Q: Many wind farms have used 3Ph distribution pad mount transformers to step up the voltage from the wind turbine to the collector system. The industry has discovered that many of these small transformers are now generating very high levels of hydrogen in the oil, in excess of 20,000 PPM from DGA tests. Please discuss the safety concerns of having high levels of hydrogen in active transformers, and safety precautions when collecting oil samples, adding oil, replacing fuses or operating the internal load break switch.
A: Transformer DGA levels >20,000 ppm are indeed high. This is >2% in the oil. Due to the volatility and buoyancy of hydrogen, I would expect that the concentration of H2 may be even higher in the transformer's gas space. It is generally recognized that hydrogen concentrations of 4-74% are flammable and at 18-59% the mixture is explosive. This means that a spark in the gas space must be avoided at all costs. If the transformer oil is at design levels, the operation of under-oil switches should be safe. However, oil levels may be low when there has been excessive gas production. Therefore I would advise extreme caution in the operation of switches -- including the extraction of bayonet fuses. Another cause for concern would be any tilting of the transformer which would cause oil to be below design levels in any part of the tank. The extraction of DGA samples should be safe, but exposure of the gas space during the process of adding oil would be another cause for concern. If the oil level is low, deenergize the transformer from another location!
Edward S. Thomas, PE
Utility Electrical Consultants, PC