Q2Power System Protection

DTOC Relay Protection of a Distribution Feeder

A Definite Time Over-Current (DTOC) relay is an overcurrent relay whose operating time, once the fault current exceeds its pick-up setting, is a fixed, pre-set time delay entirely independent of how much the fault current actually exceeds the pick-up value (in contrast to an inverse-time relay, whose operating time decreases as fault current increases, as discussed in the corresponding overcurrent coordination answer elsewhere in this paper).

Application to a Radial Distribution Feeder

Consider a radial distribution feeder supplied from a single source at one end, with multiple sequential circuit breakers (CB-A nearest the source, then CB-B, then CB-C nearest the load end, as illustrated) protecting successive sections of the feeder, each fitted with a DTOC relay. The DTOC relays are coordinated by assigning progressively shorter time-delay settings to relays located further from the source (closer to the load end): CB-C (nearest the load, protecting the final feeder section) is set with the shortest time delay (for example, 0.4 seconds); CB-B (protecting the middle section) is set with an intermediate time delay, incremented above CB-C's setting by a suitable coordination time margin (for example, 0.8 seconds, an increment of 0.4 seconds above CB-C); and CB-A (nearest the source, protecting the first feeder section) is set with the longest time delay (for example, 1.2 seconds, again incremented by the same 0.4-second margin above CB-B).

Operation for a fault in Section 3 (nearest the load): a fault occurring in the feeder section between CB-B and CB-C is detected by all three relays (CB-A, CB-B, and CB-C), since fault current flows through all three breaker locations in a simple radial feeder configuration, but CB-C, having the shortest time delay, operates first (after 0.4 seconds), clearing the fault and interrupting the fault current — once CB-C has tripped and cleared the fault, the fault current seen by CB-A and CB-B ceases, and since their own longer time delays (0.8s and 1.2s respectively) have not yet elapsed, they reset without ever tripping, correctly leaving only the smallest possible affected feeder section (beyond CB-C) de-energized.

Operation for a fault in Section 1 (nearest the source), with correct back-up if CB-A's own downstream relay fails: for a fault occurring in the section immediately beyond CB-A (between CB-A and CB-B), only CB-A (and, being further upstream, potentially further breakers beyond CB-A toward the source, if the network is not a purely two/three-breaker illustrative example) sees this fault current in a purely radial topology, so CB-A operates after its own 1.2-second delay to clear it. If instead a fault occurs in Section 2, both CB-A and CB-B see the fault current, but CB-B, with its shorter time delay (0.8s), operates first, clearing the fault before CB-A's longer 1.2-second delay elapses — CB-A's longer setting therefore only actually results in it tripping if CB-B (or its breaker) fails to clear the Section-2 fault, providing exactly the kind of relay/breaker back-up protection discussed in the corresponding primary/back-up protection answer elsewhere in this paper, since CB-A's relay continues timing throughout the fault and will trip after its own 1.2-second delay if the fault is not cleared sooner by CB-B.

Advantages and Limitation of DTOC Coordination

The principal advantage of DTOC relay coordination is its conceptual and practical simplicity, requiring only a single time-delay setting to be selected for each relay along the feeder, based purely on maintaining an adequate coordination time margin (commonly 0.3-0.5 seconds, accounting for breaker interrupting time and relay overshoot/timing tolerance) relative to the next relay downstream, without needing to account for how fault current magnitude itself varies with fault location. The principal limitation, however, is that this simplicity comes at the cost of longer fault-clearing time for relays positioned closer to the source (CB-A in this example, at 1.2 seconds), even though faults occurring closer to the source generally produce the largest fault current magnitude (and hence pose the greatest potential for equipment damage and system stability impact) — this limitation is precisely what motivates the more widely-used inverse-definite-minimum-time (IDMT) relay characteristic in practical distribution feeder protection, which achieves a similar graded-time-delay coordination principle while allowing faster clearing of the more severe, higher-magnitude faults occurring closer to the source, as discussed in the corresponding overcurrent relay coordination answer elsewhere in this paper.