Introduction
Audible sound levels in liquid-filled transformers are a critical aspect of their operational performance and environmental impact. These transformers, typically filled with insulating oil or other dielectric fluids, generate audible noise primarily due to magnetostriction in the core laminations and electromagnetic forces acting on windings under load. As power systems expand and environmental regulations become more stringent, understanding and mitigating transformer noise has gained significant importance. Excessive audible sound can lead to community complaints, regulatory non-compliance, and even indicate underlying mechanical issues that may affect transformer reliability. This paper examines the sources, measurement techniques, and influencing factors of audible noise in liquid-filled transformers, while exploring effective mitigation strategies to ensure compliance with noise standards and enhance operational sustainability.
Sources of Sound
- Magnetostriction: The core laminations expand and contract at twice the power frequency (100/120Hz for 50/60Hz systems), generating fundamental humming and harmonics (e.g., 200Hz, 300Hz). This dominates no-load noise.
- Winding Vibrations: Lorentz forces from load current cause conductor vibrations, with frequencies tied to current frequency and its multiples.
- Cooling System: Oil pumps (flow noise + mechanical friction) and fans (air turbulence) add high-frequency noise (>500Hz) in forced cooling modes.
- Structural Resonance: Loose clamping parts, bolts, or magnetic shields may resonate at specific frequencies, causing abnormal sounds (e.g., rattling).
Typical Sound Levels
Table1. 30 – 6300 kVA, 63 kV and below oil-immersed power transformers – Sound level limit values
| Equivalent capacity / Voltage level （kVA/kV） | sound level  /dB（A） | |
| Oil Natural Air Natural（ONAN） | Oil Natural Air Forced （ONAF） | |
| 30~63/6~35 | 44 | – |
| 80~100/6~35 | 48 | – |
| 125~160/6~35 | 49 | – |
| 200~250/6~35 | 51 | – |
| 315~400/6~35 | 53 | – |
| 500~630/6~35 | 54 | – |
| 800~1000/6~63 | 57 | – |
| 1250~2000/6~63 | 59 | – |
| 2500/6~63 | 60 | – |
| 3150/6~63 | 61 | – |
| 4000/6~63 | 62 | – |
| 5000/6~63 | 63 | – |
| 6300/10~63 | 64 | – |
| 8000/35~63 | 65 | 73 |
| 10000/35~63 | 66 | 73 |
| 12500/35~63 | 66 | 73 |
| 16000/35~63 | 68 | 75 |
| 20000/35~63 | 71 | 75 |
| 25000/35~63 | 71 | 77 |
| 31500/35~63 | 75 | 78 |
| 40000~63000/35~63 | – | 78 |
Table2. 6300~120000 kVA, Sound level limit for oil-immersed power transformers at 110 kV level
| Equivalent capacity / Voltage level （kVA/kV） | sound level /dB（A） | |
| Oil Natural Air Natural（ONAN）or Oil Forced Water Forced（OFWF） | Oil Natural Air Forced （ONAF）or Oil Forced Air Forced（OFAF） | |
| 6300 | 66 | – |
| 8000 | 67 | 74 |
| 10000 | 68 | 75 |
| 12500 | 68 | 75 |
| 16000 | 70 | 76 |
| 20000 | 73 | 77 |
| 25000 | 73 | 78 |
| 31500 | 75 | 79 |
| 40000 | 76 | 79 |
| 50000 | 77 | 79 |
| 63000 | 78 | 80 |
| 90000 | 79 | 80 |
| 120000 | 80 | 80 |
Table3. 31500~360000 kVA, Sound level limit for oil-immersed power transformers at 220 kV level
| Equivalent capacity / Voltage level （kVA/kV） | sound level /dB（A） | |
| Oil Natural Air Natural（ONAN）or Oil Forced Water Forced（OFWF） | Oil Natural Air Forced （ONAF）or Oil Forced Air Forced（OFAF） | |
| 31500 | 78 | 80 |
| 40000 | 78 | 80 |
| 50000 | 80 | 82 |
| 63000 | 80 | 83 |
| 90000 | 82 | 85 |
| 120000 | 83 | 85 |
| 150000 | 84 | 86 |
| 180000 | 84 | 86 |
| 240000 | 85 | 87 |
| 300000 | 85 | 88 |
| 360000 | 85 | 89 |
Factors Affecting Sound Levels
- Core Material: Amorphous metal cores are quieter than traditional silicon steel.
- Load Current: Higher load increases noise (typically by ~2–4 dB at full load).
- Cooling Method: ONAN (oil natural air natural) is quieter than ONAF/OFAN (forced cooling).
- Tank Design: Stiffening ribs, vibration dampers, and sound insulation reduce noise.
- Installation: Mounting on anti-vibration pads or barriers can minimize transmitted noise.
Mitigation Techniques
- Core Design: Low-magnetostriction steel or step-lap core joints reduce humming.
- Sound Barriers: Acoustic enclosures or tank-mounted noise shields.
- Vibration Isolation: Resilient mounting systems.
- Oil Preservation: Proper tank sealing minimizes noise from bubbling or gas evolution.
Standards & Testing
- IEC 60076-10: Defines sound level measurement methods for transformers.
- IEEE C57.12.90 & C57.154: Provide guidelines for noise limits in North America.
- Local Regulations: Many regions impose noise limits (e.g., <65 dB(A) at 1 m for residential areas).
Measurement & Testing
- Sound pressure levels are measured in decibels (dB) using A-weighting (dB(A)) to reflect human hearing sensitivity.
- Tests are conducted at no-load and rated load per standards.
- Testing Pitfalls: Field measurements without reflection correction may have 3–5 dB errors.
Conclusion
Liquid-filled transformer noise is manageable through design optimizations, proper installation, and sound-damping techniques. For critical applications (e.g., urban substations), specifying low-noise transformers or using enclosures may be necessary.
