Open Delta at an Irrigation Site After a Lightning Failure
A three-phase 240 V, three-wire irrigation site is supplied by three 50 kVA single-phase transformers connected in closed delta on a 25 kV primary. Metered peak demand last season was 105 kVA, and the three 40 hp motors are all started within the same ten minutes each morning. During a Saturday afternoon thunderstorm one unit fails; the other two continue to carry the load in open delta. The only spare on your truck is a 75 kVA unit with the correct voltage ratio but a nameplate impedance of 2.4%, against 1.8% on the two surviving 50 kVA units. The customer wants all three pumps running by morning and the dispatcher wants to know whether the spare can go in tonight. The failed unit's primary cutout is closed with the fuse element intact, and its tank is bulged with oil on the pad. The site has no phase monitor and no recent load survey.
1. What can the two surviving units carry in open delta, and can the site run at its 105 kVA peak tonight?
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Work it out from rated current, not from kVA alone. A 50 kVA unit at 240 V is rated 50,000 / 240 = 208 A. In open delta each transformer carries the full line current of its own phase, so the bank capacity is 1.732 x 240 V x 208 A = 86.6 kVA, which is the familiar 86.6% of the 100 kVA installed in the two units.
105 kVA is about 21% over that. The answer is no, the site cannot run its normal peak tonight.
The reason this matters beyond the arithmetic: the two remaining units have no overload protection sized for this. Primary cutouts protect the transformer against faults, not against sustained overload, so an overloaded unit runs hot, ages fast and fails, and a second failure turns a temporary problem into a full replacement with the site dark for days. At 105 kVA with all three 40 hp motors starting in the same ten minutes, the starting current on top of the running load will also sag the voltage hard enough to trip contactors.
Realistic options tonight:
- Run two of the three pumps (about 70 kVA) with the customer's written agreement and a documented load restriction, and stagger the starts.
- Bring in a second 75 kVA unit and run a bank that can actually carry the load.
- Arrange a mobile transformer.
Before anything is re-energized, open that closed cutout. A bulged tank with the fuse intact means the fault was not cleared by the primary fuse - the fuse may be the wrong size or the cutout may not be making good contact. Do not re-energize a faulted transformer.
2. Can the 75 kVA spare be installed with the two 50 kVA units? Explain what happens if it is.
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Not in a closed delta, and it does not rescue the open delta either.
In a closed delta, load divides by impedance, not by nameplate kVA. With 2.4% impedance against 1.8%, the 75 kVA unit takes less than its proportional share while the two 50 kVA units take more, so the bank's usable capacity is set by whichever unit reaches full load first, not by the sum of the nameplates. On top of that, mismatched impedances in a closed delta circulate current around the delta loop even with no load on the bank, which shows up as heat and as unexplained primary current.
In an open delta with one 75 kVA and one 50 kVA, both units carry the same line current, so the 50 kVA unit's 208 A caps the bank at exactly the same 86.6 kVA. The big spare buys nothing.
Other things that must match regardless: exact turns ratio (check the taps on both units), additive polarity, and phase identification.
What I would recommend to the dispatcher: bring a matched 50 kVA unit tonight for a temporary three-unit bank if the load can be held under 150 kVA, or better, replace the bank with matched units. If the site is going to keep growing, quote a single three-phase transformer of adequate rating so the mismatch question never comes back.
3. What do you check at the customer's equipment and what do you record before leaving?
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At the bank and the service:
- The two surviving transformers: oil level, leaks, arrestor condition, primary cutout and fuse size, and tank temperature. Any sign of distress on a unit that has been running overloaded means it comes out too.
- Secondary voltage at the service, phase-to-phase and phase-to-neutral where applicable, with the load on. Compare all three phases; an unbalanced reading points at a connection problem rather than at the transformers.
- Neutral and bonding integrity at the service, and the condition of the customer's main disconnect and overloads.
At the customer's motors:
- Overload heater sizing and reset all tripped overloads only after the cause is understood.
- Rotation on any motor that was re-connected.
- Starting method: confirm the staggered start schedule is actually being followed, and check voltage dip at the farthest motor during a start.
Records and handover:
- Tag the bank with the temporary capacity limit and the date, and note it in the outage record so the next crew does not assume a full 150 kVA bank.
- Record the actual running load after restart so the restriction can be verified, and the failed unit's serial number and failure evidence for the asset record.
- Arrange the follow-up replacement in writing with the customer, and confirm the site has no generation that could backfeed the bank while you are working.