- 2073 Bhadra · 8 marks
IDMT relays across 132/33 kV transformer (2000 A)
Short answer
R1 operating time = 0.56 s. R2 must operate at 1.06 s, requiring TMS = 0.272. Calculation uses relay characteristics: R1 PSM=10, t=2.8 s at TMS=1; R2 PSM=5, t=3.9 s at TMS=1, with fault current on 132 kV side = 500 A.
It is given that fault current level at 33 kV side is 2000 A. CT ratio at 33 kV side is 200:1 and 132 kV is 100:1 as shown in figure below. If both relays R1 and R2 are set for 100% plug setting, determine the operating time for both relays assuming that both the relays have same characteristics as shown in table below. For the discrimination the time grading margin between two relays is 0.5 second and the time setting multiplier for R1 is 0.2. Also determine the time setting multiplier for R2.
| PSM | 2 | 3.6 | 5 | 8 | 10 | 15 | 20 |
|---|---|---|---|---|---|---|---|
| Time in second for TMS = 1 | 10 | 6 | 3.9 | 3.15 | 2.8 | 2.2 | 2.1 |
[Figure: 132 kV bus – CB – CT 100/1 with relay R2 – 132/33 kV transformer – 33 kV bus – CB – CT 200/1 with relay R1 – fault on the 33 kV feeder, If = 2000 A.]
Both relays are IDMT relays graded by time: R1 (33 kV, near fault) is the primary relay and R2 (132 kV side of the transformer) is its back-up, set to operate 0.5 s later.
Data: If = 2000 A at 33 kV, CT of R1 = 200/1, CT of R2 = 100/1, plug setting 100% for both (pick-up = 1 A secondary), TMS of R1 = 0.2, margin = 0.5 s.
Relay R1
Pick-up (primary) = 1.0 × 200 = 200 A
PSM = 2000 / 200 = 10
t (TMS=1) from table = 2.8 s
t_R1 = 0.2 × 2.8 = 0.56 s
Fault current seen by R2
The current on the 132 kV side is reduced by the transformer ratio (ignoring magnetising current):
I(132 kV) = 2000 × 33/132 = 500 A
Relay R2
Pick-up (primary) = 1.0 × 100 = 100 A
PSM = 500 / 100 = 5
t (TMS=1) from table = 3.9 s
Required t_R2 = t_R1 + margin = 0.56 + 0.5 = 1.06 s
TMS_R2 = 1.06 / 3.9 = 0.272
| Quantity | R1 | R2 |
|---|---|---|
| Fault current seen | 2000 A | 500 A |
| Pick-up current | 200 A | 100 A |
| PSM | 10 | 5 |
| t at TMS = 1 | 2.8 s | 3.9 s |
| TMS | 0.2 | 0.272 |
| Operating time | 0.56 s | 1.06 s |
Answer: t_R1 = 0.56 s, t_R2 = 1.06 s, TMS of R2 = 0.272 (in practice the next available step, about 0.275, is chosen).
✓ Answer checked by Claude Opus 5.5 (high effort)
Related questions
- IDMT relays across 132/33 kV transformer (2700 A) · Switchgear and Protection, chapter 7
- IDMT relays across 132/11 kV transformer (3500 A) · Switchgear and Protection, chapter 7
- IDMT grading: R1, R2 with 1000/5 CTs · Switchgear and Protection, chapter 7 · asked 2×
- CT ratio and connection for 11/132 kV · Switchgear and Protection, chapter 7
- CT ratio for 33/6.6 kV star/delta (400/5) · Switchgear and Protection, chapter 7
- IDMT settings for 20 MVA transformer and feeder · Switchgear and Protection, chapter 7 · asked 2×