A transformer manufacturer and exporter China for South America and Africa is asked for one product more than any other. It is the sealed oil-immersed distribution transformer that sits at the end of a feeder, on a pole or in a small kiosk. The unit described here is the S9-M, built from 30 to 1,600 kVA.
Two continents buy it, and they do not buy the same machine. Africa runs at 50 Hz. South America runs at 50 Hz in some countries and 60 Hz in others, and the voltage class changes with the country as well. That is why the same drawing cannot simply be sent to both places.
WHAT THE UNIT IS
The S9-M takes 6, 6.3, 10, 10.5 or 11 kV on the primary and gives 0.4 kV on the secondary. Tapping is ±5 %. The connection group is Dyn11, Yzn11 or Yyn0. Impedance is 4.0 % up to 500 kVA and 4.5 % from 630 kVA.
Sealed means there is no conservator. The corrugated wall flexes as the oil heats and cools, so the oil volume has somewhere to go, and the same wall does the cooling. Leaving the conservator off also takes height off the unit, which matters when it has to fit under a kiosk roof.
Oil goes in under vacuum, so it enters without air and without moisture. That single step decides how long the insulation holds up, and it is the reason a sealed unit can sit for years without an oil top-up.
ONE PRODUCT, TWO FREQUENCIES
Africa runs at 50 Hz, with few exceptions, and its distribution is built at 11 kV and 33 kV. Capacity is still being added across the continent, and projects such as the hydropower that now feeds Ethiopian towns only matter once a distribution network carries the power to them.
South America is not one grid. Brazil, Peru, Colombia, Ecuador and Venezuela run at 60 Hz. Argentina, Chile, Uruguay, Paraguay and Bolivia run at 50 Hz. Growth in the region is real, as the expansion behind Brazil's demand shows, but the country on the order decides the frequency.
The voltage split follows the same pattern. Part of the continent distributes at 13.8 kV, 22.9 kV or 34.5 kV, and this sheet stops at 11 kV. Where the grid runs above that, the enquiry belongs with the wider oil-immersed range up to 35 kV, not with the model in this article.
WHAT 60 Hz DOES TO A 50 Hz DESIGN
Flux in the core follows voltage divided by frequency. Hold the voltage and move from 50 to 60 Hz, and the flux drops to about five sixths of what it was. No-load loss falls with it, and so does the no-load current listed in the table as 2.3 % at 30 kVA and 0.8 % at 1,600 kVA.
Reactance goes the other way, because it rises with frequency. The impedance percentage climbs, the low-side fault current falls, and the core noise changes as well, since sound follows flux. None of this is an argument for or against a build. It is the reason the figures have to be restated for the grid the unit will actually serve.
The reverse direction is the one to avoid. Put a 60 Hz design on a 50 Hz grid and it runs at about 20 % more flux, saturates, and overheats. A 50 Hz build can serve a 60 Hz grid once losses and impedance are recalculated. A 60 Hz build cannot serve a 50 Hz grid.
THE IMPEDANCE STEP AND THE BREAKER BEHIND IT
At 50 Hz the table holds 4.0 % to 500 kVA and 4.5 % from 630 kVA. Raising the percentage at the larger ratings keeps the low-side fault level from climbing in step with the kVA, which is what keeps it inside a standard breaker range.
Change the frequency and that calculation moves with it. The protection study has to run on the impedance of the build that will arrive, not on a number copied from a project on another grid.
ALTITUDE, THE LINE UNDER THE TABLE
The sheet states two service conditions. Site altitude up to 1,000 m, and ambient temperature from −40 °C to +40 °C. Both are easy to skim, and both decide whether the standard design is the right one.
Several cities in these two markets sit well above that line. Bogotá is about 2,640 m, Quito about 2,850 m, Addis Ababa about 2,355 m. At those heights the air is thinner, so it carries heat away more slowly and insulates less well. The unit needs a design review, and the altitude belongs in the enquiry rather than in a footnote after the order.
This is the clearest reason a continent is not a specification. A 630 kVA unit for a town substation in West Africa and a 630 kVA unit for a site above Bogotá carry the same rating and are not the same machine.
THE COLD END AND THE HOT END
Minus 40 °C covers the inland cold of Patagonia and the highlands. Plus 40 °C covers the hot end, and that figure is ambient air, not oil temperature. A unit standing in a metal kiosk under direct sun runs above the shade temperature the sheet assumes.
Where the site goes past either limit, say so before the order. It changes the cooling surface and, at the cold end, the materials that have to stay serviceable.
WHAT DOES NOT CHANGE BETWEEN THE TWO MARKETS
Most of the unit stays the same, which is why one range can serve both continents. Every rating carries a pressure relief valve. A gas relay can be added, with contacts that alarm and trip. Each tank has a pocket for a glass-stem thermometer, set into the cover and dipping about 120 mm into the oil, and units from 1,000 kVA upward add an outdoor signal thermometer.
Tapping stays at ±5 % on a switch mounted on the cover, and the supply has to be off before the tap is moved. That is an operating instruction rather than a design feature, and it is worth handing to the crew that will run the unit.
WHAT THE ORDER HAS TO STATE
Six lines do most of the work. Rating in kVA. Primary and secondary voltage. Connection group. Frequency. Site altitude and ambient. Acceptance of the standard impedance step, or a stated alternative.
Two offers that agree on kVA and disagree on frequency are not comparable, even when the loss figures look close. Supply of distribution transformers has been tight for long enough that a specification corrected after the order is a problem nobody needs.
A transformer manufacturer and exporter China for South America and Africa can only build to the grid it is told about. The six lines are not paperwork. They are the difference between a unit that energises on arrival and one that has to be reworked.
WHO MAKES IT, WHO SHIPS IT
The S9-M and the wider oil-immersed range are manufactured by Wenzhou Xiaopai Electric Power Technology Co., Ltd. in Wenzhou, Zhejiang, and exported to South America and Africa. The same factory builds the dry-type range for sites where oil is not allowed, and the switchgear range for the substation the transformer stands in.
The iron-and-oil distribution transformer is a mature product. The solid-state designs that may one day replace it are still at demonstration stage. Until then, the choice is settled by voltage, frequency and site conditions, which is where this article began.
