Distribution Transformer Buying Guide
A distribution transformer is selected from the network and load, not just the kVA printed on an old nameplate. Voltage ratio, load profile, fault level, losses, installation and documentation must be agreed together.
Short answer: select a distribution transformer from the actual HV and LV system, frequency, load profile, demand, power factor, starting currents, future capacity, vector group, impedance, loss limits, installation environment and required documents. A kVA rating alone is incomplete.
Collect the network data first
Record the upstream voltage and tolerance, frequency, earthing arrangement, short-circuit level, protection philosophy and the secondary distribution voltage. These values affect insulation, taps, vector group, impedance and terminals. A transformer may look similar while being unsuitable for the connected network.
Choose kVA from demand and operating pattern
Start with measured demand or an engineered load schedule, then include power factor, motor starting, harmonics where relevant, ambient temperature, load growth and the required reserve. Connected nameplate load is often larger than simultaneous demand, but undersizing can cause overheating and voltage drop during peak operation.
| Load question | Why it matters |
|---|---|
| Maximum and normal demand | Separates steady load from occasional peak |
| Motor starting and inrush | Can affect voltage dip and protection coordination |
| Power factor and harmonics | Influence current, heating and design review |
| Future expansion | Defines the economic reserve to include now |
Voltage ratio, taps and vector group must match the system
State high voltage, low voltage and frequency explicitly. Off-circuit taps allow adjustment when the transformer is de-energized; their range and step must be confirmed. Vector group affects phase displacement and paralleling. Dyn11 and Yyn0 are examples, not interchangeable defaults.
Losses and impedance are project data
No-load loss is present whenever the transformer is energized. Load loss varies with load current. Both affect operating cost and thermal performance. Percentage impedance affects short-circuit current and voltage regulation, so it must coordinate with switchgear, protection and parallel transformers. Do not request a “low-loss transformer” without stating the governing standard and loss values.
Oil-immersed installation requires site planning
Review indoor or outdoor location, ambient conditions, altitude, clearances, foundation, lifting, cable entry, fire and oil-containment requirements, ventilation, earthing and maintenance access. These are site and authority requirements, not assumptions that can be made from the product category.
Drawings and tests close the gap between a quotation and equipment
Agree the nameplate data, general arrangement, terminal layout, vector group, losses, impedance, tap positions, accessories and document scope before manufacture. Confirm routine-test records and any required type, special or witness testing against the accepted project standard.
S11/S13 as a project configuration
The Geararo S11/S13 series is listed for three-phase oil-immersed 10 kV, 20 kV and 35 kV networks with published capacities from 30 to 6,300 kVA. Published options include two-winding construction, off-circuit tap changing, self-cooling and Dyn11 or Yyn0 vector groups. Ratio, capacity, losses, impedance, terminals and accessories remain project-specific.
Information to prepare before requesting a quotation
- HV/LV voltage, frequency and earthing arrangement.
- Rated kVA, normal and peak load, power factor and expansion plan.
- Motor starting, harmonics and parallel-operation requirements.
- Vector group, impedance, tap range and loss requirements.
- Indoor/outdoor location, ambient, altitude and clearances.
- Terminal, cable, protection and accessory requirements.
- Applicable standard, drawings, test records and witness requirements.
- Destination, delivery and installation constraints.
Frequently asked questions
How do I choose transformer kVA?
Use the load profile, demand, power factor, starting currents and future load. Do not use connected nameplates alone.
Can transformers with the same kVA be substituted?
Not without checking ratio, vector group, impedance, losses, taps, terminals, environment and protection coordination.
What documents should be requested?
At minimum: accepted nameplate data, drawings, terminal details, test-document scope and the applicable standard.
Need a transformer configuration review?
Send the single-line diagram, load data, voltage levels, site conditions and document requirements. Geararo can review transformer and associated power equipment together.
Contact Geararo