Quality standards for transformer oil filtering, oil filling and hot oil circulation

During the oil filtering, oil filling and hot oil circulation process of the transformer, the oil quality index must be strictly controlled to ensure that the performance of the insulating oil meets the national standards (such as GB/T 7595-2017 or IEC 60296) and industry specifications (such as DL/T 572-2021). The following are the key quality standards and requirements for each link:

1. Oil filtering quality standards

The main purpose of oil filtering is to remove moisture, gas, impurities and oxidation products to ensure that the oil quality meets the standards for commissioning or continued use.

Key indicator requirements

| Project | Quality standard (new oil/operating oil) | Test method |

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

| Breakdown voltage | ≥50 kV (new oil) / ≥45 kV (operating oil) | GB/T 507 |

| Moisture content | ≤15 mg/L (new oil) / ≤20 mg/L (operating oil) | GB/T 7600/7601 |

| Dielectric loss factor (90℃) | ≤0.005 (new oil) / ≤0.02 (operating oil) | GB/T 5654 |

| Gas content | ≤1% (volume ratio) | DL/T 423 |

| Particle contamination | ≤NAS level 8 (or manufacturer's requirements) | DL/T 432 |

| Acid value | ≤0.03 mg KOH/g (new oil) / ≤0.1 mg KOH/g (operating oil) | GB/T 264 |

Oil filtration process requirements

- Vacuum oil filtration: vacuum degree ≤ 0.5 kPa (absolute pressure), oil temperature 50~60℃ (avoid high temperature oxidation).

- Mechanical filtration: filter element accuracy ≤ 5 μm, number of cycles ≥ 3 times.

- Oil quality acceptance: After filtering, the oil needs to be left to stand for ≥ 12 h, and the next process can only be entered after the sampling test is qualified.

2. Oil filling quality standards

During the oil filling process, air, moisture and impurities must be prevented from entering the transformer to ensure that the oil quality is not contaminated.

Key Control Points

| Project | Requirements |

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

| Oil filling speed | ≤100 L/min (avoid oil flow with electricity) |

| Oil temperature | 40~60℃ (prevent low temperature moisture absorption) |

| Vacuum oil filling | Residual pressure ≤ 133 Pa (110 kV and above transformers) |

| Standing time | 110 kV: ≥24 h; 220 kV: ≥48 h; 500 kV: ≥72 h |

| Gas content | ≤1% (≤0.5% for 500 kV and above) |

Post-oil filling inspection

- Electrical strength (breakdown voltage) ≥50 kV (new oil) / ≥45 kV (operating oil).

- Micro-water content ≤15 mg/L (new oil) / ≤20 mg/L (operating oil).

- Chromatographic analysis (DGA): no abnormal gas (such as acetylene C₂H₂ ≤0.1 μL/L).

3. Hot oil circulation quality standard

Hot oil circulation is mainly used for large transformers (such as 220 kV and above) to further dehydrate and degas and improve insulation performance.

Process requirements

| Parameters | Standards |

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

| Oil temperature | 60~70℃ (not exceeding 70℃ to prevent aging) |

| Circulation flow | ≥4~6 times the amount of transformer oil/day |

| Vacuum | ≤133 Pa (500 kV requires ≤50 Pa) |

| Circulation time | Usually 48~72 h (adjusted according to oil quality) |

Acceptance criteria

- Micro-water content ≤10 mg/L (500 kV requires ≤8 mg/L).

- Gas content ≤0.5% (500 kV and above).

- Dielectric loss factor (90℃) ≤0.005 (new oil) / ≤0.01 (operating oil).

- Oil withstand voltage ≥60 kV (500 kV and above).

4. Key points

1. Sampling specifications:

- Full circulation is required before sampling to avoid dead corners affecting the data.

- Use dry, clean glass bottles or special oil sample bottles.

2. Environmental control:

- Humidity ≤60% RH, avoid working on rainy days or in high humidity environments.

3. Safety requirements:

- Prevent oil temperature from being too high (≤70℃) to avoid fire risks.

- When vacuum filtering oil, ensure that the oil tank is strong enough to prevent deformation.

- Oil filtering: focus on removing moisture, gas and impurities to ensure that the oil quality meets the standards.

- Oil filling: control the oil filling speed, temperature and vacuum degree to prevent secondary contamination.

- Hot oil circulation: Enhance dehydration and degassing, improve insulation performance, especially suitable for ultra-high voltage transformers.

Strictly following the standards can effectively extend the life of the transformer and reduce the risk of failure.