MV Overcurrent Relay Tripping Ahead of the LV Breaker
A 500 kVA, 11/0.4 kV, Dyn11 distribution transformer serves a small industrial feeder from an 11 kV ring. The transformer has Z = 4.5 % and a magnetising inrush of about 10 × rated current decaying over roughly 0.1 s. The 11 kV vacuum circuit breaker is fitted with a digital overcurrent relay fed from 50/1 A phase CTs. Its phase overcurrent element (51) is set at Is = 60 A on an IEC Standard Inverse curve with TMS = 0.10, and a high-set element (50) is set at 400 A with 0.05 s definite time. The LV main circuit breaker is a 1000 A air circuit breaker with a short-time setting of 8 × In and 0.1 s. In service, every time a heavy fault occurs on an LV sub-feeder a few metres from the main board, the 11 kV breaker opens and the whole transformer is lost; the LV air circuit breaker shows no trip flag and the fault is cleared.
1. Using the data given, explain why the 11 kV breaker wins the race against the LV air circuit breaker. Support the explanation with calculated currents referred to the 11 kV side.
Read the worked answer
LV fault level at the transformer terminals: LV FLC = 500 000/(√3 × 400) = 722 A; Isc = 722/0.045 = 16.0 kA. A fault a few metres downstream on a heavy sub-feeder cable will still be around 10–14 kA.
Referred to 11 kV (ratio 11/0.4 = 27.5):
- 16.0 kA → 16 000/27.5 = 582 A
- 12 kA → 436 A
- 10 kA → 364 A
Against the relay settings:
- The high-set 50 at 400 A sees 582 A for a close-in LV fault and 436 A for a mid-cable fault. Both exceed the setting, so the element issues a trip after its 0.05 s definite delay. The breaker opens in roughly 50–80 ms total.
- The LV air circuit breaker is set to short-time at 8 × 1000 = 8 kA with 0.1 s delay, plus its own opening time - typically 150–200 ms before the arc is extinguished.
So the MV element is deliberately faster than the device that should clear the fault. The 50 element is doing exactly what it was set to do; the setting is simply wrong because it was chosen against the transformer's inrush and the 11 kV fault level without checking the maximum LV fault current reflected through the transformer. Once the transformer is energised there is nothing on the LV side that the 50 element should ever clear - the LV devices must always be allowed to clear first, and the MV relay is the backup.
The 51 element alone would not have caused this: at 582 A it is at 9.7 × Is, giving t = 0.10 × 0.14/(9.7^0.02 − 1) ≈ 0.30 s, which is slower than the LV breaker. The 50 element is the culprit.
2. Recommend revised settings or design changes, and state what you must check before adopting them.
Read the worked answer
1. Set the 50 element above the maximum reflected LV fault current, with margin. 1.3 × 582 = 757 A → adopt 800 A. Re-check that this is above inrush (10 × 26.2 = 262 A) - comfortably yes - and that it is below the minimum 11 kV-side fault current at the transformer terminals. With a 250 MVA source the 11 kV fault level is ~13 kA, so the 50 element still provides fast clearing for HV faults; it simply no longer reaches into the LV zone.
2. Restore time grading with the 51 element. With the 50 raised, close-in LV faults fall to the 51 at 0.30 s against an LV clearing time of 0.15–0.20 s - a margin of only about 1.5. Raise TMS to 0.15, giving ~0.45 s and a margin near 3, which is the usual target for an electromechanical-to-electronic mix.
3. Check before adopting:
- Transformer thermal withstand: 0.45 s at 16 kA must sit inside the transformer's short-circuit withstand (IEC 60076-5 typically gives 2 s at the LV fault current), which it does.
- LV switchboard Icw: the through-fault energy at 16 kA for 0.45 s must be within the board's 1 s rating.
- Inrush ride-through: at TMS 0.15 the 51 would operate in about 0.7 s at 4.4 × Is, so the 0.1 s inrush is not a problem, but confirm second-harmonic blocking or an inrush restraint feature is enabled on the relay.
- CT performance: 582 A on a 50/1 CT is 11.6 × rated; check the CT's accuracy limit factor and the lead burden, and confirm the 11 kV cable and CT are rated for the through-fault.
4. Design option: where a transformer outage is genuinely costly, add restricted earth fault or differential protection so that LV busbar faults are cleared by a dedicated element instead of by raising the backup settings indefinitely.