Medium Voltage Transformer Selection Guide
Specifying a medium voltage transformer means translating network data and load requirements into a buildable design. This guide lists the decisions in the order an engineer should make them.
It applies to 11 kV, 22 kV and 33 kV distribution transformers for industrial and commercial installations.
Specifying a medium voltage transformer means translating network data and load requirements into a buildable design. This guide lists the decisions in the order an engineer should make them.
It applies to 11 kV, 22 kV and 33 kV distribution transformers for industrial and commercial installations.
1. Collect the network data
System voltage and frequency; system earthing arrangement; declared fault level at the point of connection; required insulation level (BIL); ambient conditions and altitude; and any utility-specific requirements.
2. Define the duty
Rating, load profile, largest motor and starting method, harmonic content, expected overloads and future expansion.
3. Choose the electrical design
Vector group. Dyn11 unless there is a reason to do otherwise
Impedance. Selected from fault level and voltage drop limits, typically 4–8%
Tapping. Off-circuit ±2 × 2.5% as standard; OLTC where voltage varies continuously
Windings. Copper for lower losses and compactness; aluminium for lower cost and weight
4. Choose construction and cooling
Oil immersed ONAN or ONAF for outdoor and higher ratings; dry type AN or AF for indoor and fire-sensitive installations. Confirm the enclosure IP rating and corrosion protection for the site atmosphere.
5. Specify protection and monitoring
For oil units: Buchholz relay, pressure relief, oil level, winding and oil temperature. For dry type: PTC sensors and a temperature relay with alarm and trip. Add surge arresters at the terminals.
6. Interface with switchgear
Confirm bushing type and arrangement, cable box position and entry direction, and the signals required by the protection scheme and SCADA or BMS.
7. Confirm standards and testing
IEC 60076 or IEEE C57.12, plus any local utility approval. Agree the routine test programme and whether type test reports are required.
Frequently Asked Questions
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Not sure which rating fits your project? Send us your load list and we will size it for you, or email info@xsdfftransformer.com, call +86 158 6789 7761 or WhatsApp +852 5416 2620.