In step-up transformers (GSU), 14% of faults are attributable to bushing insulators (>100KV).
CIGRE WG A2.37 – Transformer reliability survey, Dec. 2015
By “Sampling of the oil insulator – SF6” we mean the activity of taking an aliquot of oil from the bushing insulators equipped with a compartment in oil and a compartment in SF6 (sulfur hexafluoride).
Bushing insulators are normally placed on the upper part of the transformer and are of different types depending on the voltages / currents involved and the type of external conductor to which they must be connected. HV insulators typically consist of two compartments, one bordering the transformer, usually filled with oil, the other in communication with the devices downstream of the transformer filled with air, oil, or SF6 (sulfur hexafluoride).
Maintenance actions applicable on bushing insulators
- Visual inspection
- Electrical tests
- Oil sampling, analysis and diagnosis (in the bushing)
Sea Marconi, following the support offered to the French customer EDF, has gained specific expertise on sampling, laboratory analysis and subsequent diagnosis of the oil present in the (oil) compartment of the bushing.
By analyzing the SF6 content in the oil and the subsequent diagnosis, it is possible to verify the presence of the criticality “SF6 in Oil – bushing seal defects“. Not only that, Sea Marconi is also able to diagnose the following “pathologies”:
- Chemical degradation (symptoms: acidity, interfacial tension)
- Dielectric degradation (symptoms: discharge voltage, moisture in the oil)
- Contamination (symptoms: moisture, particles, metals, SF6)
- Partial discharges (symptoms: hydrogen)
- Overheating (symptoms: ethylene, carbon oxides)
Representative sampling of an oil-SF6 isolator is a delicate and complex operation. The difficulty lies in the presence of a very low volume of oil in the compartment (oil) of the insulator, consequently it is necessary to perform the sampling by compensating with the same amount of oil (pre-treated). Furthermore, the utmost attention must be paid to avoid the slightest infiltration of air bubbles.
Sea Marconi has gained a great deal of experience in the field by carrying out this operation on the isolators of the nuclear EDF machine park.
Since there is no specific legislation or validated bibliographic references, to better support the customer in the management of the criticality, Sea Marconi has launched an internal research process that led to the development and implementation of:
- suitable operating procedures (in accordance with the specifications of the insulator manufacturer),
- specially made sampling devices
- specific training course of our technical staff
- an internal method for the analysis of the SF6 content in oil (see symptoms of the “SF in oil – insulator leaks” criticality)
- specific diagnostic algorithms for the interpretation of the analytical results
- the latter obviously aimed at identifying specific “pathologies” of the equipment and therefore at preventing breakdowns.
Sampling of the oil in the isolator must be performed according to the specifications provided by the manufacturer:
- with procedures suitable for the purpose and specific sampling devices
- with personnel possessing specific skills and training on the subject
- relying on operators able to demonstrate a wide range of applications and able to certify the interventions performed under quality assurance (ISO 9001)