Choose Cooling Configuration — Transformer Sizing Guide

ONAN, ONAF, KNAN — each impacts size, cost, and maintenance.

Step 4

Choose Cooling Configuration

ONAN is simplest and most reliable (no moving parts). ONAF adds 25–40% cooling capacity with radiator fans. Dual-rated ONAN/ONAF is common for power transformers. For dry-type: AN (natural) vs AF (forced air). Ester-cooled (KNAN) units offer superior fire safety and environmental performance.

Cooling configuration selection balances thermal performance, reliability, noise, maintenance requirements, and cost. The choice directly impacts transformer physical size and operational flexibility.

Liquid-Immersed Cooling Options

| Code | Description | Best For |

|------|------------|----------|

| ONAN | Oil Natural Air Natural | Distribution transformers, base-load applications, noise-sensitive sites |

| ONAF | Oil Natural Air Forced | Power transformers, dual-rated operation, peak load capability |

| OFAF | Oil Forced Air Forced | Large power transformers (>100 MVA), maximum thermal capacity |

| ODAF | Oil Directed Air Forced | Highest performance, oil flow directed through windings |

| KNAN | Ester Natural Air Natural | Fire-sensitive, indoor, environmentally sensitive installations |

Dry-Type Cooling Options

| Code | Description | Best For |

|------|------------|----------|

| AN | Air Natural | Indoor, noise-sensitive, lower ratings |

| AF | Air Forced | Higher ratings, ventilated enclosures |

| ANAF | Air Natural/Air Forced | Dual-rated, flexible operation |

Key Decision Factors

1. Load profile: Base-load operation favours ONAN simplicity. Variable loads benefit from dual-rated ONAN/ONAF flexibility.

2. Ambient temperature: Hot climates (>45°C) may push towards ONAF or enhanced ONAN designs with additional radiator banks.

3. Noise requirements: ONAN is silent (no fans). ONAF fans generate 55–65 dB(A). Urban sites may have noise ordinances.

4. Maintenance philosophy: ONAN has zero auxiliary equipment to maintain. ONAF/OFAF requires fan motor and contactor maintenance.

5. Fire safety: KNAN with ester fluid or AN/AF dry-type for fire-sensitive locations.

6. Space constraints: Forced cooling allows smaller transformers for a given rating.

Practical Example

A 40 MVA power transformer for a Gulf utility substation would typically be specified as 40/50/63 MVA ONAN/ONAF/OFAF — providing flexibility across three capacity ratings with progressively more cooling assistance.

Specify the Right Product

Products that match this sizing decision.

transformersMedium Power TransformersMedium power transformers for sub-transmission, industrial, and generation applications.

transformersEHV Power TransformersExtra high voltage power transformers for transmission grids, generation step-up, and interconnection applications at 220–400 kV.

transformersCast Resin Dry Type TransformersIndoor-rated dry-type transformers for clean and sensitive environments.

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