An oil-immersed and dry-type transformer manufacturer China for South America and Africa is usually asked for a rating and a vector group first. The question that decides the next twenty years is quieter. What has changed since the unit already standing in the yard was built?
Two models from one production file answer it, each set against its own predecessor at the same rating: an oil-immersed 4,000 kVA unit and a dry-type 2,500 kVA unit. Four figures carry the comparison. No-load loss, load loss, no-load current and mass.
THE MODEL NOW IN PRODUCTION
The S13 build in the file is a 4,000 kVA unit with a 33 kV primary and an 11 kV secondary, connected Yd11. It carries 70 A on the high side and 210 A on the low side, and both windings are copper.
No-load loss is 3,610 W and on-load loss is 27,300 W. No-load current is 0.45 %, mass is 8,800 kg, and the outline is 2,760 by 1,620 by 2,370 mm. The tank is corrugated, so the wall itself carries the cooling surface instead of a separate radiator bank.
The 35 kV oil-immersed range on the site covers S9 through S22, and this unit sits at the top of the kVA ladder in the file.
THE GENERATION IT REPLACES
At the same 4,000 kVA the file holds an S9 unit, built at 30 kV with a 0.8 kV secondary. Its no-load loss is 4,220 W, its on-load loss 34,890 W, its mass 8,195 kg, and its impedance 6.5 %. The winding is aluminium.
The two secondaries differ, 0.8 kV against 11 kV, so this is not a terminal-to-terminal comparison. It is the same rating built one generation apart, which is the choice a buyer faces when an old specification is still circulating in a tender.
FOUR NUMBERS, TWO FAMILIES, ONE STEP
| Build | Rating | No-load loss | On-load loss | Mass |
|---|---|---|---|---|
| S9-4000/30 | 4,000 kVA | 4,220 W | 34,890 W | 8,195 kg |
| S13-4000/33 | 4,000 kVA | 3,610 W | 27,300 W | 8,800 kg |
| SC(B)9-2500/35 | 2,500 kVA | 5,000 W | 21,500 W | 6,930 kg |
| SCB10-2500/34.5 | 2,500 kVA | 4,000 W | 21,800 W | 5,500 kg |
The SC(B)9 row is quoted at 120 °C. The other three rows carry no reference temperature in the file.
Read the two oil rows and both loss columns fall: 14.5 % on no-load, 21.8 % on load, with mass 7.4 % higher. Read the two dry rows and only one falls: no-load is 20 % lower, load loss is 1.4 % higher, and mass is 20.6 % lower.
The gain is not a better letter on the nameplate. It is 610 W off the no-load column and 7,590 W off the load column at 4,000 kVA, and 1,430 kg off the dry unit.
The gap between those two readings is the argument of this article. A generation change moves the no-load column almost by definition. It moves the load column only when the conductor changes with it.
The 35 kV cast-resin dry-type range runs SC(B)9 to SC13, and the 2,500 kVA build in this table sits in the middle of it.
THE NO-LOAD COLUMN IS A CORE DECISION
No-load loss is paid every hour the unit is energised, loaded or not. At 8,760 hours a year, 1 kW of standing loss is 8,760 kWh. The 610 W saved between the two oil builds is 5,344 kWh a year before one ampere reaches the secondary.
That figure is set in the core rather than in the winding. Core grade, lamination thickness and the flux density the design runs at decide it, and none of them can be changed after the core is stacked. A new generation earns its name here, or it does not earn it at all.
Regulators read the same column. The current round of US distribution transformer standards is aimed at exactly this number. Export buyers carry that expectation into a tender even when the local rule is looser, as the proposed DOE distribution transformer standards show.
No-load current reports the same design from the supply side. It falls from 0.6 % to 0.45 % on the oil pair. That is less reactive current drawn from the line for the same output, and less reactive power to account for at the meter.
THE LOAD COLUMN FOLLOWS COPPER
Load loss is resistive loss in the windings, so it follows conductor section, conductor material and the current the unit carries. Between SCB9 and SCB10 at 2,500 kVA the conductor stayed copper and the section stayed close. The figure moved 300 W the wrong way, from 21,500 W to 21,800 W, which is 1.4 %.
On the oil pair it moved because the metal changed. Aluminium gave way to copper, and 21.8 % of the load loss went with it. A buyer who reads only the generation letter would expect that drop on the dry pair as well, and would be wrong.
Both columns start with a resistance measurement on each winding, corrected to a reference temperature. A winding resistance meter that holds its accuracy at low values is what makes a 300 W difference visible in the first place.
ONE GOT HEAVIER, THE OTHER DID NOT
The S13 build is 605 kg heavier than the S9 it replaces, 8,800 kg against 8,195 kg. The corrugated tank is where that extra steel sits: the wall carries the cooling surface itself, which removes the separate radiator bank and puts the steel into the tank instead.
The dry pair moves the other way. SCB10 lists 5,500 kg against 6,930 kg for the SCB9 body, a fall of 1,430 kg, and the older unit adds about 100 kg more once its shell is fitted.
Mass decides the crane, the transport frame and the floor loading at the point of installation. A unit that survives the road is part of the specification, which is why heavy units travel in containment bags for transformer transport rather than on an open deck.
The oil side has its own version of the same question. A corrugated tank has fewer joints to seal than a tank with bolted radiator banks, and oil tightness decides how often the unit is topped up and inspected. Sealing oil leaks is a maintenance routine, not a design feature, and the fewer places there are to leak, the shorter that routine becomes.
THE VOLTAGES THE TWO CONTINENTS ASK FOR
The same file holds 34.5, 33, 20, 13.8, 12.47 and 11 kV primaries, with secondaries at 0.48, 0.44, 0.415, 0.4, 0.24 and 0.22 kV. The 34.5 kV and 13.8 kV builds follow South American distribution practice, and 33 kV with 11 kV follows what African utilities specify. That range is the practical side of being an oil-immersed and dry-type transformer manufacturer China for South America and Africa: the same generation has to exist at the voltage the site already runs.
A generation comparison only holds inside one voltage class. A low-loss design at 33 kV is no answer for a site that runs 34.5 kV. A dry unit wound for 34.5 kV is not interchangeable with a 33 kV unit, whatever its generation.
FIVE QUESTIONS THAT SETTLE A COMPARISON
Ask for no-load loss at rated voltage, and for load loss at a stated reference temperature. The SC(B)9 table quotes 120 °C, and the oil rows carry no temperature at all, so the basis has to be fixed before two numbers are set side by side.
Ask for no-load current as a percentage, because it sets the reactive draw at light load. Ask for mass with and without the enclosure, since the two differ by about 100 kg on the 2,500 kVA dry unit and by more on larger frames.
Ask for impedance and tap range last, because they fix the fault level the incoming breaker has to interrupt and the voltage swing the tappings can absorb.
WHO BUILDS BOTH
Wenzhou Xiaopai Electric Power Technology Co., Ltd. manufactures oil-immersed and dry-type transformers in Wenzhou, Zhejiang, and supplies both families to South America and Africa. The oil range runs S9 to S22 and the dry range SC(B)9 to SC13, both to 35 kV.
A plant usually orders the transformer together with the high-voltage switchgear that protects it, and one supplier can check the two against each other before either leaves the works.
The next step past S13 is solid-state, already demonstrated at megawatt class on a live feeder, and not yet a distribution product. A solid-state transformer changes the shape of the answer, not the arithmetic in this table.
A generation earns its name in the no-load column. It earns the load column only when the copper changes with it. Ask for both numbers at the same rating and the same temperature, and the letter on the nameplate stops mattering.
