Relay Protection Academy Module 09 of 25
Module 09 Intermediate

Overcurrent Protection
(IDMT)

⌛ ~2.5 hours 📚 IEC 60255-151 / ANSI C37.112 📑 12 slides

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Module 09 / 9.1

Discrimination Principles

Time grading (Definite Time)
Fixed delay per relay, longest at source. Simple. Source relay clears the worst faults most slowly.
Current grading
Different pick-up levels per relay. Only works on radial feeders where fault current falls with distance.
IDMT
Operating time inversely proportional to fault current. Fast for large faults; flattens at a definite minimum. Best of both above methods.
Directional control
Voltage polarisation restricts operation to one direction of current flow. Essential on ring mains and parallel feeders.
IDMT advantage Highest fault current is at the source end - IDMT automatically trips fastest there. Definite time does the opposite: slowest at source.

Module 09 / 9.2

IEC IDMT Characteristics

General form
Standard Inverse (SI)

Flat curve. Wide current range.
Very Inverse (VI)

Steeper. Better for large impedance changes.
Extremely Inverse (EI)

Matches fuse thermal response. Required when grading with downstream fuses.
IEC vs. ANSI ANSI uses Time Dial (TD), not TMS, with different constants. Mixing equation sets causes severe miscoordination.

Module 09 / 9.3

Grading Margin

IEC formula
Typical result Numerical relays + fast CB: 0.3 s. Electromechanical + slow CB: 0.4 s.
Terms
  • = relay timing error (%) - 5 to 7.5%
  • = CT ratio error allowance
  • = CB interrupting time
  • = relay overshoot
  • = safety margin
Re-evaluate at each fault current is a percentage of operating time - margin grows at low multiples. A fixed 0.4 s rule fails at low fault currents.

Module 09 / 9.4

Current Setting and Drop-off

Minimum pick-up
Drop-off ratio = 0.95. Setting exactly at max load leaves relay permanently picked up on load transients.
  • Grade with fuse: set
  • Grade with relay: use the same curve - parallel curves never cross
  • Grade with fuse: always use EI characteristic
TMS out of range If optimal TMS falls below 0.05 (minimum settable), raise to reduce the current multiple and increase the required operating time, allowing a higher TMS. This is not a grading compromise.

Module 09 / 9.5

High-Set Instantaneous

Setting rule
130% provides security margin against CT and relay errors.
  • Operates in <1 cycle - no intentional delay
  • Allows IDMT TMS to be raised - better downstream grading
  • Reduces arc energy for close-in faults
Not applicable everywhere On long cable feeders where max and min fault currents are similar throughout the zone, a high-set will overreach for external faults. Use IDMT only.

Module 09 / 9.6

Directional Overcurrent

90-30 connection (MTA = 60°)
vs. . 30° offset aligns MTA with inductive fault typical of transmission.
90-45 connection (MTA = 45°)
45° offset gives wider operating zone for resistive faults. Better suited to distribution networks.
Polarising Uses a reference voltage to determine current direction. RCA = angle of max torque. Essential on parallel feeders and ring mains.
⎓
Diagram needed

Phasor diagram 90-45 directional relay Phase A: Vbc polarising vector, Ia fault current lagging Va, 45 degree MTA, zero torque line perpendicular. Label Va, Vbc, Ia, MTA. Dark background, coloured vectors.

Module 09 / 9.7

Ring Main: Open-Point Method

  1. Conceptually open the source breaker (thought experiment only)
  2. Ring becomes two radial feeders - one clockwise, one anti-clockwise
  3. Grade each direction as a simple radial: lowest TMS at open point, highest at source
  4. Each relay has two elements - one per current direction
Open-point relay Sees faults in its short section only - no downstream relay to back up, so gets the lowest TMS (typically 0.05-0.1).
⎓
Diagram needed

Ring main loop with source at top, 4-6 substations. Two directional relay symbols at each bus (CW and CCW). Dotted cut line at source breaker. TMS values increasing from open point to source in both directions. Dark background.

Module 09 / 9.8

Earth Fault: Effective Setting and CBCT

Residual connection
Three CT secondaries summed. Only one CT drives the relay; the others load it with exciting current. Effective setting is relay setting plus all three CT exciting currents - often several times the nominal setting.
Core Balance CT (CBCT)
Toroidal CT around all three phase conductors. Phase currents cancel; only zero-sequence current produces net flux. Sensitivity below 0.5% of rated - impossible with residual connection.
Critical CBCT error Cable gland earth strap must be routed back through the CBCT window before connecting to earth. Bypassing the window cancels the fault current signal - the relay becomes blind to earth faults.

Module 09 / 9.9

Petersen Coil and Wattmetric

Tuned neutral inductor resonates with earth capacitance - earth fault current near zero. Standard OC relays cannot detect faults.

Wattmetric formula
tuned near 0° to detect the small resistive component.
Direction identifies faulted feeder Faulted feeder: active power flows away from busbar. Healthy feeders: power flows toward busbar.
Isolated neutral No earth fault current path. Healthy phases rise to times normal. Detect by neutral displacement voltage (3V0) from broken delta VT.

Module 09 / Worked Example

IDMT Coordination: Radial Feeder

Given Downstream fuse 160 A, relay F2 at 0.05 s. Relay 1: max load 400 A, CT 500/1, max fault 9.33 kA, margin 0.3 s, EI characteristic.

Relay 1 - current setting

Load: . Fuse backup: . Select , multiple = 9330/620 = 15.05.

Relay 1 - TMS

Final settings Relay 1: EI, 620 A, TMS 1.0. Relay 3 (worst-case Bus B): EI, 1060 A, TMS 0.85.

Module 09

Knowledge Check