Fault-Level Coordination on a 33 kV Feeder After Transformer Upsizing
A textile mill is fed from a dedicated 33/11 kV, 5 MVA, 8% impedance transformer through a 1.2 km 11 kV cable (positive-sequence impedance 0.18 + j0.09 ohm/km) to an 11 kV switchboard. The mill's maximum demand is 3.8 MVA at 0.88 power factor. The utility's 33 kV source fault level is 250 MVA. The existing 11 kV switchgear is rated 25 kA for 1 s. The mill now wants to replace the transformer with a 10 MVA unit of 9% impedance at the same location, keeping all downstream cables and switchgear. The protection engineer proposes retaining the existing 11 kV overcurrent relay settings, arguing that the fault current at the switchboard will rise only modestly. The plant manager wants the changeover completed over a weekend shutdown.
1. Estimate the three-phase fault current at the 11 kV switchboard before and after the transformer replacement, and judge whether the existing 25 kA switchgear and the unchanged relay settings remain acceptable.
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Refer everything to the 11 kV side. Source impedance from 250 MVA at 33 kV: Zs(33) = 33^2/250 = 4.36 ohm, referred to 11 kV by (11/33)^2 gives 0.484 ohm. Existing transformer: Zt = 11^2/5 x 0.08 = 1.936 ohm. Total = 2.42 ohm, so I_f = 11/(sqrt3 x 2.42) = 2.62 kA at the transformer terminals. Add the cable (0.18 + j0.09 per km, 1.2 km = 0.216 + j0.108 ohm) and the switchboard fault is roughly 2.5 kA.
New transformer: Zt = 11^2/10 x 0.09 = 1.089 ohm. Total = 1.573 ohm, I_f = 11/(sqrt3 x 1.573) = 4.04 kA at the terminals, about 3.8 kA at the switchboard. This is a 55-60% increase, not 'modest'.
Judgement: 3.8 kA is well below 25 kA, so the switchgear withstand is fine. The real problem is discrimination and cable thermal duty. The relay settings were chosen so the 11 kV feeder relay backs up the downstream LV main and motor relays at the old fault level; with a much stiffer source the downstream relays see higher currents and the grading margins (typically 0.3 s) may collapse, causing nuisance tripping of the incomer. Also check the 1 s short-time rating of cables against the higher let-through energy. Conclusion: switchgear acceptable, relay settings must be re-studied and the weekend changeover should not proceed until grading is re-verified.
2. The plant manager argues that because the switchgear rating is satisfied, the changeover can proceed without any protection study. Construct the technical rebuttal and list the specific checks required before energisation.
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The switchgear rating only proves the equipment will not be destroyed; it says nothing about selectivity, which is what keeps the mill running. A stiffer source raises fault current at every downstream point, so the current-grading margins between the 11 kV incomer, the 11 kV outgoing feeders and the LV main breaker shrink or invert. The consequence is a fault on one motor feeder tripping the whole mill instead of one circuit.
Checks required: (1) recompute maximum and minimum fault levels at each bus, including the minimum-generation/minimum-source case for relay reach; (2) re-plot time-current curves for all overcurrent and earth-fault relays and confirm at least 0.3 s grading margin at the maximum through-fault; (3) verify CT ratios and class are adequate for the new currents and that saturation is not reached during the DC offset; (4) check cable short-circuit withstand against the higher I^2t, especially the 1.2 km 11 kV cable; (5) check the transformer's own HV protection and the utility interface protection settings, since the utility may require the 33 kV relay to be re-graded; (6) verify the earthing system and earth-fault current, which also rises; (7) confirm the transformer's LV breaker is rated for the new through-fault. Only after these are signed off should energisation occur.