- 2074 Bhadra · 8 marks
IDMT settings for sectionalised radial feeder A, B, C
Short answer
Relay C TMS 0.1 gives 0.22 s; B TMS 0.234 gives 0.585 s; A TMS 0.310 gives 0.869 s, based on grading margin 0.5 s and IDMT curve. Pick-up: C 100 A, B 225 A, A 400 A; PSM values: C 20, B 8.89 (or fault at C), A 7.5 (or fault at B).
IDMT relays for protection of sectionalised radial feeder shown in figure below and the method of selection of their settings are given below. Time setting multiplier is 1. [Figure: Source – bus A – CB_A, CT_A, relay R_A – bus B – CB_B, CT_B, relay R_B – bus C – CB_C, CT_C, relay R_C – feeder end.]
| Relay Point | CT ratio | Fault current | Current setting |
|---|---|---|---|
| A | 400/5 | 4000 A | 100% |
| B | 300/5 | 3000 A | 75% |
| C | 200/5 | 2000 A | 50% |
IDMT Characteristics:
| PSM | 2 | 4.2 | 6.5 | 8.8 | 10 | 12.4 | 13.3 | 16.3 | 18 | 20 |
|---|---|---|---|---|---|---|---|---|---|---|
| Time in sec | 8 | 4.2 | 3.8 | 3.1 | 2.8 | 2.6 | 2.5 | 2.4 | 2.3 | 2.2 |
The relays are graded so that C (farthest from source) trips first, B backs up C, and A backs up B, with a fixed time margin.
Assumptions: curve given for TMS = 1; TMS of relay C = 0.1 (fastest relay); grading margin = 0.5 s; CT secondary 5 A; linear interpolation of the table.
Pick-up = Current setting × CT primary
PSM = I_fault / Pick-up ; t = TMS × t(curve)
Relay C
Pick-up = 0.5 × 200 = 100 A
PSM = 2000/100 = 20 → 2.2 s
t_C = 0.1 × 2.2 = 0.22 s
Relay B
Pick-up = 0.75 × 300 = 225 A
Fault at C: PSM = 2000/225 = 8.89
t = 3.1 − (8.89−8.8)/(10−8.8) × 0.3 = 3.078 s
Required t_B = 0.22 + 0.5 = 0.72 s
TMS_B = 0.72/3.078 = 0.234
Fault at B: PSM = 3000/225 = 13.33 → 2.499 s
t_B = 0.234 × 2.499 = 0.585 s
Relay A
Pick-up = 1.0 × 400 = 400 A
Fault at B: PSM = 3000/400 = 7.5
t = 3.8 − (7.5−6.5)/(8.8−6.5) × 0.7 = 3.496 s
Required t_A = 0.585 + 0.5 = 1.085 s
TMS_A = 1.085/3.496 = 0.310
Fault at A: PSM = 4000/400 = 10 → 2.8 s
t_A = 0.310 × 2.8 = 0.869 s
| Relay | Pick-up | TMS | Back-up time | Own-fault time |
|---|---|---|---|---|
| C | 100 A | 0.10 | – | 0.22 s |
| B | 225 A | 0.234 | 0.72 s (fault at C) | 0.585 s |
| A | 400 A | 0.310 | 1.085 s (fault at B) | 0.869 s |
Answer: TMS_C = 0.1, TMS_B ≈ 0.234, TMS_A ≈ 0.31; times for faults at their own sections are 0.22 s, 0.585 s and 0.869 s. If a different TMS for C or margin is used, the same steps apply.
✓ Answer checked by Claude Opus 5.5 (high effort)
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