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How to Design a Dry Type Transformer: Step-by-Step Guide with Design Calculations

2026-03-23

The design of a dry type transformer is a highly integrated multidisciplinary engineering task, encompassing electrical engineering, thermodynamics, mechanical structure, materials science, and safety standards. Unlike traditional oil-immersed transformers, dry type transformers do not rely on mineral oil for insulation and cooling. Instead, they utilize air natural cooling (AN), forced air cooling (AF), or cast resin epoxy systems. This innovative approach provides outstanding advantages such as superior fire resistance, maintenance-free operation, environmental friendliness, and suitability for indoor installations.

During the dry type transformer design process, engineers must precisely calculate electromagnetic parameters for the core and windings to optimize short-circuit impedance, load losses, and no-load losses. Thermal simulations are essential for analyzing temperature rise distribution, enabling the selection of appropriate cooling methods and insulation classes to prevent localized overheating and insulation degradation. Additionally, critical performance indicators—including voltage regulation, noise control, mechanical strength, and short-circuit withstand capability—must comply with national and international standards such as GB/T 1094 or IEC 60076 series.

Leveraging advanced CAD/CAE tools and finite element analysis, modern dry type transformer design achieves higher efficiency, lower operating costs, and extended service life. These transformers are widely applied in high-safety-demand environments like data centers, hospitals, subways, and high-rise buildings.

NPC Electric Dry Type Transformers

Step 1: Define Electrical Ratings and Application Conditions

Every transformer design begins with system requirements.

Typical inputs:

Rated power: 100 kVA – 25 MVA
Primary voltage: e.g. 11 kV, 22 kV, 33 kV
Secondary voltage: e.g. 400 V, 690 V
Frequency: 50 / 60 Hz
Cooling: AN or AF
Installation: indoor, confined space, high-rise building

Example: Basic Rating Definition

Assume:

Rated power: 1000 kVA
Primary voltage: 11 kV
Secondary voltage: 415 V
Frequency: 50 Hz
Type: Cast resin dry-type transformer

This rating determines current, conductor size, insulation class, and cooling system.

NPC Electric Dry Type Transformer Design

Step 2: Calculate Rated Currents (Primary and Secondary)

Rated current is fundamental to winding and conductor design.

Formula

Dry Type Transformer Rated Currents

Example Calculation

Primary current:

Dry Type Transformer Primary Currents

Secondary current:

Dry Type Transformer Secondary Currents

These values drive coil windings' cross-section, current density, and thermal losses.

Step 3: Selection of Dry Type Transformer Type

Based on application:

Cast resin dry-type transformer → high humidity, high voltage, fire safety
VPI dry transformer → standard indoor electrical rooms
Dry-type triplex transformers → rectifier and special industrial loads

For the example above, a cast resin transformer is preferred due to high secondary current and thermal stability requirements.

Step 4: Winding Design and Turns Calculation

Turns Ratio

Dry Type Transformer Turn Calculation

If the LV winding has 40 turns, then:

Dry Type Transformer Turn Calculation Example

Current Density Selection

Typical design values:

Copper windings: 2.5–3.2 A/mm² (dry transformer)
High reliability designs: lower current density

Secondary conductor cross-section:

Dry Type Transformer Secondary conductor cross-section

Multiple parallel copper strips or foil windings are used to achieve this area.

Step 5: Winding Resistance and Copper Loss Calculation

Formula

Dry Type Transformer Copper Loss Calculation

Assume:

Secondary winding resistance = 0.002 Ω

Dry Type Transformer Secondary winding Calculation

Copper losses are a major contributor to temperature rise, influencing cooling system design.

Step 6: Core Design and Magnetic Flux Calculation

EMF Equation

Dry Type Transformer Magnetic Flux Calculation

Where:

BBB = flux density (typically 1.4–1.6 T)
AAA = core area

Assuming:

Flux density = 1.5 T
Frequency = 50 Hz

The core cross-section is selected to keep no-load losses and noise within limits, especially critical in high-rise buildings.

Step 7: Cooling System Design (Air Cooled vs Forced Air)

Dry transformers rely on air cooling, but load and ambient conditions dictate the method.

Temperature Rise Limits

Class F insulation: 100 K
Class H insulation: 125 K

Cooling Decision Example

If calculated total losses:

Core losses: 1.8 kW
Copper losses: 3.9 kW
Total losses: 5.7 kW

Natural air cooling may be sufficient up to ~1000 kVA. If overload capacity or confined installation is required, forced air (AF) is added to increase the rating by 25–40%.

Step 8: Short-Circuit Withstand and Mechanical Design

Short-circuit forces are proportional to:

Dry Type Transformer Short-Circuit Withstand

High secondary currents require:

Rigid coil bracing
Axial and radial reinforcement
Strong resin encapsulation (for cast resin transformers)

This ensures compliance with IEC short-circuit withstand requirements.

Step 9: Voltage Regulation Calculation

Approximate Formula

Dry Type Transformer Voltage Regulation Calculation

Well-designed dry transformers typically achieve:

Voltage regulation: 4–6%

Low regulation is essential for sensitive loads and stable output voltage.

Step 10: Industry Standards and Testing

Dry-type transformers must comply with:

IEC 60076
IEEE C57
Local grid and project specifications

Routine and type tests include:

Winding resistance measurement
Temperature rise test
Partial discharge test
Sound level measurement

Dry Type Transformer Design

Application-Specific Design Considerations

Application	Design Focus
High-rise buildings	Low noise, compact size
Data centers	Forced air cooling, redundancy
Renewable energy	Voltage stability, efficiency
Industrial plants	Overload capability

Designing a dry-type transformer is a calculation-driven engineering process that integrates electrical ratings, winding resistance, cooling system performance, and mechanical integrity. By applying correct design calculations at each stage—from rated current to temperature rise—engineers ensure reliable operation, safety, and compliance with industry standards.

For critical infrastructure such as high-rise buildings, industrial facilities, and renewable energy systems, partnering with an experienced dry type transformers manufacturer ensures that theoretical calculations translate into robust, field-proven transformer designs.