A distribution transformer is the final step in the power delivery chain – it steps down medium voltage to low voltage for direct use in homes, businesses, and industrial facilities. These transformers come in ratings from 16 kVA to 2500 kVA, and can be single-phase or three-phase, pole-mounted or ground-mounted. When selecting a distribution transformer, you typically choose between two main types: oil‑immersed or dry‑type. This guide explains the differences, voltage ranges, and how to optimize losses.

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1. Two Main Types of Distribution Transformers

1.1 Oil‑Immersed Distribution Transformer

In an oil‑immersed transformer, the windings and core are fully submerged in mineral oil. The oil provides both electrical insulation and efficient heat dissipation through ONAN (Oil Natural Air Natural) cooling – the oil circulates naturally and dissipates heat through the tank walls.

Key characteristics:

• High overload capacity due to oil‘s thermal mass
• Long service life (typically 20–30 years)
• Suitable for outdoor installations
• Cost-effective for medium to large ratings

1.2 Dry‑Type Distribution Transformer

In a dry‑type transformer, the windings are insulated with epoxy resin or NOMEX® insulating paper – without any liquid coolant. Cooling relies on natural air circulation (AN) or forced air (AF) with fans.

Key characteristics:

• Fire‑safe – no oil leakage risk
• Environmentally friendly
• Lower maintenance requirements
• Ideal for indoor installations (buildings, subways, hospitals)

Therefore, choose oil‑immersed for outdoor/industrial settings and dry‑type for indoor or fire‑sensitive areas.

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2. Voltage Ratings & Tap Arrangement

Distribution transformers typically have two windings per phase:

Winding	Voltage Range	Notes
Primary (high‑voltage)	1.0 kV – 38.5 kV	Connects to the medium‑voltage grid
Secondary (low‑voltage)	≤ 1.1 kV	Supplies end‑users (e.g., 400V three‑phase, 230V single‑phase)

Off‑Circuit Tapping

The high‑voltage winding usually includes off‑circuit (de‑energized) tappings to adjust the output voltage. Common configurations:

• ± 2.5% (two steps: +2.5%, -2.5%)
• +2 × 2.5%, -3 × 2.5% (wider range for grid voltage variations)

These taps allow you to fine‑tune the secondary voltage when the primary voltage fluctuates – but only when the transformer is de‑energized.

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3. Loss Optimization & Core Materials

Transformer losses (no‑load and load losses) directly impact operating costs over the equipment‘s 20‑30 year life. We use two advanced core materials to reduce losses:

Core Material	Advantage
Amorphous alloy	Ultra‑low no‑load losses (70–80% lower than conventional silicon steel)
Silicon steel (oriented grain)	Good balance of performance and cost, lower losses than standard steel

Loss Evaluation Methods

We assign loss values using recognized standards, such as BS 7281‑1, or by applying a loss‑capitalization formula. This formula converts the capitalized cost of losses over the transformer‘s life into an economic value, helping you select the most cost‑effective design for your specific load profile.

Therefore, by selecting the right core material and optimizing the design, we deliver distribution transformers that minimize both initial purchase cost and long‑term energy waste.

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4. Summary of Key Specifications

Parameter	Oil‑Immersed	Dry‑Type
Rating range	16 – 2500 kVA	16 – 2500 kVA
Cooling	ONAN (oil natural)	AN (air natural) / AF (forced air)
Primary voltage	1.0 – 38.5 kV	1.0 – 38.5 kV
Secondary voltage	≤ 1.1 kV	≤ 1.1 kV
Tapping	Off‑circuit (±2.5% etc.)	Off‑circuit (±2.5% etc.)
Core materials	Amorphous alloy / Silicon steel	Amorphous alloy / Silicon steel
Best for	Outdoor, industrial, harsh environments	Indoor, fire‑sensitive, low‑maintenance areas