# AC Microgrids Protection: A Digital Coordinated Adaptive Scheme
## 4 Case Studies and Simulation Results
### Grid Connected Mode  
For line faults, DFR current-based PDs operates as main PD. These devices detect the fault conditions in one cycle. The PCC and feeder protection relays are considered as back up protections.  
In the grid connected mode, Fault #1 in the grid-side, Fault #2 occurred in Feeder #2 and Bus #3, and Fault #3 occurred between Feeder #1 and Bus #1 are investigated. Although all results are extracted for different types of faults, waveform results of an ABG fault at the location of Fault #2 is depicted in Figure 10.  
As can be observed from Figure 10, the upstream grid experience a voltage sag with 30% voltage drop in the location of Fault #1. Injected current from DGU #1 is shown in which, in its inner loop, a current limitation block is employed. The current limitation allows 20% over-current for fault and overload steady-state conditions, as it can be observed from the current wave forms. For the mentioned fault, DGU #1 feeds the fault. As can be expected, the current saturation of the DGU's inner loop, limits the fault current, and it leads to distortion of current waveforms. As can be observed from Boolean type waveform trip signals, the trip command coordination can be observed for Feeder PD, DGU PD, and PCC PD. They are coordinated based on their contribution for feeding the faults. It is worth highlighting that to analyze all the protection schemes, all the trip commands have been intentionally blocked, and thereby, the faults remains active for 2 s, which leads us to check the coordination procedure. As shown in subplots, after the occurred fault at t = 4 s, the DFR protection scheme for Line #11 issues the trip command after 0.003 s; then, the protection scheme for Feeder #1 operates, due to its directional OCR at t = 4.056 s, and consequently, Feeder #1's NSC protection scheme trips at t = 4.083 s. After that, the first DGU which contributes in feeding the faults is recognized as DGU #1 which its UVR and THD protection schemes trip at 4.112 and 4.150 s. After these, the protection schemes of the PCC relay, Feeder #2 and Bus #3 respond to the fault as the can be observed from Figure 10. The illustrated sequence of trip commands demonstrates a coordinated protection manned for an L-L grid-side fault.  
More faults in grid connected mode are also investigated to verify the proposed protection scheme. The investigated faults are not only symmetric/asymmetric faults in  
Figure 10: Voltage and current waveforms of DGU #2 during an ABG fault and coordinated trip signals for Fault #2 in grid-connected mode.  
the grid-side location, but also include faults occurred in the MG and downstream region. These faults and coordination assessing are given in Table 3, where for Fault #1 located in the grid side, all the fault types are considered. Faults #2 and #3 located in the downstream, as shown before in Figure 9, are also investigated the coordination point of view of the proposed protection scheme in the grid connected mode. As can be seen, DFR protection is the main PD, and all the backup PDs can operate in a coordinated way based on their contribution in feeding the fault.