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Overcurrent grading is generally a "straight forward" consideration of the maximum fault current of a downstream relay should result in a slower operation for the upstream relay.
However Delta-star transformers have a particular special consideration.
Consider a 66/11 kV Dyn transformer
Assume 1000 turns secondary
LV volts per turn = (11000/√3) / 1000 = 6.35 V/turn
On the HV delta side, there must be the equivalent V/turn = 6.35 V/turn
However at 66 kV phase-to-phase, this means there must be a total 10392 turns.
1-phase faults on LV
| LV 1-ph fault | 1000 A | ||
| Ph-ph LV | 11000 V | ||
| Ph-n LV | 6351 V | ||
| LV Turns | 500 | ||
| LV ampere.turns | 500000 | ||
| LV volt/turn | 12.7 | HV volt/turn | 12.7 |
| HV Ph-Ph | 11000 V | ||
| HV Turns | 866 | ||
| HV ampere.turns | 500000 | ||
| HV winding current | 577 A | ||
| HV line current | 577 A |
Ph-Ph fault on LV
| LV ph-ph fault | 866 A | LV overcurrent operates slower than for 1-ph fault | ||
| Ph-ph LV | 11000 V | |||
| Ph-n LV | 6351 V | |||
| LV Turns | 500 | |||
| LV ampere.turns | 433013 | |||
| LV volt/turn | 12.7 | HV volt/turn | 12.7 | |
| HV Ph-Ph | 11000 V | |||
| HV Turns | 866 | |||
| HV ampere.turns | 433013 | |||
| HV winding current | 500 A | |||
| HV line current "1" | 500 A | two phases operate slower than for a 1-ph LV fault | ||
| HV line current "2" | 1000 A | one phase operates faster than a 1-ph fault .. and faster than the LV relay for this ph-ph fault |
- Protection Systems Engineering
- IEC 61850 Engineering
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