_DGj_max = 460A |
| RBDG-j | 150kVA, impedance – 12 %, 380V, maximum load current – 230A | |
| DG-k | IBDG-k | 100kVA, impedance – 10 %, 380V, maximum load current – 150A The minimum fault current, Ifault_DGk_min = 180A The maximum fault current, Ifault_DGk_max = 300A |
| RBDG-k | 100kVA, impedance – 10 %, 380V, maximum load current – 150A | |
| DG-m | IBDG-m | 100kVA, impedance – 10 %, 380V, maximum load current – 150A The minimum fault current, Ifault_DGm_min = 180A The maximum fault current, Ifault_DGm_max = 300A |
| RBDG-m | 100kVA, impedance – 10 %, 380V, maximum load current – 150A | |
Note: A three-phase fault is assumed to occur in the microgrid as seen in Figure 5; thus only positive-sequence impedances are given in the table.
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