What is overrunning action in relay protection

Overrunning action is a relay protection strategy where upstream relays are allowed to operate after downstream relays have cleared a fault, ensuring proper coordination and selective tripping.

Definition and Purpose

Overrunning action occurs in overcurrent relay systems to maintain selectivity and coordination between relays installed at different points in a power network. When a fault occurs, the downstream relay closest to the fault operates first. If the fault persists, the upstream relay is allowed to trip after a predetermined time delay, effectively "overrunning" the downstream relay's operation if necessary . This ensures that only the faulty section is isolated, minimizing disruption to the rest of the system.

How It Works

1. Downstream Relay Operation: The relay nearest to the fault detects the overcurrent and initiates a trip after its set time delay.
2. Time Coordination: Upstream relays are set with longer time delays than downstream relays. This delay, often denoted as t1, ensures the downstream relay has sufficient time to clear the fault before the upstream relay acts .
3. Overrunning Action: If the downstream relay fails to clear the fault, the upstream relay trips after its time delay, providing backup protection. This is the essence of overrunning action, which prevents prolonged fault conditions and protects equipment from damage .

Advantages

• Selective Isolation: Only the faulted section is disconnected, maintaining service continuity elsewhere.
• Backup Protection: Upstream relays act as a safety net if downstream relays fail.
• Improved Reliability: Reduces the risk of cascading outages in complex power networks.

Practical Example

Consider a feeder with relays at points B, C, D, and E. If a short circuit occurs at location F:

• The relay at B (downstream) trips first after its set time.
• Relays at C, D, and E (upstream) are set with longer delays.
• If the relay at B clears the fault, upstream relays do not operate.
• If B fails, the relay at C trips after its delay, demonstrating overrunning action .

Relation to Other Relay Actions

• Instantaneous (50) Relays: Operate immediately for severe faults, with minimal inherent delay .
• Definite-Time (51) Relays: Operate after a fixed time delay once the current exceeds the set threshold .
• Overrunning Action: Specifically refers to the time-coordinated backup operation of upstream relays relative to downstream relays, rather than immediate or fixed-time tripping. In summary, overrunning action ensures proper coordination in relay protection systems, allowing upstream relays to act only if downstream relays fail, thereby enhancing system reliability and minimizing unnecessary outages .