Relay protection current coordination time

Relay coordination time ensures that the protective relay closest to a fault operates first, with upstream relays acting as backups after a deliberate time delay known as the Coordination Time Interval (CTI).

Overview of Relay Coordination

Relay coordination is the process of setting protective devices so that only the faulted section of a power system is isolated, preventing unnecessary outages in other parts of the network . This is achieved by grading the operating times of relays in series: the downstream relay (closer to the load) operates first, while upstream relays (closer to the source) operate later if the downstream relay fails .

Time-Current Coordination

Time-current coordination uses time-current characteristic (TCC) curves to determine relay operating times based on fault current magnitude . There are two main types of overcurrent relays:

• Definite-time relays: Operating time is fixed and independent of fault current magnitude.
• Inverse-time relays: Operating time decreases as fault current increases, providing faster response for higher fault currents . The time grading ensures that the relay nearest the fault trips first, while upstream relays provide backup protection.

Coordination Time Interval (CTI)

The Coordination Time Interval (CTI) is the deliberate time margin between the operating times of upstream and downstream relays at the same fault current . It ensures that the downstream relay clears the fault before the upstream relay operates. Typical CTI values depend on relay technology:

• Electromechanical relays: ~0.40 s
• Static relays: ~0.35 s
• Microprocessor relays: ~0.20 s
• Mixed technologies: ~0.30 s CTI is calculated as: CTI = Upstream relay operating time − Downstream relay operating time This interval must be sufficient to account for breaker clearing time and ensure selectivity .

Practical Considerations

• Relay settings are determined using system data: single-line diagrams, transformer impedances, short-circuit currents, motor starting currents, and CT performance .
• Plotting TCC curves for all relays on a common scale helps visualize coordination and avoid curve crossing, which can compromise selectivity .
• IEC and IEEE standards (IEC 60255, IEC 60947, IEEE C37.112) provide guidelines for relay performance, time grading, and selectivity margins .
• Radial networks benefit from inverse-time relays, which speed up operation for high fault currents and simplify coordination with fuses .

Summary

Relay protection current coordination time is critical for selective fault isolation. By using time-current curves, inverse or definite-time relays, and a properly calculated CTI, engineers ensure that the relay closest to the fault operates first, while upstream relays act as backups. Adhering to IEC/IEEE standards and plotting TCC curves are essential steps for reliable and safe protection system design .