11kV vs 33kV Oil-Immersed Transformer for Africa: How to Choose
Choosing an 11kV vs 33kV oil-immersed transformer starts with the electricity network, not the transformer capacity. The incoming voltage at the installation point decides the high-voltage winding.
Do not choose 11kV or 33kV only because another project in the same country used it. Both voltage levels may exist in one national grid, and they may serve different areas or stages of distribution.
A real Kenya procurement case
Kenya Power published tender KP1/9A.3/OT/34/24-25 on 18 June 2025 for pole-mounted oil-type distribution transformers. The schedule included both 11kV and 33kV units in the same procurement.
For the Nairobi South delivery lot, the document listed 213 units of 100kVA, 11/0.42kV, Dyn11 oil-type transformers. The same lot also listed 100 units of 100kVA, 33/0.42kV, Dyn11 oil-type transformers. Delivery was stated as six months after the purchase order.
The capacity, low-voltage output and vector group were the same, but the primary voltage was different. This is a useful example of why capacity alone cannot decide the voltage class.
This was a public Kenya Power procurement case, not a XINGNIU order. It is included only as verified market evidence.
1. Confirm the supply voltage in writing
Ask the utility, EPC contractor or electrical consultant for the voltage at the exact connection point. Use the approved single-line diagram, grid connection letter or project specification.
A country name is not enough. Different areas may use 11kV or 33kV, while a large site may receive 33kV and distribute 11kV internally. Write the ratio clearly, such as 11/0.415kV, 33/0.415kV or 33/11kV.
2. Understand the three common arrangements
An 11kV oil-immersed transformer with an 11/0.4kV ratio steps an 11kV supply down to the low-voltage system used by the site. A 33/0.4kV transformer performs a similar final distribution job from a 33kV supply.
A 33/11kV transformer normally supplies an 11kV network, which then feeds other transformers or medium-voltage equipment. These ratios are not interchangeable because the insulation, windings, bushings and protection are different.
3. Do not use kVA to guess the primary voltage
Transformer capacity is selected from the load. Primary voltage is selected from the grid connection.
A 100kVA unit can be designed for an 11kV or 33kV supply, as shown in the Kenya Power schedule. A larger transformer is not automatically 33kV, and a smaller unit is not automatically 11kV. Confirm voltage separately from the utility documents.
4. Check the required low-voltage output
African projects may specify 400V, 415V, 420V, 433V or another output. Do not write only “low voltage” in the inquiry.
The Kenya Power case used 0.42kV for the three-phase 100kVA units. Your project may require a different output, so copy the exact value from the approved design and confirm whether a neutral is required.
5. Confirm the other electrical parameters
The quotation should also state frequency, phase, vector group, tap range, transformer impedance and system earthing. These parameters affect voltage adjustment, protection and connection to the network.
Send the complete consultant specification when available. Guaranteed values belong in the approved technical sheet, and measured values should be checked in the factory test report.
6. Match the transformer to the site
Tell the manufacturer whether the unit will be pole-mounted, ground-mounted or installed in a substation. Provide ambient temperature, altitude, dust, humidity and corrosion conditions.
For an existing site, send foundation dimensions, terminal direction and cable information. Also confirm transport and lifting limits before production.
7. Compare an 11kV vs 33kV oil-immersed transformer quotation
An 11kV quotation and a 33kV quotation are not directly comparable. Their insulation, high-voltage components and clearances differ.
Check that every offer uses the same voltage ratio, winding material, cooling, tap changer, protection, tests and documents. Ask each supplier to list exclusions before comparing price.
Information to send with the inquiry
- Rated capacity in kVA
- Primary and secondary voltage
- Frequency and phase
- Vector group
- Tap range and tap-changing method
- Required impedance
- Cooling method
- Installation type and environment
- Applicable utility or project specification
- Quantity and destination
- Required drawings and test reports
- Delivery schedule
If one item is not confirmed, mark it as pending. Do not replace missing project data with an assumed standard value.
How XINGNIU supports overseas projects
XINGNIU is a source factory and can customize oil-immersed transformers to the confirmed voltage, capacity and site requirements. The design focuses on full rated capacity, low loss and controlled temperature rise.
Normal production lead time is 15 to 25 days after technical confirmation. The warranty is one year. Send your single-line diagram and specification so the team can review missing data before quoting.
FAQ
Is a 33kV transformer better than an 11kV transformer?
No. The correct voltage is the one required by the grid connection.
Can a 33kV transformer replace an 11kV transformer?
Not directly. The voltage ratio, insulation, bushings and protection are different.
Can the same 100kVA capacity be made for both voltage classes?
Yes, when the project design requires it. The Kenya Power procurement included 100kVA units at both 11/0.42kV and 33/0.42kV.
What document should I send first?
Send the approved single-line diagram or grid connection specification. It is the clearest source for the required primary and secondary voltage.
Conclusion
The choice between an 11kV vs 33kV oil-immersed transformer should come from the approved grid and project documents. Confirm the full voltage ratio, capacity, vector group, impedance, installation and testing requirements before comparing prices.
Planning an 11kV or 33kV transformer project? Send XINGNIU the single-line diagram, voltage ratio, capacity, site conditions and destination. We will review the requirements and prepare a custom quotation without guessing missing values.
