What are the limitations of using traditional overcurrent protection schemes in microgrids with bidirectional power flow?

Traditional overcurrent protection schemes, which rely on detecting excessive current flow to isolate faults, face significant limitations in microgrids with bidirectional power flow. In a traditional radial power system, power flows in one direction from the source to the load, and overcurrent relays are designed to trip based on the magnitude and direction of the current. However, microgrids often have multiple sources of generation, including distributed generation (DG) units like solar PV, wind turbines, and battery energy storage systems (BESS). These DGs can inject power into the grid from various locations, creating bidirectional power flow. This bidirectional power flow makes it difficult for overcurrent relays to accurately determine the fault location. For example, a fault on a feeder might be fed from both the main grid and a local DG unit. The overcurrent relay closest to the fault might not see a sufficiently high current to trip, because the fault current is being supplied from both directions. This can lead to delayed fault clearing or even failure to clear the fault, increasing the risk of equipment damage and system instability. Another limitation is the potential for sympathetic tripping. In a microgrid with multiple DGs, a fault on one feeder can cause overcurrent relays on other feeders to trip unnecessarily, leading to a widespread outage. This is because the fault current from the DGs can flow through multiple relays, causing them all to see an overcurrent condition. Furthermore, the fault current contribution from inverter-based DGs is typically limited to protect the inverter's power electronic devices. This means that the fault current might not be high enough to activate traditional overcurrent relays, especially for faults that are located far from the source. To overcome these limitations, microgrids with bidirectional power flow require more sophisticated protection schemes, such as adaptive protection, differential protection, or distance protection. These schemes can take into account the bidirectional power flow and the limited fault current contribution from inverter-based DGs to provide more reliable and selective fault clearing.