Relay protection 3 and 2 in parallel

Paralleling relays requires careful design because unequal current sharing and contact differences can lead to relay failure, while proper protection of parallel feeders uses directional and inverse-time overcurrent relays for selective tripping.Paralleling Relays: Risks and ConsiderationsConnecting relays in parallel to increase current capacity is generally not recommended. In an ideal scenario, both relays would share the current equally, but in practice, differences in contact resistance or timing can cause one relay to carry more current, potentially leading to overheating or failure. If one relay fails, the other may be overloaded, and during load interruption, the arc may concentrate on a single relay, increasing the risk of damage. Protective measures such as fuses or current sensors can mitigate this, but manufacturers like OMRON do not guarantee safe operation in parallel configurations .Protection of Parallel FeedersWhen two feeders are connected in parallel from a source to a load, selective isolation of the faulty feeder is critical to maintain continuity of supply. The protection scheme typically includes:Non-directional inverse-time overcurrent relays at the source end of each feeder to detect overcurrent conditions.Directional or reverse power relays at the load end to detect reverse power flow, which indicates a fault in the parallel path. For example, if a fault occurs on one feeder, the fault current splits between the two feeders. The directional relay on the feeder experiencing reverse current trips instantaneously, isolating only the faulty feeder, while the inverse-time relay on the other feeder may not operate because the current direction remains normal . This ensures selective tripping and prevents unnecessary disconnection of healthy feeders.Practical ImplementationTime grading: Relays closer to the fault operate first, while backup relays operate after a delay to ensure proper coordination.Directional relays: Instantaneous operation at the load end prevents reverse power flow from affecting the healthy feeder.Current sharing: In parallel relay setups, ensure each relay has a margin above the expected load current, or use individual fuses to prevent overcurrent damage .Key TakeawaysParalleling relays for increased current is risky due to unequal current sharing and potential relay failure.Parallel feeder protection requires a combination of inverse-time overcurrent relays at the source and directional relays at the load for selective tripping.Proper time grading and coordination are essential to maintain system stability and continuity of supply during faults. By following these principles, relays 3 and 2 in parallel can be integrated safely into a protection scheme, ensuring both reliability and selective fault isolation.