Background
Hydrogen in power transformers is a combustible gas that is produced within a power transformer and may provide an indication of incipient faults.
Chemical Structure
The following is the chemical structure of Hydrogen (H2)
Hydrogen is the simplest and lightest element with an atomic number of 1. It usually exists as a hydrogen gas consisting of two hydrogen atoms with the formula H2.
Formation
What causes the production of Hydrogen in transformer oil?
Hydrogen (H2)is usually formed from ionization reactions of low-energy faults such as partial discharges of the cold plasma type (corona discharges). These are generally from the scission of the weakest C-H bonds requiring around 338 kJ /mol of energy [1].
These partial discharges (PD) usually form in gas bubbles or voids trapped in paper from poor drying or impregnation of oil in paper [2]. Discharges (D1 and D2) of the sparking type can form in the liquid (oil) or solid (paper) insulation producing a high level of acetylene in addition to hydrogen [2].
Hydrogen gas may also be produced by the chemical reaction of the steel and galvanized steel within the transformer in contact with water and high levels of Oxygen. This was experienced on some power transformers where large quantities of Hydrogen were produced even though the transformers had never been energized. This is noted as being part of stray gassing [1].
Fault Analysis
Hydrogen as a key gas is usually taken as a sign of partial discharges. Depending on the amount of Hydrogen it is also found that high levels of Hydrogen with Acetylene (C2H2) are usually an indication of arcing.
Hydrogen is one of the first dissolved gases that is produced within the transformer and as the fault temperature increases the Hydrogen gas levels in oil increase accordingly. The IEEE guide for the interpretation of gases generated in oil-immersed transformers prescribes a limit of 100 ppm above which the transformer may have a developing or fault condition present [2].
Hydrogen gas can be trended over the period of operation where the gas production rates can also be assessed to identify the severity and progression of the fault. The gas production rates are usually calculated by taking the difference between ppm levels and dividing it by the period between samples in days.
Hydrogen can be used in a health index as applied in the Transformer Age Index Model to assess the health of the transformer
References
| 1. | IEC60599, “Mineral oi-filled electrical equipment in service – Guidance on the interpretation of dissolved and free gases analysis,” 2007. |
| 2. | IEEEC57.104, “IEEE guide for the interpretation of gases generated in oil-immersed transformer,” IEEE Power Energy Society, 2019. |
| 3. | Cigre-Technical-Brochure-771, “Advances in DGA interpretation,” JWG D1/A2.47, 2019. |